Experin ~ental and Analytical Development of the Application of a

Transit Laser Velocimeter

A. E. Smart and W. T. Mayo, .Ir. Spectron Development Laboratories, Inc.

3303 Harbor Boulevard Costa Mesa, California 92626

November 1 980

Final Report for Period March 1 9 7 9 - September 1 9 7 9

Approved for public release. distribut~on unlimited.

ARNOLD ENGINEERING DEVELOPMENT CENTER ARNOLD AIR FORCE STATION, TENNESSEE

AIR FORCE SYSTEMS COMMAND UNITED STATES AIR FORCE

NOTICES

When U. S. Government drawings, specifications, or other data are used for any purpose other than a defmitely related Govemment procurement operation, the Govemment thereby incurs no responsibility nor any obligation whatsoever, and the fact that the Government may have formulated, furnished, or in any way supplied the said drawings, specifications, or other data, is not to be regarded by implication or otherwise, or in any manner licensing the holder or any other person or corporation, or conveying any rights or permission to manufacture, use, or sell any patented invention that may in any way be related therelo.

- Qualified users may obtain copies of this report from the Defense Technical Information Center.

References to named conlnicrc~al products in this report are not Lo be considered in any sense 1 as an indorsement of the product by the Unitcd Stales Air Force or thc Government. I

This final repor1 was submit~ed by Spcctmn Developn~ent L:tbor:itories. Inc., 3303 Harbor Boulevard, Costa Mcu, California 92626. undcr cont~.act F40600-79-C-0003, with thc Arnold Engineering Dcvelopnicnr Ccntcr, Air Force Systcms Conlrnand, Arnold Air Force Station. Tennesscc. M r . Marshall K. Kingcry, DOT, was ttic Air Force I'rojcct hlanagcr.

This report has been reviewed by the Office of Public Affairs (PA) and is releasable to the National Technical Information Service (NTIS). At NTIS, it will be available to the general public, including foreign nations.

APPROVAL STATEMENT

This report has been reviewed and approved.

MARSHALL K. KlSGEKY - ./ Project Manager Directorate of' Technology

Approved for publication:

FOR T H E COMMANDER

MARION L. LASTER Director of Technology Deputy for Operations

UNCLASSIFIED REPORTDOCUMENTATION PAGE READ INSTRUCTIONS

BEFORE COhlPLETISG FORM 1 REPORT NUMBER 12 GOVT ACCESSION NO. 3 RECIPIENT'S CATALOG NUMBER

I

4 T ITLE ( a d Subrlrle)

EXPERIMENTAL AND ANALYTICAL DWELOPMEYT OF THE APPLICATION OF A TRANSIT LASER VELOCIMETER

7. A J T u O R l r J

I A. E. Sna r t and W. T. Mayo, Jr.

96 31STRIBJTION STATEMENT (01 rhla Raporr) I

5 T V P E O F REPORT h PERIOD COVERED

Fina l Report T March 1979 - September 1979

6. PERFORMING ORG. R E P O a T NUMBER

SDL NO. 79-6501 8 . CONTRACT OR GRAhT NUMBER/s;

F40600-79-C-0003 I 9 PERFORMING ORGANIZATION N W E AND ADDRESS

Spectron Development Labora tor ies , Inc . 3303 Harbor Boulevard Costa biesa, Ca l i fo rn i a 92626

Approved f o r pub l i c r e l ea se ; d i s t r i b u t i o n unl imited.

?O PROGSAH ELEMEN'. PROJECT. 'ASK AREA h WORK UNIT NUMBERS

Progran: Element 65807F

17 JIS'RIBUTION STATEMENT (of rho mb8tracf entered I n B lock 10. I 1 d l l b r c n t born R w r t ; 1 18 S l iPPLEHENTARY NOTES

11 COWTROLLINS OFFICE NAME AND ADDRESS ' 12 R E P 0 9 T C I T E

Arnold Engineering Development CenterfDOS [ Xovember 1980

I Avai lab le i n Defense Technical I n f o r m t i o n Center

Ai r Force Systems Command Arnold Ai r Force S t a t i o n , Tennessee 37389

1 1 MONIrDSING AGEYCY NAME h ADDRESS(l1 df l fersnl from Conrrolf ln$ O I l ~ e e j

19 K E Y WORDS fCmrlmum on mvsrae r rde I f necensuy s l d l den f l l y by block number) 1

13 NCMBER O F PAGES

192 15 SECURITY CLASS (01 rhfa report)

l a s e r v e l o c i n e t e r s anernome t e r s d i s c r i n i n a t o r s ~ e r f ormance t e s t s i i b e r o p t i c s

20. ABSTRACT ( C ~ n t l n u e an revsrae 8rds I! necssmary and l dcn t l l y by block number) I 1 Spec t ron Development Labora tor ies , Inc. has designed, cons t ruc ted and I

demonstrated a rugged, compact, and r e l i a b l e coax i a l backscat t e r l a s e r t r a n s i t anemometer system. Measurements have been made through a 1.25" t h i c k window without seeding a t a range of 400 mm from very c l ean a i r a t 1200 m / s wi th 80 mW of l a s e r power i nc iden t on t he sampling volume and a 100 mm diameter c o l l e c t i o n l ens . The system employs many r e f i ned and/or new techniques which i nc lude s p e c i a l l y prepared s i n g l e f i b e r o p t i c l i g h t guides, s p a t i a l f i l t e r i n g of f l a r e

1 I FORM DD JAN ,3 1473 EDITION O F 1 NOV 6 5 IS OBSOLETE

w

20. ABSTMCT (Continued)

l i g h t from t h e oppos i te spot , high ga in photomult ipl ier tubes wi th preampli- f i e r s , f i l t e r / d i s c r i m i n a t o r s which r e j e c t s i n g l e photon events and e s t ima te pulse cen t e r t h e , microcomputer con t ro l of o p t i c s and e l e c t r o n i c s , wi th on-l ine flow parameter es t imat ion and graphics d isp lay .

A d i g i t a l c o r r e l a t o r was used t o f u r t h e r demonstrate t he s e n s i t i v i t y of combining nonl inear f i l t e r d i s c r imina t ion techniques wi th s i n g l e b i t c ros s c o r r e l a t i o n . The 50 ns time c lock was adequate f o r 0.6% abso lu t e agreement wi th tunnel c a l i b r a t i o n a t Mach 4.5 by i n t e r p o l a t i o n of t he cor re logran s t o r e occupation. This de l ay increment was inadequate f o r supersonic turbulence i n t e n s i t y measurenents with t he o p t i c a l conf igura t ion used. Repea t ab i l i t y of flow ang le measurenents was approximately 0.10 wi th t h e p r ina ry l i m i t a t i o n being number of p a r t i c l e events a v a i l a b l e i n a reasonable time. Previous Mie s c a t t e r i n g and photon r e a l i z a t i o n s imula t ions i n d i c a t e t h a t p a r t i c l e diam- e t e r s of 0.2 - 0.4 micrometers were observed a t Mach 8.

A F I T A r m l l AFII Tern

4

PREFACE

The work reported herein was conducted by the Spectron

Development Laboratories, Inc., Costa Mesa, California, under Con-

tract No. F40600-79-C-0003 with the Arnold Engineering Development

Center, Air Force Systems Command. Air Force Project Manager for

this contract was Marshall K. Kingery, AEDC/DOT. The work was

conducted between March and September 1979, and the manuscript for

this report was submitted to AEDC on October 30, 1979.

The reproducibles used in this report were supplied by the

authors.

The authors wish to thank Ernestine Badman and Marshall

Kingery for their support in administering the contract, and also

Joe O'Hare, Bill Strike, and Willard Templeton, ARO, Inc., for their

assistance during the tests at the Von Karman Facility.

TABLE OF CONTENTS

1.0 INTRODUCTION

1.1 LASER TRANSIT ANEMOMETRY DEVELOPMENTS

1.2 IRTERVE?IIIRG DEVELOPVENTS

1.3 CONTRACT SCOPE

2.0 ELECTRONIC DEVELOPMENTS

2.1 DISCRIMINATORS

2.2 MOTOR COSTROL FOR IMAGE ROTATOR

3.0 INTERNAL OPTICAL DEVELOPMENTS

3.1 ABERRATIONS AND OUTPUT LENS CHOICE

3.2 B M 1 PARALLELISM EFFECTS

3.3 FIBER OPTICS

3.4 CLEANING AND ALIGMENT

3.5 ALIGNMENT AIDS AND APERTURES

3.6 COMPONENT REDESIGN

3.7 IMAGE ROTATOR PRISM

3.8 COATINGS

3.9 DUST FREE PACKAGING

4.0 FLARE AND WINDOW THICKNESS EFFECTS

4.1 INTERNAL FLARE REDUCTION STOPS

4.2 INTERNAL REFLECTIONS AND SCATTER

4.3 CROSS STOP WALL FLARE REJECTION

4.4 WALL PROPERTIES

4.5 WINDOW THICKNESS EFFECTS

5.1 EQUIPMENT SETUP

5.2 TUNNEL A RESULTS (FUCH 4.5)

5.3 TUNNEL B RESULTS (MACH 8)

5.4 DISCUSSION ,

Page

7

7

8

8

6.0 CONCLUSIONS

7.0 REFERENCES

Page

63

64

APPENDICES

I -- "LASER TRANSIT ANEMOMETER WITH MICROCOMPUTER AND SPECIAL DIGITAL ELECTRONICS: MEASUREMENTS IN SUPERSONIC FLOWS" 6 7

II -- "SmICLASSICAL PROCESSING OF LASER TRANSIT ANEMOMETER SIGNALS I' 7 7

111 -- IIDISCRIMINATOR TESTS" 109

IV -- SECTION 7.0 THROUGH 7.3 OF "LASER TRANSIT ANEMOMETER SYSTEM OPERATION MANUAL" 123

v -- SECTION 7.4 OF "LASER TRANSIT ANEMOMETER SYSTEM OPERATION MANUAL1' 135

VI -- "THICK WINDOW ABERRATION EFFECTS ON A LASER TRANS IT ANEMOMETERII 169

LIST OF FIGURES

No. - Title or Description

Head and Data Management System

Typical Signal from Large Particles

Photograph of Discriminator Card and Plug In Modules

Photograph of Yotor Control Board

Prism Motor Angular Rates

Formation of Two Spots by Wollaston Prism

Relay of Spots into Test Space

Mounting for Fiber Ends

Photograph of Image Rotator Prism

Image Through Stray Light Stops

Image of Output Lens Behind Cross Stop

Comparisons of Axial Rejection

Sources of Cross Talk

Mechanism of Cross Channel Scatter

Received Image Pattern with Flare

Spot Appearance Through Normal 4" Window

Spot Appearance Through lSO Inclined 4" Window

Spot ~lp?earance Through 25' Inclined 4" Kindow (Disk of Least Confusion)

Spot Appearance Through 25O Inclined 4" Window (Near)

Spot Appearance Through 25' Inclined 4" Window way) Installation of System in Tunnel A (Mach 4 . 5 )

Optical Head with Beams Entering Fog in Tunnel A

Installation of Optical Head in Tunnel B (Mach 8 )

Printout of Correlogram and Angle Plot from Tunnel A Discriminated Output Signal from Tunnel A.

Data Rate and Data/Background Ratio Versus Threshold

Non-correlation of Measured Velocity with Threshold

Angular Plot for Tunnel B

Three Correlograms from Tunnel B

Page

A E DC-T R-80-28

SECTION 1.0

INTRODUCTION

i.I LASER TRANSIT ANE~IOMETRY DEVELOPMENTS

Laser transit, time of flight, 2-spot, and L-2-F anemometry

are all related names which have been used to describe different

implementations of a class of laser techniques for measuring two-

dimensional vector velocities. These techniques were developed in-

itially as a method to reject noise produced by flare light better

than has been achieved with the now conventional fringe laser Doppler

velocimetry technique. However, we have demonstrated backscatter single-

particle sensitivity and high supersonic velocity capabilities under this

contract which show the technique to be suitable for many windtunnel

applications, and others have shown that low-speed applications at ranges

up to hundreds of meters may be possible.

The authors and others at Spectron Development Laboratories (SDL)

have developed and demonstrated a prototype laser transit anemometer (LTA)

system under Arnold Engineering Center funding in 1978, and the reader

may wish to be familiar with the final report AEDC-TR-79-32 (May 1979)

which is available from the NTIS. In order to provide che

reader with an immediate introduction and review with additional descrip-

tions of more recent developments and measurements, we have included

reproductions of two recent conference papers as Appendices I and If.

In addition, a llst of references is provided as Section 7.0 of this

report which ~ncludes the references of AEDC-TR-79-32.

The prototype demonstration with an unseeded supersonic jet at

the AEDC in September 1978 was successful, but revealed a number of

areas of improvement including electronic speed and reliability, optical

efficiency and ease of cleaning and alignment, the understanding of thick

window effects, the operation of the microcomputer control and data

analysis system, and system and procedures documentation. During the

7

period September 1978 to September 1979, extensive improvements have

been made and these are documented here.

1.2 INTERVENING DEVELOPMENTS

The instrument (Figure l),which was created in 1978 with the

partial support of AEDC Contract No. F40600-78-C-0002,was modified

and used prior to the beginning of the present contract for super-

sonic feasibility measurements at NASA Ames Research Center (Refer-

ence 25) and for subsonic axial compressor measurements at the

General Electric Company (Reference 24). In addition, SDL funds

were expended in the further development of the microprocessor based

data management system and the correlation detection theory. Most

of the results of these activities are described in the appendices

of this report.

The present contract was awarded in March 1979. The effort

addressed the areas identified by the first AEDC contract, other

than the data management system.

1.3 CONTRACT SCOPE

During the period from March to September 1979, the optical head

of the LTA prototype system was extensively redesigned and a new unit

constructed and tested to a standard where it could be applied to the

hypersonic windtunnels at the Von Karman Facility (VKF) at the AEDC in

late September. The resulting optical head was superficially similar

to the prototype. The several and various optical and electronic changes

occupied the period until September 4, when the equipment was shipped to

the Arnold Center and installed on VKF tunnels with the assistance of

Mr. Joe O'Hare, Bill Strike, and other ARO, Inc., personnel. Feasibility

tests and measurements were conducted on Tunnel A during the night of

September 10111 and on Tunnel B the following night. Data were obtained

from both experiments and were in good agreement with the calculated tunnel

conditions.

kO

Figure i. Head and Data Management System.

m

o

oo

This report describes the results of the present AEDC contract,

i.e., the details of the design changes, the study of optical window

thickness effects on spherical aberration, the laboratory tests of

certain instrument parameters, and the feasibility demonstrations

to Mach 8 with unseeded backscatter from 80 mW of laser power

incident on the sampling volumes.

A E D C-T R -80-28

SECTION 2.0

ELECTRONIC DEVELOPMENT

Several significant changes were made to the first AEDC

prototype system of Reference 20. Pulse discriminators were redesigned

and tested. For this contract~ photomultlpliers with 14 stages and one

hlgh-speed preamplifier were used and we have noted that for improved

operation in the presence of significant flare it is better to use two

preamplifiers. In the latter case~ the anode overload detection circuits

are connected to a point between the preamplifiers. There is some

discussion yet about whether to use 12 or 14 stage tubes for the flare

constrained situation.

The Motor Control Card for the image rotator was redeslgned and

reduced to a printed circuit card; a set-up and calibrate procedure was

documented. These items are described in Section'2.2 below.

Internal optical head wiring and connectors were standardized

and "hardened" for reliability of operation in hostile envirouments,

particularly vibration. A high voltage cutoff for tube protection was

designed and fitted with care given to the safety aspect of high voltage

when the side panels are removed from the head. No further discussion of

these items is provided here.

2.1 DISCRIMI.NATORS

Figure 2 is an oscilloscope photograph of the summed signal

from both monitor channels* triggered from that one upon which the event

occurs first. Several features here indicate why the discriminators are

necessary and what they should do. They should overcome the jitter caused

by fixed level triggering of pulses of different heights. They should

reject the pulses produced from single photon events while detecting all

In the new design, the monitor output is taken from the output of the detection filter for the range selected by use of digitally selected reed

relays.

I!

A E DC-TR -80-28

Figure 2. Typical Signal from Large Particles.

]2

those whose photon bunching indicates a high probability of coming from

a scattering particle. This photograph was taken in a low speed flow

(100 m/s) using large particles (>1 vm diameter) to show the salient

effects.

The effect of separating multiphoton events associated with

particle transits and single photon events from background light pulses

by proper selection of detection filter threshold has been discussed

earlier. (See Appendix I1 and References 20 and 25 for discussions of

the detection and pulse center estimation techniques.) The first

prototype system included a wire-wripped card with plug-in integrated

circuits and seven overlapping filter ranges going down to 100 ns pulse

width for the integrated pulse center estimation filter. For the present

tests, the electronic discriminator system was redesigned as a printed

circuit board (Figure 3) with six plug-in filter/detector modules. These

originally corresponded with the fastest six ranges in the prototype

system, but for this contract, two even faster plug-in cards were

developed. These were designated ranges 00 and range 0 and had pulse

center estimation filter time constants of 25 ns and 50 ns respectively

(the corresponding detection filter widths were approximately 15 ns and

25 ns respectively). The 00 board had no prefiltering other than that

associated with the limited bandwidth of the photomultiplier and pre-

amplifier combination. In Appendix 111, we present some results of

laboratory tests of the pulse delay accuracy and jitter of the discrimi-

nator circuits. The mean systematic errors and rms jitter are seen to

be each on the order of 2 ns from those results for the fastest ranges

00 and 0.

2.2 MOTOR CONTROL FOR IMAGE ROTATOR

The circuits for motor instruction and driving to a specified

position were translated from prototype breadboard to printed circuit

construction with necessary and sufficient potentiometer adjustments

for zero and calibration of the correspondence of mechanical position

A E DC-TR-80-28

Figure 3. Photograph of Discriminator Card and Plug-ln Modules.

]4

with electronic indication. A photograph of this board is included

as Figure 4. It is connected by a Scotchflex ribbon connector for

all power and data services and to the motor/prism combination by an

eight-pin plug. Several monitor points are provided for checkout,

calibration and troubleshooting.

The 0.05% linearity potentiometer was retained as adequate for

the 0.1" resolution required of the device. Some more careful circuit

adjustments enabled the position achieved by the prism to be consistently

with +O.1° arc of the requested position except in some extreme vibra-

tion environments where the error has been observed to be as large as

0.3" arc, but this is not normal and has not often been observed. The

absolute accuracy is as high as the combination of the potentiometer

linearity and the care taken in calibration will allow, but is typically

within 0.4O over 360°.

At one stage, the new prism assembly exhibited stiction (before

the oil formula was changed) and the residual error was a function of

distance traveled. This was overcome before the thixotropy of the

lubricant was discovered to be the culprit by using a second pass through

the software to give a second dispatch instruction if the residual error

on the first attempt was larger than 0.5" arc. This feature is retained

as a safety measure even though the problem was solved by other methods.

Occasionally, the V.D.U. reports that a second attempt was necessary and

has been used to attain the final precision.

The angular speed achieved when moving to a new angular location

has two domains dictated by the tracking loop conditions. For a change

in angle of less than 20" arc then the speed to new location is 6O/s,

station to station. If the angular change is greater than 20°, then the

speed also varies with direction being 100°/s for decreasing angle and

130°/s when the angle is going towards higher numerical values. This

is because the gear train from the motor experiences more resistance in

the direction of decreasing angle.

The motor behavior is shown in Figure 5 which shows graphs on two

scales 0 - 20" and 0 - 400" and the time for an angular traverse in these ranges regardless of starting position.

AEDC-TR-80-28

Figure 4. Photograph of Motor Control Board.

16

Figure 5 . Prism Motor Angular Rates

SECTION 3.0

OPTICAL DEVELOPXENTS

The prototype instrument was built using general principles of

good optical design. Only after some experience of the machine performance

was it possible to select those critical areas where more careful study

would produce significant improvement in performance, either in efficiency

or in rejection of flare.

3.1 ABERRATIONS AXD OUTPUT LENS CHOICE

The prototype system (used for the initial AEDC test - Reference 20 - and the hues test - Reference 25) had an output lens system of 3"

diameter lenses. These cemented achromats of fair quality and price were

shown to limit the ultimate performance of this instrument very severely.

There were two restrictions - residual spherical aberration and inadequate diameter. Because it was necessary to cure the former and this involved

great effort, we decided to cure both sources of limitation by making a

4" system at increased cost to increase collecting power. The internal

collimation now had an £14 requirement and so far the only lens appro-

priate has been the Tropel 100 m, 400 mm focal length, collimator which

has diffraction limited performance at any single visible wavelength,

but is not achromatic. This last limitation carries a penalty for

fluorescence work, but for such applications, the focusing qualities

are less important and an inferior f/4 100 m diameter cemented doublet

achromat may be installed. For a throw of about 400 nun, an output lens

of the same specification as the internal collimator lens may be used.

For 600 mm, a special lens has been obtained and proved very good. It

is a cemented doublet which achieved near diffraction limited performance,

actually slightly better than the Tropel, but it was working at £16

which is much easier than f/4. Other lenses which proved to be adequate

were 900 m and 1600 mm air-spaced doublets, but many commercial lenses

were rejected before these were found. KO realignment is required

except as discussed under Section 3.2 for very high precision in low

turbulence.

The quantification of freedom from spherical aberration is further

analyzed and discussed under Section 3.3.

3.2 BEAM PARALLELISM EFFECTS

The focusing lens images two points from the diverging but almost

collimated small diameter beams emerging from the Wollaston prism. The

center of the beamsplitter is near to the back focal plane of this lens

which acts as a Fourier transforming element. The transform planes are

not exact and the focusing lens may be axially translated to make the

final output spots of light parallel to each other and the axis. The

beams will suffer divergence if there is not due allowance for the magni-

fication variation with axial position for an extended object in the

projection system.

Figure 6 shows that for parallel spots in the output focal plane,

it is essential to have the equivalent point of divergence in the back

focal plane. Allowance must be made for the equivalent fore-shortening

due to the refractive index of the prism. Because of the longitudinal

variation of magnification shown in Figure 7, it is necessary to compensate

this divergence with converging input spots. The system is designed to

accept different output lenses and the first internal collimator (A in

Figure 7) is set to produce parallel light so that the change of external

lens B incurs minimum realignment problems. However, an output lens

exchange, for reasons shown by inspection of Figure 7 will change the

equivalent amount of required compensation to a small degree. The

effect is only a few milliradians for typical dimensions but for low

turbulence applications should be removed by readjustment of the Wollaston

prism and focusing lens separation.

Reference 24 discusses this same point.

Figure 6. Formation of Tko Spots by Wollaston P r i s m .

Figure. 7. Relay of Spots into Test Space.

3.3 FIBER OPTICS

The original idea of using fiber optics to relay the light from

the field image to each photomultiplier was good but required some

refinement. Several areas needed attention, the matching of equivalent

field size with final return image, allowance for all aberration sources,

the reduction of insertion losses and protection from damage.

The mounting method for adjustable fiber optics was selected to

be within hypodermic needles and both the needles and fibers are only

available in a limited number of sizes. After some deliberation about

topics such as mechanical handling, strength, bending radius and packaging,

230 pm diameter fused silica single fibers were selected. Yultiple

fibers immediately incur a high insertion loss because of interfiber

spaces. The magnification availahle from any of a number of microscope

objectives available as an enlarging lens conpensates partially for the

fixed choice at this point. 4 x7 objective was adequate although

calculations show that the diffraction limited image would thus use less

than all of the available fiber diameter. The additional size compensates

for some aberration and even a small amount of misalignment. At the same

time, it is sub-optimal in the theoretical rejection of light scattered

from the axial regions near the probe volume. There is a trade-off here

between collecting all the light and restricting the probe volume length.

There are other trade-offs related to these same parameters (see Sections 3.5

and 3.6).

k'e chose to use this configuration as an all-around optimum but

note that exchange of the enlarging lens will either increase the focal

volume definition at the expense of light collection efficiency and

difficulty of alignment with potential for adverse effects from alignment

drift, or will improve the light gathering power (slightly) and improve

the tolerance to misalignment but sacrifice some restriction of the probe

volune length.

Initially, the fiber ends were cleaved normal and mounted to

protrude slightly from their fragile low refractive index sheath and to

stand proud of the needle end. This was poor, being inadequate fiber

support and leaking light. A second batch of fiber was obtained with a

much super ior and more r i g i d low index shea th and t h e same cons t ruc t ion

now was phys ica l ly sound but o p t i c a l l y made poor sense. The pro t ruding

f i b e r was being used a s t he f i e l d s top but was not l im i t ed i n i ts

acceptance of l i g h t by the l i m i t s of t he co re f i b e r . The cladding permit ted

l i g h t t o l e a k i n t o t he core.

Figure 8a shows t h e i n i t i a l cons t ruc t ion a s i t suf fered from poor

s t o p d e l i n e a t i o n and had extreme tendencies f o r t he edge of t h e cleaved

f i b e r t o break o f f . The l o s s of t he se corners a f f ec t ed the near f l a t

normal end face and caused t h e i n s e r t i o n l o s s t o be up t o a f a c t o r of 20

which w a s t o t a l l y unacceptable.

With t h e improved cons t ruc t ion , Figure 8b, t he f i b e r s were r i g i d ,

wel l p ro tec ted from acc iden ta l damage and, cur ious ly , showed l e s s tendency

t o a c c r e t e dus t . The b lack rubber surrounding t h e f i b e r end was an

e f f e c t i v e s t o p t o r a d i a t i o n ou t s ide the a r e a of t he fused s i l i c a and

a l s o made alignment much e a s i e r . When the f i n a l image i s focused upon

t h e now f l a t end of t he e n t i r e hypodermic assembly, i t is easy t o s e e t he

b r i g h t best-focused p a t t e r n on e i t h e r t he metal r i m of t h e hypodermic

needle o r t h e b lack rubber . When the adjustment is c o r r e c t , t he end of

t h e f i b e r g ives r i s e t o l e s s s ca t t e r ed r a d i a t i o n because t he co re of t h e

f i b e r appears t o t a l l y dark provided t h e o the r end i s i n darkness.

The f i b e r s a r e about 18" long s o t h e t ransmission l o s s e s , even

i n a f i b e r , poor by corrmunication s tandards , i s q u i t e neg l ig ib l e . For

a spot of low divergence smal le r than the f i b e r end and placed s o a s t o

be wliolly w i th in t h e f i b e r edge, bu t no t neces sa r i l y centered , t h e o v e r a l l

t ransmission is t y p i c a l l y 82%. Any reduct ion from t h i s impl ies damage and

t h e f i b e r u n i t must be discarded. I n the conf igura t ion of t h i s instrument ,

t h e output from the f i b e r s i s r e s t r i c t e d t o a cone of about 10' t o t a l

angle . It is not pos s ib l e t o misal ign the f i b e r incidence angle s u f f i -

c i e n t l y badly t h a t t h i s output w i l l d iverge t o a much l a r g e r o r dark

centered cone and f o r t h i s reason i t is not necessary t o r eco l l ima te t he

l i g h t a t t h e photomul t ip l ie r end p r i o r t o any narrow band f i l t e r i n g which

may be des i r ed .

1 Steel Hypodermic Needle 2 Epoxy Adllcsive 3 Low Refractive Index Cladding 4 Optical Fiber 5 Well Cleaved End 6 Black Rubber Cement

Figure 8. Mounting for Fiber Ends

3.4 CLEANING AND ALIGNMENT

The cleaning procedures f o r t h e instrument a r e out l ined i n

Appendix I V which i s Sect ion 7.0 through 7.3 of t h e opera t ing manual

f o r t he instrument. There a r e s e v e r a l new design f ea tu re s here which

make cleaning e i t h e r e a s i e r o r less necessary than i n the prototype

instrument.

Because of t he extremely c r i t i c a l o p t i c a l performance of t h i s

machine, t h e o p t i c a l alignment is very important. It i s designed t o

preserve the c o r r e c t alignment once i t is f u l l y a l igned and t h e r e a r e

small and s p e c i f i c adjustments t o co r rec t f o r l a s e r aging and o the r

long-term d r i f t e f f e c t s . The d e t a i l s , which w e have developed under

t h i s con t r ac t , a r e long and tedious and a r e out l ined i n Appendix V

which i s a l s o Sect ion 7.4 from the operat ing manual. This s e c t i o n i s

not y e t completely foolproof. There is a d e t a i l e d sequence f o r a l i gn ing

the image r o t a t o r prism but minute e r r o r s i n t h i s may i n t e r a c t wi th

the b a s i c system alignment d e t a i l e d here. The undesirable consequences

of t h i s are t h a t e i t h e r t h e spo t s may be co r rec t with the l e n s output

pos i t i on sus t a in ing a smal l o r b i t of t he shape of t he Limacon of Pasca l ,

o r v i c e versa. A s y e t , we have not completely analyzed t h i s e r r o r but

i t may be removed by c a r e f u l manipulation of the adjustments on the

prism and turn ing mirror supports.

3.5 ALIGMENT AIDS AND APERTURES

The sequence of Appendix I V is f a c i l i t a t e d by design changes

such a s t h e i n s e r t i o n of a c i r c u l a r s top j u s t above the Wollaston

prism. Not only does t h i s serve t o keep t h e prism r e l a t i v e l y f r e e

from gravity-deposited d i r t , i t serves a s an intermediate alignment

aper ture f o r t he l a s e r so t h a t i f its cav i ty is changed t o r e s t o r e

maximum power f o r any reason, it is very easy t o r e s t o r e the alignment

of the system without recourse t o t h e f u l l procedure. This may be done

by gen t l e adjustment of t he f i r s t mirror encountered from the l a s e r .

Centration of beams is made possible at several points in the

system, image rotator input and output and final lens aperture, by use

of the target patterns of Figure 7.4.11 in Appendix V printed on to

overhead projection transparency material and cut to suitable diameter.

The input aperture from laser to optical system is arranged so that the

system is aligned when the beam is normally incident and centered on the

hole; the condition is assured by multiple reflections from an inserted

microscope cover slip.

3.6 COMPONENT REDESIGN

The turning mirror and its mount were extensively redesigned

for two reasons: (a) to reduce the size of the assembly so that it

could be brought so close to the image rotator that the latter became

the limiting £/No aperture, (b) to facilitate critical final alignments

by kinematically separating the available degrees of freedom and, (c) to

allow for insertion of a new type of stop (Reference 24 and Section 4.3

of this report). The new design is smaller and simpler (See Figure 7 . 4 . 7

of Appendix V, Items I through L) . The output lenses and mountings were significantly changed to

allow up to 4" free diameter lenses in standardized cells. The new

lens cell yokes are precisely centered so that no realignment is

necessary on the exchange of the outer lens. A small amount of axial

adjustment is present to allow perfect collimation after the beam

parallelism has been corrected by translation of the focusing lens

with respect to a fixed Wollaston prism. There is a difference in

free length of Wollaston prisms of different divergence angles and

only the shorter ones (1/2O and lo) may be translated axially in the

support. There was substantial redesign of the optical head assembly

to accommodate the new electronics cards and to be the basis of stan-

dardization of connections. These services are now sufficiently

hardened to provide high reliability.

3.7 IMAGE ROTATOR PRISH

The image r o t a t o r prism has been redesigned inc luding glassware,

mechanics and coat ings.

The new r o t a t o r prism conforms t o the design and desc r ip t ion of

References 20 and 23 (Figure 9) but has a few small and s i g n i f i c a n t

improvements. The p rec i s ion b a l l bearings were replaced with p l a i n

bear ings which had a mixed e f f e c t . Side f l o a t was e f f e c t i v e l y removed

giving more p rec i se loca t ion of the mechanical a x i s and b e t t e r freedom

from backlash which gave minute cyc l i c e r ro r s .

The change was not a t o t a l cure because a small amount of end

f l o a t now necessary t o provide f r e e r o t a t i o n gave a very s l i g h t g l i t c h

a t one poin t of t h e cycle. The magnitude of optical /mechanical a x i s

misalignment on the occurrence of t he g l i t c h va r i ed between zero and

about 30% of the spot separa t ion . This does not cause problems i n any

app l i ca t ion so f a r performed o r envisaged. The bear ings were lub r i ca t ed

with th ixo t rop ic o i l a t f i r s t , but t h i s was l a t e r replaced by ref ined

watch o i l wi th the opportunity f o r re -o i l ing with age and use. The

b r a k e could now be dispensed wi th , but a mechanical end s top was s t i l l

incorporated.

The glassware of t h e l a r g e base prism was redesigned t o give a

l a r g e r ape r tu re without o the r penalty. This redesign a l s o eased manu-

fac ture . Both t h i s p r i s n and the mir ror a r e now hard coated wi th s i l v e r

and a MILSPEC overcoat so t h a t they may be cleaned. The normal r e f l e c t i v i t y

w a s reduced t o between 95% and 96%, bu t t h i s was thought acceptable i n l i e u

of increased d u r a b i l i t y . A l l non-active sur faces of glassware were

painted wi th o p t i c a l b lack p a i n t t o suppress r e f l e c t i o n s of l i g h t t rans-

mit ted by the nominally r e f l e c t i v e coat ing. I n systems such a s t h i s ,

where the re is g rea t emphasis on s t r a y l i g h t suppression, a number of

such t a sks produced b e n e f i c i a l r e s u l t s .

The f l a t mir ror mount was changed from simple b o l t i n g t o a

reference counterbore t o a 3-point kinematic mount t o avoid the d i s t o r t i o n

which became evident on in t e r f e rome t r i c t e s t i n g of t h e completed assembly.

A EDC-TFI-80.28

Figure 9. Photograph of Image Rotator Prism.

28

A EDC-TR-80-28

The motor gearing ratio was increased to aid better response

to small signals from residual errors. There was some difficulty in

maintaining equality of rotational resistance at any angular position

as the entire assembly was not perfectly rigid for reasons of lightness.

Selective shimming on assembly and three-polnt mounting to the spine plate

minimized any unsatisfactory consequences.

3.8 COATINGS

There is a requirement for high optical efficiency, low flare,

and practicability of cleaning optical components and this places a

strict specification on reflective and anti-reflectlve coatings. For

the 45 ° mirrors, the Newport Research Coating DM.2 was specified. This

is a cleanable hard dielectric coating of 99.8Z reflectivity at 45 °

for both argon wavelengths, 488.0 nm and 514.5 nm. Unfortunately, it

was not practicable to obtain a similarly excellent coating for the

turning mirror. Such a coating is not suitable for the image rotator

prism whose incidence angles are predominantly 60 ° and 30 ° with an f/4

angular spread of 7.2" half angle. For this and the turning mirror, we

specified hard overcoated silver and accepted the loss of 4Z per reflec-

tion per surface in order to obtain mechanical cleanabillty and insensi-

tivity to polarization (hence the metal). With any sensitivity to rota-

tion of the polarization vector, the spots will change their relative

intensities and impair the statistics of data obtained at a range of

angles.

Wollaston prism, focusing lens and all output lenses were broad-

band coated with best pcrformance in the green. The 1/4Z reflection of

the Tropel was a considerably better performance than that of other components.

The cleaved fiber ends were not coated, nor were the photomultipller faces.

Although there is considerable emphasis on not losing light, the more

important property is not to scatter nor reflect unwanted light into

places where it could result in detected photons or bunches of photons.

29

AEDC-TR-80-28

All the above coatings are sufficiently durable to be cleaned

by the methods of Appendix IV and show considerable resistance to

accidental mechanical damage. Because this is not an imaging system

'per se', small flecks of deposited dust or scratch and dig damage do

not contribute primary malfunction. As they worsen, the scattered light

may rise but it may be rejected over a wide dynamic range of photon rate

by the electronic methods outlined in References 20 through 22.

3.9 DUST-FREE PACKAGING

The exclusion of dust and other contaminating media is of

importance for long-term functioning of the optical system. To this

end, the head is sealed by velvet tape, heavily compressed at each

metal Joint. A stepped construction has also been adopted which assists

in the rejection of foreign matter (and llght, of course!). The side

panels are pre-flexed so that pressure is maintained upon the seals while

preserving the ability to remove the panels easily with only four thumb

nuts each. The front panel at the top of the head is quite easily

removable so that small mirror adjustments may be made without interfering

with the main components, nor opening the head in a test environment.

30

SECTION 4.0

FLARE AND WINDOW THICKNESS EFFECTS

The system r e l i e s b a s i c a l l y upon c l a s s i c a l de t ec t ion of bunches

of photons sca t t e r ed from p a r t i c l e s i n t he flow through the t e s t space.

Any s t r a y l i g h t i s e s s e n t i a l l y s ca t t e r ed l a s e r l i g h t and i s thus a Poisson

d i s t r i b u t e d stream of photons. I n p r i n c i p l e , as long a s t h e s t a t i s t i c a l

f l uc tua t ions of photon r a t e give a very low p robab i l i t y of two o r more

s t r a y l i g h t photons occurring wi th in one p a r t i c l e t r a n s i t time, and multi-

photon p a r t i c l e s c a t t e r i n g occu r s , t h i s source of e r r o r may be e n t i r e l y

r e j ec t ed by thresholding. Actual ly, t h e cutoff is not unambiguous and

i t is always important t o reduce f l a r e a s much a s poss ib l e from both

i n t e r n a l and ex te rna l sources. This becomes e spec ia l ly t r u e when the

s i g n a l from p a r t i c l e s i s an average of only two o r l e s s photons and the

threshold d iscr iminator w i l l no longer r e j e c t noise. I f , i n t h a t condi t ion ,

t h e noise from sca t t e r ed photons is low enough, t he system w i l l s t i l l

e x t r a c t d a t a from the t e s t s i t u a t i o n . F l a re r e j e c t i o n is degraded by a l l

sources of sphe r i ca l a b e r r a t i o n , as discussed previously f o r l ens e f f e c t s .

I n t h i s s ec t ion , we a l s o present a study of the sphe r i ca l abe r ra t ion

e f f e c t s of t h i ck f l a t windows. This is discussed i n g rea t e r depth i n

Appendix V I .

4.1 INTERNAL FLARE REDUCTION STOPS

The two ape r tu re s , t he l i g h t admission ho le i n t o t h e box and t h e

s top r e t a ined by O.P. on Figure 7.4.7 of Appendix V each minimize f l a r e

from s c a t t e r o r d i f f r a c t i o n sources from the l a s e r t o t h a t s top. This

l a t t e r ape r tu re is dual funct ion i n t h a t i t is an alignment a i d and a

f l a r e suppression s top . Its second funct ion becomes more re levant i f

t h e mir rors preceding i t i n the o p t i c a l t r a i n become dusty o r damaged.

The beam incident upon the turning mirror has a Gaussian

spread function and an associated group of haloes from multiple

reflections and forward scattered light in the Wollaston prism and

focusing lens. Residual transmission through the coating and diffraction

from the edges of the turning mirror are suppressed by a small square

baffle fixed behind the mirror. This becomes the limiting center stop

of the return system (Reference 13) and may have different configurations

according to function. For windtunnel tests, it was not fitted with

'wings' (See Section 4.3) as collection of light was more important

than rejection of external flare.

The projected silhouette of the turning mirror is square

because we have chosen to use a right-angled prism of 2.7 mm side as

a readily available and adequately flat component. Accordingly, the

guard stop under discussion is square and incurs typical 'square stop'

diffraction patterns from scatter and reflections in the output optical

train. At their 4S0 displacement, the receiving apertures cannot observe

the bright lines of this pattern and thus no additional diffraction penalty

is incurred.

The stop is made from 0.005" brass shim stock with carefully

finished edges and chemically blackened surface and is slightly adjustable

in position to compensate for any manufacturing or other system assymetries.

Observation through the return apertures will quickly assure that this stop

is optimally located and further identify any flare sources in the system.

The return picture is quite complicated and exists as a different view

according to which plane is focused.

4.2 INTERNAL REFLECTIONS AIiD SCATTER

The construction of this instrument is such as to have very

little of the source laser light returned to the photocathodes by unwanted

means, particularly those due to optical components in the optical head

system itself. The laser produces typically 3 x 1017 photonsls while

typical flare light from all internal sources (i.e., with the lens capped 4 6 on the outside) is between 10 and 10 photons/s. Thus, the internal

design achieves a flare suppression of 11 to 13 orders of magnitude.

On typical high speed ranges, the probability of a photon

coincidence within a transit time is low and thus two-photon particles

are detectable above the background as was predominantly so in the tests

reported in Section 5 of this report.

To achieve these levels, it is necessary to have small, well

chosen stops and to arrange that reflections from even fifth and sixth

order images in glass components are geometrically excluded from receiving

apertures. There is a curious effect from diffraction of the return light

at the pinhole apertures. This is manifest very obviously with a scatterer

of surface texture in the range of prn placed at the sampling volume, and

is also observable as an integrated pattern from sufficiently dense

particles. Figure 10 is a photograph of the image of the return stops

showing that the focus is significantly smaller than the stops. The radial

diffraction from the stop edge is not finally included in the detected

light for the most restrictive field stop is the end of the fiber optic

but it is an effect which should perhaps be considered further. If the

pinholes were to be made smaller to reduce scattered light more, then

this diffraction becomes worse. The patterns of diffraction may be seen

in Figure 11, an image of the output transceiver lenses, taken through

the center stop and receiving system up to the fiber ends. These are

light random bands just discernible in the area blocked by the cross stop

(see Section 4.3 and remember this figure) .

4.3 CROSS STOPWALLFLAREREJECTION

Light returned to the instrument may be from small particles or

from walls or other physical objects in or near the sampling volume.

The rejection of this light is very important when measurements are to

be made near model surfaces (in boundary layer regions, for example) or

in reentrant cavities such as between blades in turbomachinery.

A EDC-TR-80-28

Figure I0. Image Through Stray Light Stops.

Figure ii. Image of Output Lens Behind Cross Stop.

34

A s a r e s u l t of making measurements i n a compressor, the a x i a l

r e j e c t i o n of f l a r e was seen t o be good from i t s ohm channel, but t h e r e

w a s c ros s t a l k from the o the r channel. Tes t s were performed and Figure 12

w a s obtained by scanning a Lambertian s c a t t e r e r , with t ex tu re of a micron

o r two and f a i r opaci ty , through the a c t i v e region of an LTA system. The

f a r f i e l d s c a t t e r p a t t e r n has a s c a l e which was small compared with the

rece iv ing ape r tu re except wi th in a few t ens of microns of the b e s t focus.

The r e tu rn s i g n a l was p l o t t e d t o show t h e s e n s i t i v e region with only t h e

cen te r and f i e l d s tops i n place and we see t h a t these a r e e f f e c t i v e i n

con t ro l l i ng the a x i a l ex tent of s e n s i t i v i t y t o i t s own channel illumina-

t i o n down t o two-and-one-half orders of magnitude below the peak. From

t h i s do t t ed curve i n Figure 12 , i t i s obvious t h a t t he re i s a r e tu rn of

s e n s i t i v i t y t o f l a r e wi th increas ing a x i a l d i s t ance from the s e n s i t i v e

volume i n both d i r ec t ions . Pigure 13 shows how t h i s spurious l i g h t a r i s e s

from i l l u n i n a t i o n i n the o the r channel and Figures 14 and 15 t h e mechanism

of production and appearance of the annular f l a r e on the f i e l d s top. The

e f f e c t of c ros s channel f l a r e i s manifest when the halo from one beam

crosses t he r e t u r n ape r tu re from the o the r a s t h e ha lo expands and defocuses.

The e f f e c t is analogous but inverted a s t he s c a t t e r source is

t r ans l a t ed from one s i d e of t h e s e n s i t i v e volume (a d i r t y window, say) t o

the o ther (a wal l o r o the r mechanical boundary). The cure f o r t h i s most

de t r imenta l l i m i t t o w a l l proximity is t o a t t a c h wings t o the center s top

which occlude the f l a r e p a t t e r n a s shown i n Figure 7.4.35 of Appendix V.

This ' c ros s s top ' then suppresses the f l a r e t o t h e s o l i d curve of Figure 12.

The r e a l appearance corresponding t o Figure 7.4.35 of Appendix V

is shown as Pigure 15, a photograph of t h e e f f e c t of t he c ros s s top on the

l i g h t c o l l e c t ion cone.

This o p t i c a l improvement we have ca l l ed a ' c ross s top ' because i t

e l iminates channel c ros s t a l k , i t i s a c ros s between a f i e l d s top and an

ape r tu re s top , and i t i s shaped l i k e a c ros s . It is poss ib le t o use the

empir ica l d a t a of Figure 12 t o es t imate the achievable wal l proximity

knowing the p a r t i c l e s i z e s a v a i l a b l e t o s c a t t e r . I t must be noted t h a t

the r e j e c t i o n shown is re levant t o a s c a t t e r e r which covers t h e whole

-- A1.L STOPS

---- CRhTER AKD FIELD STO?S ONLY

unit

FIGURE 12. COMPARISONS OF AXIAL REJECTION

PIE1.D ' / IMAGE

FIGURE 13. SOURCES OF CROSS TALK

BEST FOCUS

FLARE IMAGE ON

FIELD STOP

LENS

FIGURE 14. MECHANISM OF CROSS CHANNEL SCATTER

FIGURE 15. RECEIVED IMAGE PATTERK FITH FLARE

CROSS CHANNEL SCATTER

BEST FOCUS IMAGE

illuminating beam: this is usually true for the flare source but the

ratio of useful scatter to flare must be reduced by both area and

efficiency factors for particles, and, hence, the closeness to the wall

in an active experiment has other factors involved. The measurements of

focal depth corresponding to the quantification of geometrical properties

are summarized on Table 7.4.37 of Appendix 111.

4.4 WALL PROPERTIES

Recent tests (Reference 24) and a demonstration in a transonic

axial compressor at the Naval Postgraduate School, Monterey, confirmed

the desirability of having the internal surfaces close to which measurements

are required finished in matt black. Aluminum may be black anodized satis-

factorily; otherwise, a black paint which dries to a texture similar to

that of the base material is acceptable. In the tests reported in Reference

24, the original blade finish was translucent fiberglass and this was not

acceptable. Black painting produced an adequate improvement. Data were

obtained without seeding within 0.2" of walls 'square on' in this test,

but that was before (and a significant motivation for) the invention of

the cross stop. Seeding was finally used to increase data rate, but the

size of particle which may be used as a signal source depends on how much

external flare must be tolerated.

For windtunnel applications, flare is much less of a problem

since the sampling volume need not normally be so close to a material

surface.

4.5 WINDOW THICKNESS EFFECTS

An analytical and computational study of the spherical aberrations

introduced by windows of different thicknesses and material is presented

as Appendix VI of this report. The Appendix presents graphic simulations

and parameter design curves. The results show that typical applications

in windtunnels with windows less than 2" thick do not suffer appreciable

degradation for near normal incidence. For either thicker windows or

incidence in the neighborhood of 25", there is serious degradation from

tangential and sagittal astigmatism and the qualitative effects are shown

here as Figures 16 through 20.

Figure 16 shows the appearance of the spots at normal incidence

through a 4" window of optical glass of Schlieren quality. Figure 17

shows the 'disc of least confusion' for 15" incidence and Figure 18

shows the same for 25" off axis inclination. The Figures 19 and 20 are

the tangential and sagittal foci, respectively, and show that the system

will not operate at all satisfactorily in this geometrical configuration.

Quantification of the astigmatic defects has not been carried to

the same lengths as the spherical defects in Appendix VI.

A EDC-T R-80-28

@

@

16.

@

17.

18.

i

16.

17.

18.

19.

20.

Spot Appearance Through Normal 4" Window.

Spot Appearance Through 15 @ Inclined 4" Window.

Spot Appearance Through 25 ° Inclined 4" Window. (Disk of Least Confusion)

Spot Appearance Through 25 ° Inclined 4" Window. (Near)(Tangentlal)

Spot Appearance Through 25 @ Inclined 4" Window. (Away)(Saglttal)

19. 20.

42

SECTION 5.0

A. E .D . C . DEMONSTRATION

Equipment constructed along the lines of the earlier part of

this report was shipped to A.E.D.C. on September 4, 1979, preparatory

to feasibility tests on whatever of the VKP tunnels was available in

the following two-week period. In the event Tunnel A (at a fixed

Mach No. of 4.5) was available for several hours on the night of

September 10th and Tunnel B at Mach No. 8 available for the 40 minutes

run up to condition and 5 minutes at condition on the night of September

11th.

5.1 EQUIPMENT SETUP

The first few days were spent in laboratory checkout and per-

formance verifications with some last minute improvements in motor

calibration precision, discriminators and spot spacing calibration.

This last was probably good to about 1% or better. The angle of spots

was not calibrated for this exercise as we were interested in precision,

not absolute accuracy - there was also no horizontal datum referred to the tunnel because of the type of stand used.

Two output lenses were used, the standard 400 nnn Tropel and a

newly obtained 600 mm, whose performance as a lens appears excellent,

but in this installation was less effective because of its larger focal

length. The performance of this machine depends typically on the

fourth power of the focal length and is thus five times worse with the

600 m lens. The way that the performance is defined is extremely

complex and this simple rule is not totally correct, but may be a

useful guide. Effects of increasing focal length are larger spots and

separation (same ratio), longer probe volume (squared), smaller collection

aperture and possible small changes in aberrations. The final configura-

tion used on these tests is summarized in Table I.

Table I.

Laser Power

Throw

Spot Size

Spot Separation

Size to Separation Ratio (For 600 mm lens separation was 875 urn with other parameters scaled)

P.M.T. High Voltage A

P.M.T. High Voltage B

Window Thickness

Tunnel A

Tunnel B

Discriminator Ranges Used

Preferred I I Tunnel A Range (s) Tunnel B

1.75"

1.25"

00, 0, 1

0

00 and 0

During the Tunnel A t e s t , t he o p t i c a l head was mounted upon a

cant i levered platform which w a s prone t o v i b r a t e so t h a t i t w a s q u i t e

unpleasant t o p lace a hand on t h e u n i t . The head suf fered t h i s l e v e l

of v i b r a t i o n throughout t he e n t i r e s h i f t - 7 hours more a f t e r t h e r e s u l t s

reported here were obtained - without any not iceable d e t e r i o r a t i o n of

alignment nor e l e c t r o n i c u n r e l i a b i l i t y . This gave us some confidence

i n t he head engineering and i l l u s t r a t e d s u i t a b i l i t y f o r long-term opera-

t i o n without ser ious maintenance.

Figure 21 shows the i n s t a l l e d system i n Tunnel A and Figure 22

a b e t t e r view of the o p t i c a l head with the beams c l e a r l y v i s i b l e i n the

fogging condit ion a t s t a r t u p and f o r 77 minutes u n t i l the fog c leared

such t h a t a s igna l was obta inable , and the l a s e r beam became almost

i n v i s i b l e . Figure 23 shows the i n s t a l l a t i o n i n Tunnel B where the

o p t i c a l head was s l i d on t o the r a i l s of an e x i s t i n g t r ave r se assembly

which was a l ready s e t up f o r the t e s t . This simple mounting meant

t h a t t h e head and a l l the r e s t of the equipment could be removed i n

f i v e minutes, a f t e r t he tunnel reached condit ion and reverted t o t he

prime user . I n t h i s f i v e minutes, the r e s u l t s reported here were

obtained . The o p t i c a l head was supported by the stand-alone d a t a management

system shown i n Figure 1 and in ter faced t o a 96 s t o r e 50 ns K7023 Malvern

c o r r e l a t o r kindly loaned by Yr. J. F. Meyers of NASA Langley who wit-

nessed these t e s t s .

The simple program i n t h e present da t a management system was

used t o es t imate mean speed and angle and i t is noted t h a t b e t t e r

es t imates may be made with more soph i s t i ca t ed software. This was no t

s i g n i f i c a n t here a s t h e prec is ion was more dominated by the s t a t i s t i c a l

p rope r t i e s of the smal l numbers of events leading t o e s t i n a t e s . Some-

what su rp r i s ing ly , f o r low turbulence flows, t h e p rec i s ion of ve loc i ty

es t imate may be much g rea t e r than the s t o r e spacing of t h e co r re l a to r .

A t some of these measurements the peak w a s only -9 s t o r e s from zero ,

but a s long a s t he occupation number of each s t o r e i s adequate, a v e l o c i t y

es t imate t o a f r a c t i o n of a percent may be made by in t e rpo la t ion .

4~

> m

O

do o

0o

Figure 21. Installation of System in Tunnel A (Mach 4.5).

Figure 22. Optical Head with Beams Entering Fog in Tunnel A.

>

m

O

o

rfl

AE DC-TR-80-28

F 4~

i

Figure 23. Installation of Optical Head in Tunnel B (Math 8).

48

The printout from an experiment performed with the present soft-

ware yields a data record of which Figure 24, taken from the Tunnel A

sequence is a fair example. An explanation of the legend for this figure

is as follows: The top graph is a print of the first 25 correlogram stores

normalized to 5 units on the ordinate. The abscissa is store number with

tick marks every ten. The vertical line represents the nearest resolution

element of the pointer to the calculated peak taking one store on either

side of the maximum store as a potential contributor to the mean velocity

once the background is subtracted. DTHETA is the angular step through

which the machine thinks it should search to find the best estimate of

flow angle. This is based on an estimate of longitudinal turbulence

from the correlogram and assumed isotropic variation. It is not critical

to the correct estimate of angle so long as the probability of transit

declines on either side of an angle at which a measurement is made

(see the lower graph on this figure).

T-ACT is the angle at which the measurement was made and T-REQ

is the angle to which the prism was sent and should have settled to.

The readout precision is f0.1' arc.

V is the estimated velocity based on mean transit time inversion

using knowledge of the spot spacing.

TI is an estimate of turbulence intensity and is quite meaningless

in the context of this experiment and software. It should be ignored.

N is the number of angles at which experiments are to be performed

and is a manual key entry.

X is a new number identifier for the test.

Y & Z are spare identifiers, normally associated with atraverse

if used.

DISC is the discriminator card slot selected (2 here) and, in this

case, corresponded to Range 0 (See Appendix 111, Table 111).

IDEAL is the range which the machine calculates to be most

appropriate and is wrong in this case because the divergence of the

Wollaston prism was different from that in the software. If the spot/

separation ratio is changed, the calculation which infers transit

time of a spot from the transit time between spots must have its input

constraints changed.

Figure 24. Printout of Correlogram and Angle Plot from Tunnel A

DATA RATE i s the number of co r r e l a t ed events received per second

of observat ion.

BACKGROUND RATE is the mean occupation number increase pe r

second of t he s t o r e s corresponding t o background (away from the corre-

l a t i o n peak). See Reference 25 f o r more d e t a i l s of t h i s .

DATA/BACKGROUND is the quo t i en t of t h e above two p rope r t i e s

and is a somewhat o r b i t r a r y parameter which does g ive an ind i ca t ion

of t he q u a l i t y of t he correlogram; t h a t i s , t h e number of co r r e l a t ed

events which were observed compared wi th t he number which c o n s t i t u t e s

background.

An analogous f i g u r e is used t o compute the curve of t he lower

graph where t h e f i g u r e of mer i t is now the v i s i b i l i t y r a t i o which is

defined and explained i n Appendix I. This is usua l ly s u f f i c i e n t l y

accu ra t e t o enable es t imates t o b e t t e r than 0.1' a r c i f enough events

a r e ava i l ab l e .

The t i c k marks on t h e absc i s sa have t h e i r s i z e l i s t e d under

t h e graph. It is usua l ly an odd quan t i t y because of t he method of

ca l cu l a t i on and t h e simple software graphics . The mean flow angle is

obtained from a pa rabo l i c f i t of t he h ighes t t h r e e ad jacent po in t s ,

which, provided s u f f i c i e n t da t a is obtained t o reduce s t a t i s t i c a l no i se ,

is a very good measure of most probable angle d e s p i t e t he non-linear

assumptions i n t h e angular f i t .

5.2 TUNNEL A RESULTS (NACH 4.5)

On s t a r t u p , Tunnel A was f i l l e d wi th dense fog t h a t produced

opac i ty , mu l t i p l e s c a t t e r i n g and many p a r t i c l e s i n t he probe volumes

simultaneously. A s t he fog c leared a f t e r about an hour, i t became

s u f f i c i e n t l y o p t i c a l l y t h i n t o al low c l ean observa t ion of t he probe

volumes which were s t i l l f i l l e d with very many, very small p a r t i c l e s .

This is i n marked c o n t r a s t with previous windtunnel r e s u l t s i n t he

6' x 6' Transonic Facility at NASA Ames (Reference 25) where the fog

cleared to fewer particles which were rather larger and less than

one of them in the volume at one time.

As Tunnel A became even cleaner, signals were obtained (77 min-

utes after startup) and were from very small particles. Our hypothesis

is that the tunnel fogging serves as a mechanical condensate scrubber

which removes most particles, upon which moisture might condense,

with the water as part of the drying process. Later, the tunnel con-

tains only very small particles, and only those in relatively small

numbers. As measurements were obtained, they were typically giving rise

to no more than two or three collected photons against a background of

single photon events. Fortunately, this condition is tolerated by the

SDL discriminators and data was retrievable. Some care was required

with threshold selection and tube voltage balancing, but data were

then quite acceptable.

Oscilloscope photographs were not adequate to show the signal

from the monitors, but Figure 25 shows a picture of the discriminator

output. The arrow shows the second events which are only just discernable.

The correlation, however, demonstrates these perfectly as shown by

Figure 24, one of the correlograms obtained, together with some house-

keeping data and an angular search, as listed in the previous section.

Many such points were obtained at various conditions of

experimental parameters and those which merit discussion are identified

in Table11 and the following notes: Photomultiplier voltages were varied

a little to fine tune A and B rates, but finally selected a 1667 V for A

and 1750 V for B and held constant thereafter. The experiments listed

in this table were by way of preliminary attempts and demonstrate clearly

that the experiment is feasible and that moderately consistent data may

be obtained over a range of equipment parameters. To quantify these

parameters for maximum precision, the next few runs were performed with

various threshold levels to explore and find optimum levels. Table I11

shows the raw data and Figure 26 is a plot obtained from certain aspects

A E D C - T R - 8 0 - 2 8

Figure 25. Discriminated Output Signal from Tunnel A.

53

T a b l e I I . Tunnel A Data (9/10/79)

Run No. - Time Veloci ty Angle Data Rate D/B Rat io Notes C.S.T. -1 'arc -1 ms ( a rb i - s

t r a r y )

Notes :

a: Range 00 A l l o t h e r Range 0.

b: m n g e 1 1 c: 600 mm l e n s , a l l o t h e r s 400 m l e n s .

( d i f f e r e n t c a l i b r a t i o n , e r r o r i s pos s ib l e ) .

Fogging

a

d

a

b , d

d

b, d

c , d

c , d

c , d

d: Smooth angular search curves.

Table 111. Tunnel A Data (9/10/79)

Run No. - Time Event ate Threshold Velocity Data Rate D/B Ratio C . S . T . -

A+B kHz -1 -1

2 ms s

Included i n Final Average

(Data Rate) (~ata/~ackground Ratio)

Figure 26. Data Rate and Data/Background Ratio vs Threshold

of the data quality. Here are plotted data rates and data to background

ratios for a range of thresholds over which adequate data was obtainable.

The range over which these measurements are meaningful at all is plotted

as Figure 26 and this range will be seen to be already restricted from

ranges in TableIIIwhose rates outside this range make no sense because

of the nature of the software and signal statistical properties. It seems

that the data obtained within a certain (rather narrow) threshold range

is good in that there is no reason to exclude any of it in a systematic

basis. Hence, we conclude that experiments at a data-to-background

ratio of between 1.0 and 100 seem to yield measurementsconsidered not

to contain serious systematic errors.

Figure 27 was a confirmation check to show that there was no

correlation between threshold level and mean velocity calculated.

Although there is no reason to to expect such a correlation, we

felt it desirable to show that there was none because the threshold

is a manual set and to some extent is not quantified and is thus subject

to the experience of the user.

The actual results of these experiments yield a mean velocity of

724.5 m/s with a standard deviation of 0.6%. The stated tunnel velocity

calculated from thermodynamic conditions was 720.5 m/s which corresponded

to a difference of 0.55%. We have made no attempt to explain this

disparity since it could very probably be within the statistical

fluctuation expected, or it could be a calibration factor error or non-

uniform tunnel properties. We feel that the agreement is well within

that which may be expected.

With respect to angular precision, the data on TableIIsummarizes

the results of many angular searches and all lie in a region where the

data-to-background ratio has shown that the data is fairly trustworthy.

We could have included the velocity data but the tunnel conditions were

not quite stable until the results reported in TableIIIwere taken. We

did not think that a minute change in velocity could affect the angle.

Even the velocity taken over these runs, mean 727.8 t 0.55I,is not very

different from that which was compared with the calibration condition,

making the angular figure acceptable.

Mean Velocity

ms -1

Threshold

Figure 27. Non-Correlation of Meaeured Velocity with Threshold

Angle (Degrees of Arc)

12

10 * ~ a t a / Background Ratio

8 -

6 -

4 -

Figure 28. Angular Plot for Tunnel B (Mach 8)

/ - - 1

/ / b\

/ \ \

/ \

/ \

/ \ \

/ \ /

/ / \

/ \

1 \

There is no interest in the value of the angle 'per se' because

the stand for the optical head could not easily sustain a horizontal

calibration but the standard deviation of angle is of great interest

and concern. Values of Table11 yielded an angle of 274.01' arc with

a standard deviation of 0.11' arc which seems a sufficiently precise

estimate in view of the small numbers of particles available. Not all

the printed angular searches are as smooth as the one shown in Figure25.

If we restrict the above measurement to those with a smooth curve only,

then the results are 273. 98'arc with a standard deviation of 0.09' arc

which is not significantly different.

Each of the results from Tunnel A are based on five-second experiments

with numbers of events between 50 and 250 which are relatively small

compared with typical numbers considered adequate for most laser

anemometry applications.

5.3 TUNNEL B RESULTS (MACH 8)

Tunnel B was available on the night of September 11th during

the run-up to condition and for five minutes on condition. The initial

operation exhibited fogging similar to Tunnel A, but somewhat less severe.

The tunnel took 30 minutes to clear and when it did, no signals were

at first visible. An observer (M. Kingery) reported that from the other

side of the tunnel, looking toward the sampling volume, no scattered

light was visible after fog clearing and during the acquisition of

measurements. At 21:26 hr, shortly after the fog cleared, we were given

five more minutes and obtained four 50 s runs at four pre-set angles.

Table IV repeats the salient features of these. -1 The measured velocity from the four runs was 1205.4 ms with

a standard deviation of 0.6% compared with a stated figure from thermo-

dynamic tunnel calibration calculations of 1184 ms-l. Thus, we have

a disparity of 1.8%. This is not likely to be purely a statistical

(3 sigma) deviation. The window was less thick than in Tunnel A and

should not have affected the calibration. We cannot explain this

discrepancy. A s these runs were taken a t somewhat d i f f e r e n t ang le s ,

we have p lo t t ed Figure 2 8 . Even wi th these s can t d a t a , t he angle is

c l e a r l y between 273 and 274 degrees and thus may be spec i f i ed t o a

ha l f degree wi th f a i r confidence. Note t h e r e p e a t a b i l i t y of runs 32

and 3 3 with r e spec t t o d a t a r a t e .

It was not pos s ib l e t o t ake photographs of t h e s i g n a l from t h e

monitor from t h i s t e s t a s t h e r e were i n s u f f i c i e n t l y f requent events

t o r e g i s t e r on t h e Polaro id 3000 f i l m used. The c o r r e l a t i o n , however,

w a s q u i t e a b l e t o p r ~ d u c e a we l l resolved s i g n a l such as Figure 29

from which good measurements were ob ta i r ab l e .

Table I V .

Run No. Veloci ty -1

m s

Data Background Prese t -

Data Rate Ratio Angle -1

S

Figure 29. Three Correlograms from Tunnel B

Run 32

Run 33

Run 34

5.4 DISCUSSION

From experience wi th e a r l i e r s imulat ion experiments, the par-

t i c l e s from which d a t a were obtained appeared i n the s i z e range of

0.2 u m t o 0.4 diameter with almost none l a r g e r than t h i s top l i m i t .

The numbers of events from which these da t a were obtained a r e

from 20 t o 50 depending upon the p rec i s ion wi th which the spot a l ign-

ment corresponded wi th the mean flow d i r ec t ion . For both a small

number of s t o r e s and a low occupation number f o r each s t o r e , the

ve loc i ty p rec i s ion is impaired t o t he ex ten t of the f l u c t u a t i o n i n

occupation number caused by small number s t a t i s t i c s .

There a r e two s t a t i s t i c a l e f f e c t s t o consider which l i m i t t he

accuracy of a mean v e l o c i t y measurement when only a small amount of

d a t a i s present: f i r s t , i n d i v i d u a l t r a n s i t t i n e con t r ibu t ions have

f clock i n t e r v a l e r r o r s (which are t h e cause of t h e t r i angu la r i n s t ru -

ment broadening funct ion discussed b r i e f l y i n Appendix 11). Secondly,

t h e number of t r a n s i t time r e a l i z a t i o n s which occur i n t h e s t o r e s

e i t h e r s i d e of t h e t r u e ( fo r a s teady flow) delay obey small number

s t a t i s t i c s . It is thus easy t o s e e t h a t when d a t a c o l l e c t i o n time

is a premium, and the turbulence l e v e l is low, the re i s an advantage

i n using a f a s t e r c o r r e l a t o r with smal le r + clock i n t e r v a l e r r o r per

t r a n s i t so t h a t fewer events a r e needed t o produce an average of a

prescribed s t a t i s t i c a l devia t ion .

SECTION 6.0

CONCLUSIONS

Spectron Development Laboratories has demonstrated a laser

transit anemometer system with improved optical and mechanical design.

Aberrations are minimised, flare is well rejected by geometrical

stops and the system is rigid and stays aligned. Initial alignment

is by a documented logical sequence of operations.

Studies of spherical aberration caused by thick flat windows

indicate that system performance is not much degraded for thicknesses

below 1.5 inches for the shortest focal length (400 mm) and thicker

for longer focal lengths.

New transit event detection principles and circuitry further

improve the acquisition of signal in the presence of what background

light remains.

This system has made measurements in unseeded flow at Mach 8.0

in full coaxial backscatter with 80 mW of argon laser light (A-514.5 nm)

at the AEDC VKF Facility. The ambient particles are thought to be

between 0.2 pm and 0.4 pm diameter based on earlier computer simula-

tion studies.

Mean velocity measurements of high precision are possible by

interpolation, even with the poor time resolution of the 50 ns corre-

lator. For high precision where only small numbers of events are

available a faster correlator of higher precision is desirable.

Angular precision of flow direction to 0.5" arc is relatively

easy to achieve with few events for low turbulence flows: 0.1" arc

is achievable with larger numbers of events, higher rates or longer

experiment times. Accuracy depends on calibration and geometrical

levelling.

SECTION 7.0

KEFERENCES

1. B a r n e t t , D . 0 . and G i e l , T. V . , "Laser Velocimeter Meas- urements i n Moderately Heated Jet Flows , I 1 AEDC-TR-76-156, A p r i l 1977.

2. Johnson, D. A . , Bachalo, W . D . , and Moddaress, D . , "Laser Velocimetry Applied t o Transon ic and Supersonic Aerodynamics," i n

- - -

AGARD Conference Proceedings No. 193 on A p p l i c a t i o n s o f Kon-Intrus ive I n s t r u m e n t a t i o n i n r l u i d Flow Research, pub l i shed September 1976.

3 . Mayo. kr. T . . J r . , "Yodelina Lase r V e l o c i n e t e r S i g n a l s a s - - - - T r i p l y S t o c h a s t i c Po i sson Processes , " i n Proceedings of Minnesota Symposiu13 on Lase r Anemometry, h e l d October 22-24, 1975.

4. >fayo, #. T . , J r . , " D i g i t a l Photon C o r r e l a t i o n Data Process- i n g Techniques ," AEDC-TR-76-81, Ju1.y 1976.

5 . Tl-~o~~pson , D. H. , "A Tracer P a r t i c l e F l u i d V e l o c i t y Ye te r I n c o r p o r a t i n g a L a s e r , " 3. S c i . l n s t . ( J . Phys. E. ) Se r . 2 , Vol. 1, 1968, pp. 929-932.

6. Tanner , L. El . , "4 P a r t i c l e Timing Lase r V e l o c i t y Meter ," O p t i c s and Lase r Technology, June 1973, pp. 108-110.

7. Schodl , X., "On t h e Ex tens ion of t h e Range of S p p l i c a b i l i t y of LDA by Means of t h e Lase r -ha l -Focus (L-2-F) Technique," P roceed ings of t h e LDA-Symposium Copenhagen, 1975.

8 . Cummins, H. 2. and P i k e , E . R . , Photon C o r r e l a t i o n Spec t ros - copy and Lase r Velocimetry , Plenum P r e s s , New York, 1977.

9. Mayo, W. T. , Jr . , "Study of Photon C o r r e l a t i o n Techniques f o r P r o c e s s i n g o f Lase r Velocimeter S i g n a l s , " NASA CR-2780, February 1977.

10. Abbiss , J . B . , Chubb, T . W . , and P i k e , E. R., "Supersonic Flow I n v e s t i g a t i o n s w i t h a Photon C o r r e l a t o r , "Proceedings of t h e Second I n t e r n a t i o n a l Worlcsl~op on Lase r Velocimetry , Purdue U n i v e r s i t y , LaFaye t t e . I n d i a n a , March 22-29, -1974.

11. Smart, A . C . , "Spec ia l Problems o f L a s e r Anemometry i n D i f f i c u l t Appl ica t ions , " ACARD L e c t u r e S e r i e s KO. 90, 25-26 August 1977.

12. Smart, A. E., "Applications of Digital Correlation to the Measurement of Velocity by Light Scattering," Conference on Laser and Electro-Optical Systems, San Diego, California, 9 February 1978.

13. Smart, A. E., "Data Retrieval in Laser Anemometry by Digital Correlation," Third International Worksl~op on Laser Velocimetry, Purdue University, LaFayette, Indiana, 11-13 July 1978.

14. Smart, A . E., "Laser Anemometry Close to Walls," presented at DFCf78, Baltimore, FlD, Sep 1978. (Published as Appendix to Ref. 20 below)

15. Mayo, W. T., Jr., "Ocean Laser Velocimetry Systems: Signal Processing Accuracy by Simulation," Third International Workshop on Laser Velocimetry, Purdue University, LaFayette, Indiana, 11-13 July 1978.

16. Smart, A. E. and Mayo, W. T., Jr., "Applications of Laser Anemometry to High Reynolds Number Flows," Conference on Photon Corre- lation Techniques in Fluid Mechanics, Stockholm, Sweden, 14-16 June 1978.

17. Mayo, W. T., Jr., "A Two Component LDV System with Photon Counting for the NASA Langley V/STOL Tunnel: Preliminary Design Study," Final Report for NASA Contract NAS1-13737, October 12, 1976.

18. Mayo, W. T., Jr., "Fringe LV Photon Correlation Interpreta- tion Program Software Manual," Final Report for NASA Contract NAS1-14963, 11 November 1977.

19. Oliver, C. J., "Correlation Techniques," in Photon Correla- tion and Light Beating Spectroscopy, H. F. Cummins and E. R. Pike, eds., RAT0 Advanced Study Institute Series, Series B; Physics, Volume 3, Plenum Press, Kew York 1974.

20. Mayo, W. T., Jr. and Smart, A. E., "Comparison of Data from the Transit Time Velocimeter with Other Systems Now in Use for Velocity Xeasurement," ;1EDC-TR-79-32, Yay 1979.

21. Eiayo, W. T., Jr., "Semiclassical Processing of Laser Transit Anemometer Signals," presented at the 3rd International Conference on Photon Correlation Techniques in Fluid Yechanics, Churchill College, Cambridge, United Kingdom, 21-23 March 1979.

22. Mayo, W. T., Jr., and Smart, A. E., "Laser Transit Anemometer with Microcomputer and Special Digital Electronics: Measurements in Supersonic Flows,'' Proceedings of the 8th International Congress on Instrumentation in Aerospace Simulation Facilities, Naval Postgraduate School, Monterey, CAY September 1979. (Included here as Appendix I)

23. Smart, A. E., "A Compact Wide Aperture Image Rotator Without Aberrations," submitted to J. Phys. E., April 1979.

24. Smart, A. E., Wisler, D. C. and Mayo, W. T., Jr., "Optical Advances in Laser Transit Anemometrp," submitted to ASME 1980 Spring Annual Meeting Symposium on Measurement Methods in Rotating Components of Turbomachinery, New Orleans, Louisiana, 1979.

25. Efayo, 14. T., Jr. and Smart, A. E., "Feasibility Study of Transit Photon Correlation Anemometer for Ames Research Center Unitary Wind Tunnel Plan," Final Report for NASA Contract No. CR 152238, 7 February 1.979.

APPENDIX I

LASER TRANSIT ANEMOMETER WITH MICROCO>[PUTER AITD SPECIAL DIGITAL ELECTRONICS: ME~ISUREPIENTS IN SUPERSONIC mows

by W. T. Mayo, Jr., A. E. Smart and T. E. Hunt

Spectron Development Laboratories, Inc. 3303 Harbor Boulevard, Suite G-3 Costa Mesa, California 92626

Prepared For:

8TH ICIASF

INTERNATIONAL CONGRESS ON INSTRUMENTATION IN AEROSPACE SIMULATION FACILITIES

NAVAL POSTGRADUATE SCHOOL MONTEREY, CALIFORNIA, USA SEPTEYBER 24-26, 1979

Sponsored by the Aerospace & Electronic Systems Society of the INSTITUTE OF ELECTRICAL AX3 ELECTRONICS ENGINEERS

LASER TRANSIT ANEMOmTER WITH MICROCOMPUTER AND SPECIAL DIGITAL ELECTRONICS: MEASUREMEMS I N SUPERSONIC FLOWS

by W. T. *yo, J r . . A. E. Smart a n d T . E. Hunt

Spectron Development Labora tor ies , Inc. 3303 Harbor Boulevard. S u i t e 0 3

Costa Mesa, C a l i f o r n i a 92626 Sumnary improvements t o both t h e o p t i c a l and s i g n a l condi-

t i o n i n g systems. The d a t a management system (DMS) An advanced l a s e r t r a n s i t anemometer system has been improved under SDL p r i v a t e funding. Corn

has been developed f o r d i f f i c u l t o p t i c a l flow mea- mercial copies of t h i s even more advanced " four th surement appl ica t ions . This co-axial backsca t te r generat ion" system have been cons t ruc ted and system measures mean v e l o c i t y , flow angle, and tur- de l ivered . bulence i n t e n s i t y . The use of highly focused spots . s p e c i a l pulse d e t e c t i o n c i r c u i t r y , a f a s t s i n g l e b i t d i g i t a l c o r r e l a t o r , and a microcomputer d a t a X

managemnt system al low unseeded subsonic measure- ments t o ranges g r e a t e r than two meters and super- sonic measurements wi th n a t u r a l l y occur r ing submi- cron p a r t i c l e s a t s h o r t e r ranges. The system has e x c e l l e n t w a l l and window f l a r e r e j e c t i o n capabi l i - t i e s and nay be used f o r boundary l a y e r measure- ments near s o l i d m d e l s . It is compact and rugged and has been d e m n s t r a t e d f o r supersonic measure- ments i n high noise and v i b r a t i o n environments a t both USAF Arnold Center and t h e NASA Ames Research Center. This LTA system has a l s o been demonstrated f o r subsonic a x i a l compressor measurements. I n re- cen t d e v e l o p x n t s , t h e speed, s e n s i t i v i t y and r e l i - a b i l i t y have been f u r t h e r improved i n prepara t ion f o r measuremnts up t o Mach 5 a t t h e Arnold Center Von b n n a n F a c i l i t y i n l a t e summer 1979.

1. INTRODUCTIOL / I Laser t r a n s i t anemometry (LTA) o r "two-spott'

techniques have received a t t e n t i o n r e c e n t l y due t o s e n s i t i v i t y . hlgh v e l o c i t y c a p a s i l i t i e s , and s u p e r l o r f l a r e r e j e c t i o n a d ~ a a t a ~ e s l - ~ . An LTA system measures t h e d i s t r i b u t i o n of t h e t imes of f l i g h t of s c a t t e r i n g p a r t i c l e s a s they pass success ive ly through t ~ u , h igh ly focused p a r a l l e l l a s e r beams a s i l l u s t r a t e d i n Figures 1 and 2. The appropriate- ness of us ing pulse c e n t e r es t imat ion f i l t e r s with r e a l - t i - s i n g l e b i t c r o s s c o r r e l a t i o n and t h e need f o r c o r r e l a t i o n processors witn 5 t o 1 0 nano-second time r e s o l u t i o n f o r supersonic flows have both been documented recent ly6- '.

A n advanced co-axial backsca t te r LTA system was developed and denonstrated by Spectron Develop- ment Labora tor ies under sponsorship by t h e U.S.A.F. Arnold Engineering Development ~ e n t e r 5 , NASA ~ m e s 6 , and p r i v a t e funding. This system included a t h i r d generat ion compact and rugged o p t i c s head with a 135 mW l a s e r . o p t i c s . spot-pair r o t a t i o n , real- t ime graphic d i s p l a y of correlograms, prel iminary on- l i n e d a t a a n a l y s i s of mean speed, flow angle, tu r - bulence i n t e n s i t y , and mass d a t a t r a n s f e r o r s t o r - age f o r f u r t h e r o f f - l i n e computer processing.

Nore r e c e n t l y , a d d i t i o n a l deve lopwnt funding from t h e U.S.A.F. Arnold cen te r8 has r e s u l t e d i n

Fig. 1 - Laser Beam Geometry

Fig. 2 - Operating P r i n c i p l e V i = d / r i

A EDC-TR-80-28 2

Following sections describe various aspects of the system and our experience to date. These in- clude the system configuration, signal conditioning electronics, the microcomputer based control and data acquisition subsystem, and display and data reduction algorithms. This is followed by a brief description of supersonic measurements and conclu- sions about the applicability of this technology.

2. SYSTEM OVEKVIEW

Figure 3 is a photo~ravh of the LTA ovticai head and the data manazement system (DMS). A digi- tal cross correlation processor (not shown) and interconnection cables complete this stand-alone system, although traverse-mechanism and other inter- faces are available. The optical head contains several sets of printed circuit card electronics. These subsystems include a rotator prism control card, two sets of filter discriminators, a hlgh vol- tage relay circuit, and photomultiplier tube hous- ing circuits. Figure 4 is a photograph of one of the discriminator cards.

F i g . 5 - LTA D a t a Management S y s t e m a n d O p t i c a l Head

F i g . 4 - Example o f O p t i c a l Head E l e c t r o n i c s : D i s c r i m i n a t o r Ca rd

D u r i n g o u r LTA d e v e l o p m e n t , we h a v e u s e d a M a l - v e r n K7023 50 n s c o r r e l a t o r t o p r o d u c e a s l n g l e - b l t c r o s s c o r r e l a t i o n o f t h e o u t p u t s o f p u l s e - c e n t e r e s t i m a t i o n f i l t e r s ( d i s c r i m i n a t o r s ) . A f a s t e r " m u l - t i p l e station" correlation processor is presently under development at SDL for LTA and other applica- tions. Figure 5 is an example cross correlogram of transit delay intervals from a 48 store correlator. The rms turbulence intensity for this example is 7%.

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F i g . 5 - An Example C r o s s C o r z e l o g r a m w i t h 7% RMS T u r b u l e n c e I n t e n s i t y

The DNS a u t o m a t i c a l l y r o t a t e s t h e two f o c u s e d beams a b o u t a comm>n c e n t e r t h r o u g h a s e q u e n c e o f a n g l e s w i t h t h e a n g u l a r s t e p s i z e d e t e r m i n e d o n l i n e f r o m t h e t u r b u l e n c e l e v e l e s t i m a t e d f r o m t h e f i r s t d a t a r u n . The c o r r e l a t o r i s s t a r t e d a n d t h e d a t a t r a n s f e r r e d t o t h e DMS i n p r o p e r s e q u e n c e . The o p e r a t o r i s shown a g r a p h i c d i s p l a y o f t h e c o r - r e l o g r a m s , a p l o t o f s i g n a l v i s i b i l i t y r a t i o f r o m w h i c h mean f l o w a n g l e i s e x t r a c t e d , a n d a p r i n t o u t o f t h e e s t i m a t e s o f mean s p e e d , mean a n g l e , a n d a p - p a r e n t turbulence intensity.

3. OPTICAL HEAD ELECTRONICS

T h r o u g h p r e v i o u s e x p e r i e n c e , o n e o f t h e a u t h o r s ( S m a r t ) f o u n d t h a t l a r g e e l e c t r o m a g n e t i c i n t e r f e r - e n c e f i e l d s w e r e o f t e n p r e s e n t i n common t e s t e n v i - t o r m e n t s i n a d d i t i o n t o h i g h l e v e l s o f n o i s e a n d vibration. Thus, in a d d i t i o n t o d e s i g n i n g f o r me- c h a n i c a l r i g i d i t y , we c h o s e t o l o c a t e t h e a n a l o g e l e c t r o n i c s i n s i d e t h e o p t i c s h e a d w h e r e s h o r t l o w - i m p e d a n c e l e a d s a n d s h i e l d i n g c o u l d he u s e d t o m a i n - t a i n s p e e d , s e n s i t i v i t y a n d good i m m u n i t y t o i n t e r - f e r e n c e . O n l y TTL l e v e l s l g n a l s a r e t r a n s m i t t e d o v e r t h e i o n S s i g n a l a n d c o n t r o l c a b l e s ( 3 0 m a t e r s o r m o r e ) .

The p h o t o m u l t i p l i e r t u b a c i r c u i t s o f f e r e d q u i t e a c h a l l e n g e . We d e s i r e d good s l n g l a - p h o t o n r e s p o n s e ( p u l s e s up t o two v o l t s i n m a g n i t u d e a n d l e s s t h a n I 0 u s w i d e w i t h l i t t l e o r no r i n g i n g a n d w i t h good s i n g l e p h o t o e l e c t r o n p u l s e h e i g h t r e s o l u t l o n ) w h i l e a l l o w i n g much l o n g e r c l a s s i c a l s i g n a l s . T h i s mus t b e a c c o m p l i s h e d i n t h e p r e s e n c e o f w a l l f l a r e a n d b l a d e f l a s h w h i c h , f o r b r i e f p e r i o d s , d r i v e t h e t u b e

70

3 AE DC-TR-80-28

output far beyond the allowed average anode current. The PMTs selected were 12 stage, 2 inch diameter EMI 9817B llnear fast focused tubes with S-20 pho- tocathode material. These tubes have high cathode maximum current limits such that the cathode is pro- tected by the inclusion of a series i00 M~ resistor. Good single photoelectron pulse height response and first stage gain is assured regardless of the high- voltage setting by two series 150V Zener diodes be- tween the cathode and the first dynode.

The inputs and outputs are illustrated in Fig- ure 6. The gain suppression does not disconnect

CATHODE PROTECT RESISTOR

GAIN SUPPRESSION GATE 100 M

INTEGRATOR &

HIGH VOLTAGE •

[ANO OVERLDAD E

LIGHT

I PHOTOMULTIPL]ER

D.C. COUPLED PREAMPLIFIER

(fOx) D.C. COUPLED PREAMPLIFIER

(tOx)

OUT TO DISCRIMINATORS

Fig. 6 - Photodetection Electronics.

the high voltage but removes the interstage gain at the first dynode thus preventing predictable tran- sient optical overload from fatiguing the dynode electron emitters. The PMT anode load is 50 G. There are two direct coupled stages of preampllfi- cation which together give a gain of x i00 and do not change the polarity of the output signal. These preamplifiers saturate at about - 6 volts. In gen- eral, the use of such preamplifiers is not desira- ble, and for wind tunnel appllcatlons away from model surfaces, a 14 stage tube with one 10x preamp is a better selection. However, the applications of interest often require proximity to walls; thus, the prea~pllflers are required to avoid dynode fa- tlgue through excessive average current. Each pre- amplifier output drives a 50 G co-axial cable termi- nated on a dlscrlm/nator board. Figure 7 is a pho- tograph of two phototube assemblies which are par- tlally disassembled to show the circuitry.

Between the two preamplifiers there is an in- tegrating monitor, located physically on the high voltage relay board, which detects a condition which could, if sustained, damage the later dynodes or anode of the photomultiplier tube. This trip condition corresponds to a de anode condition of 200 ~A for a period of 0. Ss. The high voltage re- lay responds to the integrating monitor (actually an active low-pass filter) by disconnecting the high voltage to protect the anode and later stages of the dynode chain. Each tube has a separate sense llne and a separate hlgh voltage relay but a red overload light on both the optical head and the DMS

will come on when either tube is tripped. The red light is incorporated in a switch which resets the relays when pushed.

There is a separate discriminator card for each of the two separate photomultlplier tubes. Each discriminator card is a "mother" board with six sep- arate plug-ln "daughter" modules as illustrated in Figure 4. Each module is for a pulse width with a dynamic range of 4:1. The ranges overlap, as they increment in powers of two with range 1 being 25 ns to 100 ns, range 2 being 50 ns to 200 ns, etc. Fig- ure 8 illustrates one of the plug-in modules funct- ionally. The low-pass filters are designed to have

Fig. 7 - Photomultiplier Tube Assemblies

RANGE SELECT

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Fig. 8 - Sche-~tlc of Single Filter Discriminator Plug-in Module.

71

n e a r l y synnnetric, non-ringing. Gaussian impulse rc- sponses w5ose durac ion a r e kT and Bk'l f o r t h e dc- t e c t i o n f i l t e r and t h e p u l s e :enter es?imation iil- t e r , r e s p e c t i v c l y . The rangc s c l c c t c d is k. from 1 t o 6. The l / e Z p u l s e v i d t h ; f a r t h e f a s t e s t rang? is 25 n s f o r t h c s t a n d a r d u n i t s ; s p e c i a l higher- speed plug-in m d u l e s havc bcen developcd8 f o r ap- p l i c a r i o n s up t o ?lacl~ 10.

I n o p e r a t i o n , t h e u s e r e l e c t r o n i c a l l y x c l c c t s t h e d i s c r i m i n a t o r range [or whlch mst of ttie s p o t t r a n s i t d u r a t i o n s Tj w i l l l i c v i t h i n :hc range kr <Tjc4kr . Under t h e s e condlcions. ciie d e t e c t i o n f i y t c r (kro) has a wldcr bandwidth chan t h e c l a s s i - c a l s i g n a l s t o bc d e t e c t e c , s o t h e r e is no s t a t i s - tical b i a s s i n g due t o a t t e n u a t i o n of t h e f a s t e r pul- ses. Once t h e detection t h r e s h o l d is exceeded. Lhe pulse-ccntcr f i l t c r s a r e ezabled. S incc Ltle Jnlpulse response of t h e s e f i l t e r s is longer i n duracion than t h e s p o t t r a n s i t timfs. t h e s lngle-photon p u l s c s a r e smootlied, even when on ly one o r a few such p u l s e s occur i n a s p o t t r a n s i t group. The coapara to r t h u s l o c a t e s t h e vc igh ted average c c n t e r of a photon p u l s e c l u s t c r .

The d i s c r i m i n a t o r c a r d s l n c l u d e a second l e v e l d e t e c t i o n c i r c u i t j u s t Following t h e b u f f c r ampli- f i e r which is n o t stlown i n t h e s i u y l e block d i a g r a a T h i s c i r c u i t i n h i b i t s t h e d i s c r i a i n a t o r o u t p u t s i f t h e s i g n a l s exceed a p r c s e t l e v e l . The p r e s e t l e v e l is t y p i c a l l y -2.5 v o l t s t o avoid very l a r g e impulses (from l a r g e s c a t t e r e r s o r o t h e r causes ) . By a d j u s t - i n g t h e P!4T h igh v o l t a g e supply, t h e average pu l se l e v e l o u t 01 t h e >reirmpllf ieru is sc: Detween -0.2 and -2.0 v o l t s .

The motor c o n t r o l board s u p ? l i e s power t o t h e d c geared nocor whicli d r i v e s t h e pr ism r o t a t o r such a s t o z e r o t h e d i f f e r e n c e bctween t h e l o g i c word provided by t h e microprocessoc and t h e analog-to- c i g i t a l conversion of t n e s l g n a l fram :lie pu ten t io - m i t e r connected wi thou t backlash t o t h e pr ism ro ta - t o r . For i n c r e l e n c a l a n g l e s t e p s less than 2 de- g r e e s , 0 .1 dcgree p rac i s io r . i s ob ta ined i n l e s s than 0.5s. The v a l u e of t h e i n s t r u c t i o n d i s p a t c h w c d is d i sp layed a t t h e microprocessor a s is t h e word whicli corresponds t o t h e a n g u l a r p o s i t i o n ul- t i m a t e l y ob ta ined . The prism n o t o r may a l s o be d r i v e n i n e i t h e r f a s t o r slow mode i n e i t h e r Cirec- t i o n from t h e mandal s w i t c h e s on t h e D.% console. I n t h i s m d e t h e readout of a c t u a l p o s i t i o n t o t h e DMS s c r e e n is s u s t a i n c d and updated m n y times p e r second.

4. DATA MiLVAGLWhT SYSTEH ( D S )

The UMS ( scc F igure 3) is nornally s iLua ted remote from t h e o p t i c a l head by up t o 30m and con- t a i n s t h e mlcroprocessor , keyboard, v i s u a l d i s p l a y u n i t , two ~ i n i f loppy d i s k u n i t s , o p e r a t i n g s o f t - ware and a number of manual c o n t r o l s . F igure 9 shows t h e l a b e l e d f r o n t panel l a y o u t which i d e n t i - f i e s t h e i t ems shown i n t h e photograph o f F igure 1. I n s i d e , t h e r e a r e low v o l t a g e power s u p p l i e s and o t h e r e l e c t r o n i c hardware t o augment t h e micropro- cessor . These a r e 48k b y t e s o f RAM, i n t e r f a c e s , and t h e window and d e l a y u n i t (one channe l of which c o n t r o l s t h e " l i v e time" of one d i s c r i m i n a t o r and t h e o t h e r t h e "blanking time" o f both photornulti- p l i e r s f o r p e r i o d i c f low a p p l i c a t i o n s . ) 'The

Fig. 9 - Data Management System Front Panel

. I IGC 11.1 1.1GE

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? a r a l l e L p o r t s [interfaces) ~ r o v i d e d l a t h e DYS in - c lude a buf fc rcd 8 b i t duplex computer i n t e r f a c e wi th f o u r a d d i t l o n a l c o n t r o l l i n c s and a 48 b i t c o r r e l a t o r i n t e r f a c e . I n e c o r r e l a t o r p o r t s a r e p r w granrmable wi th r e s p e c t t o ttie d i r e c t i n n of informa- t i o n flow. Thus, i n t c r f a c e u t o a v a r i e t y o f o t h e r compiltcrs and c o r r e l a t o r s may be c o n f i g ~ r e d by sof t - ware without hardware changcs. S ince a l l so f tware is nrovidcd on d i s k r a t h e r than permanent read-only mcmory, t h e system is q u i t e E lex ib lc .

Software a s providcd is read from Disk 1 on s t a r t u p and Disk 2 is rese rved f o r d a t a s t o r a g e . E i t h c r w i l l s e r v e both f u n c t i o n s , i f necessary. A l l i n i t i a t i o n and d a t a heading f u n c t i o n s a r e e n t e r e d fro- t h e keyboard i n response LO q u e s t i o n s posed on t h e s c r e e n a s i l l u s c r a t e d i n tlie example given i n 'I'able 1. These keyboard e n t r i e s a r e w r i t t e n a s i n i - t i a l i z a t i o n heading before each d a t a record. Fur- t n e r d c t a i l s concerning t n e d a t a p roccrs ing a l g o r i - thms a r e p resen ted i n Sec t ion 5 below.

The ? t w t o m u l t i p l i e r s a r e each supp l ied from t h e i r own hig: v o l t a g e s u l ~ p l y s o t h a t f i n e t u n i n g on g a i n may be performed. The remote l a s e r s t a r t / s a f e is a removable key swi tch wi th l a se r - ready i n d i c a t i o n . 'I'his p rcven t s t h e necd f o r n c r e s s t o t h e l a s e r power supply cxcept when one wishes t o change ?over l e v e l s o r t h e c o n t r o l mode. There is a l a s e r outpuc power meter on t h e c o n t r o l console. Manual c o n t r o l s which may o v c r r i d c t h c computcr a r e

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0nS PROW1 TO USER E;IAIIRE OF ENTRY

'PUT IN DATA DISKETTEg' Confirm that t h i s i s done

olrrr (tmr~, OAV) 12. 25

N3. or STO~ES = 48 (48 s tore cor re la tor )

SPOi SEPAR ( W ) ' 0.27

NO. OF BLADES = 7 29

Y:hXN U:Xh 'OR S T E S iGhU ' ' DLL4I FO? 3 - E JI?LG. I w a t t and Celaj. 'n Hlc-oseconds

d!HDOd dIDTH FOP BLhHKlYC SICYPL ' ' DELAY FCR OLANKlhE SIGNAL ' 7

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provided t o make s e t t i n g up t h e system e a s i e r . There is a switch which al lows e i t h e r manual o r corn p u t e r d i sc r imina tor range s e l e c t i o n . Mode switches f o r motor c o n t r o l t o f a c i l i t a t e alignment of t h e spot o r i e n t a t i o n with t h e flow a r e provided. Even under manual c o n t r o l , t h e c o r r e c t prism o r i e n t a t i o n is read t o t h e sc reen o f t h e LM.5. The threshold c o n t r o l f o r t h e d i s c r i m i n a t o r s sets over a wide range, t h e l e v e l which changes t h e ranges t h e r a t i o of s i n g l e photon events t o multlphoton p a r t i c l e events .

The o p t i c a l and e l e c t r o n i c s y s t e n f o r t r a n s i t ane3met ry b a s i c a l l y acqui res two c:-.annels of d ig i - t a l events whose only information is c a r r i e d a t event times. To measure t h e d i s t r i b u t i o n of event times, i t is optimal t o c r o s s c o r r e l a t e t h e two channels where t h e delayed channel corresponds go t h a t which a p a r t i c l e c r o s s e s f i r s t . This subjec t has been previously discussed7. The subjec t of op- timum d a t a e x t r a c t i o n from correlograms can get very de ta i led . Here we w i l l exp la in what we have e lec ted t o do with t h e system t o date. Further software re- v i s i o n s a r e p o s s i b l e e i t h e r by t h e user o r from SDL on a new minidisk. The presen t software functions:

a ) Hake a v i s i b l e record of d a t a q u a l i t y s o t h a t t h e u s e r may p ike on-line improvements t o opera t ing proccdure, th reshold and vol tage s e t t i n g s ,

b) Give an approximate va lue f o r veloc- i t y , turbulence and angle i n minimum time,

c ) P r i n t t h e s e e s t i m a t e s on paper f o r b r i e f assessment.

d) S t o r e o r t ransmi t and i d e n t i f y a l l acquired d a t a f o r l a t e r processing with s u p e r i o r algori thms i n a l a r g e computer.

Figure 1 is a schena t ic v i w of t h e sampling volume and shows how t h e v e l o c i t y is resolved i n t h e a x i a l d i r e c t i o n but only g ives rise t o a mea- surement of t r u e speed i f it is i n t h e ?lane o f t h e tw p a r a l l e l cy l inders . More simply, t h i s can be viewed a s i n Figure 2. Because t h e r e is no way of i d e n t i f y i n g a s p e c i f i c p a r t i c l e , ve have chosen t o decode the d a t a on a s t a t i s t i c a l bas i s . By comput- i n g t h e tims between a l l poss ib le ' A then B' p a i r s such t h a t t h e delay does no t exceed t h e displayed de lay time of t h e c o r r e l a t o r , then It is poss ib le t o o b t a i n a peak from those p a r t i c l e s which crossed A and then B and a background from a l l o thers . Over s h o r t de lay i n t e r v a l s i t is permiss ib le t o assume t h a t uncor re la ted events from p a r t i c l e s which c r o s s only one spot o r f r o n o t h e r random events a r e Pois- son d i s t r i b u t e d . This i s not , i n genera l , t r u e f o r longer c o r r e l a t i o n times; and. indeed, provides a z t h o d of a s s e s s i n g turbulence s c a l e by c o r r e l a t i n g t h e e x i s t e n c e of p a r t i c l e concent ra t ion f luc tua- t ions9 . Over t h e s h o r t time involved i n a t y p i c a l t r a n s i t , these e f f e c t s a r e n o t manifest and i t is permissible t o assume t h a t t h e "background" is f l a t . Figure 5 showed a t y p i c a l correlogram f o r about 7% turbulence and a 48 s t o r e c o r r e l a t o r . Upon t h i s p l o t a r e marked t h e var ious opera t ing parameters which a r e used by t h e DMS t o eva lua te t h e flow parameters.

There a r e many s i m p l i f i c a t i o n s i n t h e follow- i n g a n a l y s i s . It is not t o be scorned f o r t h a t , a s i t is intended a s a quick-look device and, i f used c o r r e c t l y wi th proper precautions, i s capable of much b e t t e r than 1% accuracy i n v e l o c i t y , par t icu- l a r l v f o r low turbulence, and no more than a few percent e r r o r on turbulence f o r turbulence up t o about 15%. For very low turbulence more s t o r e s and/ o r smal le r AT c o r r e l a t o r r e s o l u t i o n may be required. The low turbulence l i m i t a l s o depends on t h e degree of deconvolution of t h e i n s t r u a e n t broadening func- t i o n 7 cne may wlsh t o a t t e 3 p t i n post-de:ection rrrocessing. The s tandard a lgor i thms a r e presented balow.

Based on experience and "a p r i o r i " knowledge of t h e flow, t h e opera tor s e l e c t s a number of s t o r e s hT which he expects t o bound t h e h ighes t tu rbulence condi t ions of t h e correlogram. The reg ion R is then t h e s e t of s t o r e s loca ted wi th in NP s t o r e s e i t h e r s i d e c f t h e peak IMX of t h e correlogram. The re- gion R is a l l t h e r e s t of t h e s t o r e s a s s h a m i n Figure 5. The f i r s t s t e p i n t h e d a t a reduction is t o compute t h e a r i t h m e t i c average of t h e va lues ac- c u m u l a t e d i n t h e s t o r e s of reg ion R. This is an es t imate of t h e "white" l e v e l due t o uncor re la ted background pulses. This va lue is subt rac ted from the correlogram. Thus, I f Y(1) a r e t h e correlogram values. we o b t a i n

where Q is t h e t o t a l "area" under t h e peak (see Fig- u r e 10) which normalizes t h e func t ion a s a probabi- l i t y d e n s i t y function es t imate ; i . e . ,

Fig. 1 0 - Def in i t ion of V i s i b i l i t y Ratio

The f i r s t o rder mean speed es t imate ( a s s u d n g t h a t t h e s p o t p a i r a x i s i s d i r e c t e d soolewhat along t h e mean flow d i r e c t i o n ) is obtained from t h e mean de- l a y number KBAR q u i t e simply a s

d " a (KBAR . AT) where d is the phys ica l d i s t a n c e between t h e s p o t s and AT is t h e c o r r e l a t o r clock i n t e r v a l . This e s t - imate of mean speed based on t h e i n v e r s e of t h e mcan t r a n s i t t i m e is s i m i l a r t o t h e mean v e l o c i t y compon- e n t es t imate obtained from b u r s t counter processing of f r i n g e Llr d a t a without invers ion . A s is w e l l knownll, t h e use of t h e i n v e r s e of the rean t r a n s i t tl3e provides f i r s t - o r d e r c o r r e c t i o n of t h e p a r t i c l e r a t e biassing. Thus, f o r low turoulcnce flows t h e aean speed e s t i m t e is exce l len t . A t higher turbu- lence l e v e l s , say g r e a t e r than lo%, nigher o r t e r d a t a processing witk t h e i?.clusion of t h e "p ie s l i c e " e f f e c t (accepted f l a - a n g l e a t any one angle depends on p a r t i c l e s i z e f o r a f ixeZ tl1resho:d) and o t h e r e f f e c t s may be included i n a more prec ise model.

For s i x ? l i c i t y che presen t tnrbulence lnten- s i t y es t imate , TI, is c a l c u l a t e d a s a r o o t mean square devia t ion of t h e t r a n s i t t i m e d i s t r i b u t i o n ins tead of t h e v e l o c i t y d i s t r i b u t i o n . This m y be inproved i n the fu ture . I n terms of t h e e f f e c t i v e width, F, of t h e correlogram peak we obta in

where t h i s somewhat unusual no ta t ion is used i n t h e d e f i n i t i o n of t h e s i g n a l v i s i b i l i t y r a t i o function below:

The above formula f o r mean speed is q u i t e in- s e n s i t i v e t o t h e va lues o f NP chosen. I f t h e d a t a .acquisi t ion time i s s o s h o r t t h a t t h e s c a t t e r i n

t h e b a s e l i n e is cons iderab le a s i l l u s t r a t e d i n Fig- u r e 5, then t h e turbulence i n t e n s i t y es t imate be- comes s i g n i f i c a n t l y a f f e c t e d by t h e noise-on t h e b a s e l i n e and, hence, t h e choice of R and K. Under such condi t ions , more d a t a should be acquired o r more e l a b o r a t e curve f i t parameter e x t r a c t i o n algo- r i thms should be employed f o r optimum r e t r i e v a l of information from t h e da ta .

The DMS uses t h e e s t i m a t e of turbulence in ten- s i t y t o compute an incremental angle be f o r spot- p a i r r o t a t i o n which proceeds au tomat ica l ly u n l e s s another A 3 i s chosen. The computation ofA9 assumes t h a t t h e turbulence is i s o t r o p i c t o o b t a i n a crude es t imate of a "good" b3. Thus, i f t h e number of angles a t which observa t ions a r e t o be made is N, then C,e i s computed a s i n Table 2.

Table 2 - Selec t ion Formula f o r bO

Unless T I = 0, i n which c a s e

and TI 1s displayed a s 1 0.

During t h e automatic angle incrementat ion se- quence, t h e s i g n a l " v i s i b i l i t y r a t i o , " VR(L), i s computed. The angle i s i l l + Id0 where 9* is t h e i n i t i a l a n g l e s e l e c t e d iaanually o r v i a computer. The quant i ty VR(L) is t h e r a t i o of t h e a r e a under t h e delay curve, Q, and t h e sun of t h i s a r e a with t h a t por t ion of t h e background wi th in t h e "width" F of t h e correlogram:

These q u a n t i t i e s a r e i l l u s t r a t e d i n Figure 10. Tire value of t h e func t ion VK(:) l i e s between zero and unity. Tnc zero c o n d i t i c n is o j t a i n e d when t h e spot p a i r a x i s i s r o t a r e d s o fzr away fro? t h e mean flow angle t h a t no c o r r e l a t e d e v e n t s occur and Q goes t o zero.

The mean flow angle occurs i n t h e v i c i n i t y of t h e peak of t h e ploc of VX(L1. The present DYS software uses a 3-?oint parabol ic f i t of t n e peak and t h e va lue of VR(L) on each s i d e of t h e peak t o ob ta in t h e mcan flow-angle es t imate a s i l l u s t r a t e d i n Figure 11. The idea behind t h e d e f i n i t i o n of t h e v i s i b i l i t y r a t i o func t ion is t h a t i t removes t h e e f f e c t s of p a r t i c l e r a t e f luc tua t ionn when t h e threshold i s s e t s u f f i c i e n t l y high f o r a l l de tec ted pu lses t o be f r o n p a r t i c l e s . A t lower th resholds and p a r t i c u l a r l y when t h e probe volume is near a f l a r e producing sur face , many of t h e de tec ted pul- s e s a r e s i n g l e photoelectron background pulses whose r a t e may be s u b j e c t t o angular o r i e n t a t i o n of the spots . Under such condi t ions , the raw d a t a r a t e Q which has background subt rac ted ou t . may be

- - - . .- - DATA R4TE.471 07 BACKGROUND U T C = & O 18 DAiAIBPCKGROUYD=5 59

4 . . . * , , I . # rn (L , PWGLE F?3H PLOT.50.82 IYCREMtNT Ch X-4%15=2.41

Fig. 11 - Example Screen P lo t of VR(L) and Display of Reduced Data.

a b e t t e r parameter t o f i t f o r mean flow angle. A l l of t h e flow es t imat ion software is w r i t t e n i n BASIC t o al low e a s e o f f u t u r e modificat ion. I n f a c t , a l l of t h e opera t ing software i s provided on d i s k , ra- t h e r than permanent mernoryand can e a s i l y be changed.

6. MEASUREMENTS

A t t h e t ime of t h i s w r i t i n g (July 1979) addi- t i o n a l supersonic measurements a r e scheduled t o be conducted a t t h e Arnold Engineering Development Cen- t e r Von Karman F a c i l i t y , but these w i l l no t be con- ducted before t h e submission deadline. These t e s t s w i l l be through windows two inches t h i c k , without seeding, i n Tunnel A a t Mach numbers i n t h e range 1.5 t o 5.0. The r e s u l t s of these t e s t s w i l l be des- c r ibed a t t h e Congress i f they a r e a v a i l a b l e a t t h a t time. and w i l l be provided l a t e r i n o t h e r publica- t ions . In t h i s s e c t i o n we w i l l b r i e f l y review t h e supersonic measurements made with t h e prototype sys- t e m a t t h e Arnold Center , September 1978, and a t NASA Ames, October 1978. These experiments a r e d e s c r ibed i n more d e t a i l i n t h e c o n t r a c t reports'16.

The o b j e c t i v e o f t h e AEDC measurements was t o d e m n s t r a t e a n LTA system wi th a d i g i t a l c o r r e l a t o r and t o conpare t h e neasurcments wi th simultaneously obtained f r i n g e velocimeter measurements. The pro- to type system was t ranspor ted t o t h e Arnold Center on Septezber 18, 1978. Denonstrat ion measuresents along t h e c e n t e r l i n e of a supersonic j c t wi th a Yach d i s k were made, with t h e L i A s y s t e z ; however, due t o equipment d i f f i c u l t i e s experienced by t h e Arnold Center personnel and time and funding con- s t r a i n t s which precluded f u r t h e r SDL measurements, no simultaneous measurements were accolrplished.

The M D C t e s t s were conducted using a f r e e j e t t e s t (unseeded) i n s t a l l a t i o n loca ted i n Propulsion Research C e l l (R-1A-1) of t h e Engine Tes t F a c i l i t y a t t h e Arnold Engineering Development Center. As- s i s t a n c e was provided t o SDL personnel by Mr. T. V. C i e l , ETF. Mr. V i r g i l Cline, PWr, and t h e f a c i l i t y technic ians . A 1" diameter underexpanded unheated jet with a pressure r a t i o of 4 was used. This j e t has been t e s t e d ex tens ive ly i n t h e pas t wi th both l a s e r velocimetry and shadowgraph techniques a s re- ported by Barnett and ~ i e l l ' i n 1977. The f i r s t and second Mach d i s k s were loca ted 1 .73 inches and 3.39 inches downstreamlo. Due t o o t h e r a c t i v i t i e s which had occurred s i n c e t h e c a l i b r a t i o n s of t h e

j e t , a r i g h t angle bend had been i n s t a l l e d i n t h e j e t supply pipe. It is thus p o s s i b l e t h a t d i f f e r - ences i n t h e flow would occur wi th r e s p e c t t o t h e e a r l i e r masurements.

The output l e n s used f o r t h e j e t t e s t was a 622 m f o c a l l e n g t h l e n s which r e s u l t e d i n a throw, o r range, from t h e f r o n t of t h e instrument of 597nm) a spot separa t ion of 0.489 mm a s c a l i b r a t e d p r i o r t o instrument shipping, and a s p o t s i z e of approximately 20 micrometers diameter. The a x i a l speed and appar- e n t turbulence i n t e n s i t i e s a s determined by t h e op- e r a t o r and t h e d a t a management system were recorded f o r s e v e r a l l o c a t i o n s along t h e c e n t e r l i n e of t h e jet. Figure 12 is a p l o t of t h e a x i a l speed mea- sured along t h e jet c e n t e r l i n e .

SCO

.;I, 3 0 C

2 . 2

i * 2 0 0

Fig. 12 - Axial Speed on J e t Centerl ine.

Careful s tudy of a l l t h e MDC d a t a and t h e ap- p a r a t u s ind ica ted t h a t t h e s e l e c t i o n of t h e c o l l i - mator l e n s f o r t h e prototype system was inadequate t o proper ly focas r e t u r n s c a t t c r e d r a d i a t i o n on t o the f i b e r o p t i c l i g h t guides ezployed i n t h e system. The r e s u l t i n g l o s s of s c a t t e r e d r a d i a t i o n r e s u l t e d i n a r e c e i v e r e f f i c i e n c y of less than 5%. Calcula- t i o n s had i n d i c a t e d t h a t measurements from ? a r t i - c l e s d o n t o 0.3 micrometers i n diameter would be achievable. The p a r t i c l e l a g ev ident i n t h e p l o t of d a t a a c r o s s t h e Xach d i s k ind ica ted t h a t t h e f i r s t prototype system d id no t achieve t h i s sens i - t i v i t y . Contractual time and funding l i m i t a t i o n s prevented f u r t h e r d a t a c o l l e c t i o n o r a n a l y s i s a t t h a t time.

The purpose of t h e NASA Ames experiment was t o ob ta in d a t a f o r design of a l a r g e r f u t u r e system f o r t h e NASA h s 11-foot t r a n s o n i c tunne l , i f such a system appeared f e a s i b l e from t h e t e s t s . The t e s t was more d i f f i c u l t than t h e AEDC t e s t i n t h a t t h e r e was a 2-inch t h i c k window between t h e LTA system and t h e flow f i e l d , and t h e l a r g e r p a r t i c l e s impact ou t of a r e c i r c u l a t i n g tunnel . The measurements were conducted i n a piggyback t e s t a t t h e 6-foot t ransonic tunne l without seed ing a t Mach numbers from 0.6 t o 1.6. Repea tab i l i ty of t h e mean flow

measurements was a s good a s 0.15%. The tunne l was no t seeded. Minor improvements i n t h e o p t i c a l sys- t e m had been made t o improve t h e o p t i c a l e f f i c i e n c y of t h e r e c e i v e r s t o near ly 10% but i n s u f f i c i e n t time had been a v a i l a b l e t o rep lace t h e output co l l imator . Computer s imula t ions of t h e s i g n a l s us ing Hie s c a t - t e r i n g codes and measured o p t i c a l parameters indi- ca ted t h a t d a t a was obtained from p a r t i c l e s down t o about 0.5 microneters i n diameter, but no s tanding shocks were a v a i l a b l e f o r r e l a x a t i o n tests i n t h i s exerc i se .

In December 19711, t h e prototype system was used t o make measurements i n a research a x i a l com- pressor a t t h e General E l e c t r i c Co. Some oE t h e d a t a was obtained unseeded and some was obtained us- i n g 0.25 m i c r o ~ a t e r s diameter s o l i d seed material . For t h a t t e s t , which w i l l be reported i n d e t a i l a t tire Spring 1980 ASW Conference i n New Orleans. a d i f f r a c t i o n l i m i t e d c o l l i m a t o r l e n s was used wi th marked increase i n o p t i c a l e f f i c i e n c y and thus sys- tem s e n s i t i v i t y .

An advanced microcoaputer automated l a s e r tran- sit anemometer system (LTA) has been designed, con- s t r u c t e d , and de l ivered . This LTA system has been demonstrated t o be capable of coaxia l backsca t te r measurements o f unseeded flow near w a l l s and sur- faces and a t v e l o c i t i e s up t o Hach 1.6. Fur ther t e s t s a t speeds up t o Mach 5 a r e planned. Tlle pro- to type system has been t ranspor ted t o t h r e e sepa- r a t e r e a l world t e s t s with high noise and v i b r a t i o n environments and performed w e l l i n a l l cases . The successes support the design choices which inc lude t h e e x t e n s i v e use of p r in ted c i r c u i t e l e c t r o n i c s in- s i d e t h e compact o p t i c a l head. The usual complaint concerning d a t a c o l l e c t i o n time with an LTA system has been s u b s t a n t i a l l y overcome by dcrocompucer au- tomation of t h e system c o n t r o l and t h e provis ion oE on-l ine flow f i e l d parameter e s t i m t i o n and graphic opera tor o r ien ted sc reen d isp lays . The LTA system is more s e n s i t i v e than f r i n g e LV systems and is ex- pected t o f i n d wide a p p l i c a t i o n t o measurement prob- lems requi r ing good s e n s i t i v i t y t o t h e suxnicron p a r t i c l e s which abound i n most ins tances without seeding. A p r a c t i c a l LTA syscen v i t h snique fea- t u r e s has been developed and demonstrated.

The development of t h e o p t i c a l h e a d e l c c t r o a i c s deacribed i n t h i s paper was funded f o r t h e most p a r t by t h e U.S.A.F. Arnold Engineering Development Cfn- t e r under Contracts F40600-78-C-0002 and F40600-79- C-0003. The Air Force p r o j e c t nanager f o r both con- t r a c t s was Marshall N. Kingery.

2. Smart, A. E., "Data Ret r ieva l i n Laser Anemometry by D i g i t a l Correlat ion." Third I n t e r - n a t i o n a l Workshop on Laser Vclocimetry, Purdue Universi ty. LaFayette. Indiana. 11-13 J u l y 1978.

3. Smart, A. E., "Lascr Anemometry Close t o Walls." Dynamlc Flow Conterence 1978. Baltimore, Maryland. 18-21 September 1978.

4. a a r c l e c t , K. b., m u SIII?. C. Y . , "Single- P a r t i c l e Corre la t ion Techniques f o r Renote Measure- ncnt of Wind Speed: Aerosol Condition and Measure- ment Rates, JOSA. 69, 455 (?la(arcil 1979).

5. Kayo, W. T.. and Smart. A. E.. "Comparison of Data from t h e T r a n s l t Time Velocimeter with Other Systems Now i n Use f o r Velocity Y!asurements," MDC-TR-79-32, May 1979 (F ina l Report on Contract F4060S78-C-0002).

6. Mayo, W. 'I'., and Smart, A. E . , "Feasibi l- i t y S t d y of Trans i t Photon Corre la t ion Anemometer f o r h ~ c s Research Center Unitary Wind Tunnel Plan." NASA CK 152238, February 1979, (F ina l 3cport on Conrracr: KO. KAS2-10072).

7. Mayo. Y. T., Jr., "Semiclassical Process- ing of Laser I ' ransi t Anemometer S igna ls , " Proceed- ings of t h c 3rd I n t e r n a t i o n a l Conference on Photon Corre la t ion Techniques i n Fluid Mechanics, Churchil l College. Cambridge, U. K., March 1979.

8. F ina l Report on AEDC Contract F40600-79-C- 0003 t o be published.

9. Erdmann, J. C., and C e l l e r t , R. I., "Recur- rence Rate Corre la t ion i n Sca t te red Light Intensity: ' JOSA 68, p. 787. (June 1978). --

10. Barnett , D. 0.. and C i e l , T. V., Jr., "Laser Velocimeter Measurements I n Moderately Heated Jet Flows." Arnold Engineering Development Center Report No. AEDC-TK-76-156. (Apr i l 1977).

11. McLaughlin, D. K., and T i e d e n a n , W. C., "Biassing Correct lon f o r Ind iv idua l R e a l i s a t i o n Laser Anemometer ?leasureri?ents i n Turbulent Flows, Phys. of Fluids. l.6, p. 2082 (December 1973).

Following Equation ( I ) , p l ea se i n s e r t :

Next, t h e mean de lay s t o r e number i s computed:

Y(I)-B 1 KBAR = - Q

9. REFERENCES

1. Sct~odl . R.. "On t h e Extension of t h e Range of Appl icab i l i ty of LDA by Means of t h e Laser-Dual- Focus (L-2-F) Technique." Proceedings of t h e LDA- Symposium Copenhagen. 1975.

APPENDIX I1

SDIICMSSICAL PROCESSING OF

LASER TRANSIT ANEYOMETER SIGNALS

by

W. T. Mayo, Jr.

Presented at:

3rd International Conference On Photon Correlation Techniques in Fluid Mechanics

Churchill College, Cambridge, United Kingdom 21 to 23 March 1979

Abstract :

Laser transit anemmetry (LTA) systems find many practical applications where the statistical variability noise produced by wall flare light limits signal detection. Under these conditions, non-linear threshold-detection and pulse-center-estimation, along with cross correlation processing, is superior to either photon correlation or to classical pulse pair timing. We have developed a photon-limited pulse-center estimation discriminator and demon- strated the efficiency of this with cross correlation processing in recent unseeded backscatter wind tunnel tests with the new SDL LTA system.

This paper introduces ideas concerning cross correlation of non-linearly discriminated LTA signals, some effects of correlator parameters on the measurement of turbulence, and supportive experi- mental results from steady transonic flow.

INTRODUCTION

Laser T rans i t Anemometer (LTA) systems a r e now rece iv ing

considerable a t t e n t i o n ('-l4) because they seem t o be more s e n s i t i v e

i n t he es t imat ion of v e l o c i t y o f very small p a r t i c l e s i n t h e presence

of f l a r e l i g h t than comparable f r i n g e LV systems (11-14). spec t ron

Development Labora tor ies , Inc. has r ecen t ly developed a d i g i t a l l y

automated LTA system wi th funding a s s i s t ance by the U.S. Air Force

Arnold Engineering Development Center (I3) and used t h e system t o

ob ta in unseeded backsca t t e r s e n s i t i v i t y and d a t a rate information f o r

t h e NASA Ames s ix-foot t r anson ic tunnel(14). Demonstration measure-

ments have a l s o been made i n a General E l e c t r i c research compressor

(unpublished). The d e t a i l s of t h e spec t ron LTA system are provided

i n the AEDC r e p o r t (13) and w i l l n o t be repea ted here.

The experimental d a t a from the Ames measurements support a few

ideas of the author concerning LTA s i g n a l process ing us ing a d i g i t a l

c o r r e l a t o r . In t h i s paper we w i l l f i r s t b r i e f l y desc r ibe t h e exper i -

ments and then use some of t he r e s u l t s a s i l l u s t r a t i o n s f o r d i scuss ion

of LTA s i g n a l process ing concepts. The conclusions from t h i s discus-

s i o n have impl ica t ions of a dual na tu re f o r f r i n g e LV s i g n a l process- (15) i n g , some of which may be i n f e r r e d from an e a r l i e r paper .

EXPERIMENT

Description of System Used for NASA Anes Experiment

The SDL LTA system illustrated schematically in Figures

1 and 2 was transported to NASA Ames in October 1978, and set up.

A 50 ns Halvern correlator (45 store) was used for the high speed

delay processing of the SDL pulse discriminator output signals.

The correlator was used in both the "photon" and the "pulse"

correlation modes and all the indistinguishable states in between

as discussed further below. The laser was a Lexel Model 75-.2

operating at about 135 mW at 514.5 nm with 100 mi4 in the

transmitted beams.

The parameters of the SDL optical head were as follows.

The spot separation was measured with a calibrated microscope 2 objective as 0.376 mm with l/e spot diameters estimated at 1/25

of the spot separation, i.e., 15x10-~ micrometers. The "throw" or

range from the front face of the optical head to the probe volume

center was 473 m. The transmitter/receiver lens pair Isere 40 mrn

in radius. The outer annular portion of the lens from radius

20 m to 40 mm radius was used for the receiver, while the

inner area from 0 to 20 mm radius was used for the transmitted

beams. The transmitter/receiver image rotator prism assembly has

digital-to-analog DC drive with DC feedback sensing, and a separate

linear potentiometer and analog-to-digital readout. The precision

of this angular control system was 0.1 degree.

The co-annular backscattered radiation was imaged around a

small transmitter beam turning mirror to two pinhole aperture stops.

These transmitted the desired signal radiation and some diffuse back-

ground radiation to a nicroscope objective which magnified the

images of the transmitted beams illuminating scatterers onto the

ends of two fiber optics assemblies. The purpose of the fiber

optics was to separate the light from the two spots to two separate

LTA Optical Components

r P.M.T1s Laser 7 r Wollaston Prism

- l ~ o t a t o r Prism L I M a ~ n Frame Structure / \ LTrarirceiwr Mirror Assembly

Enlarging objectiveJ L ~ i e l d Stop

Figure 1. Schematic Diagram of Optical Components in SDL Optical Head.

- - 5.0 cm THICK 11

Figure 2. nlock Diagram of SDL LTA Systcrn for Ames Transonic Measurements.

- I - LASER

LASER - WINDOW POWER SUPPLY

LOW -

m AMES 6 I x 6 ' VOLTAGE SUPPLY OPTICS, PMT'S, TRANSON 1 C

ROTATION, AND WIND TUNNEL Y DISCRIMINATORS - *

r

- ;-?I- - - - - HIGH VOLTAGE of u REWOTE FROM

W 2 MEASUREnENTS

r - CORRELATOR z g 2 == Z

<5 E I zg 2 MANUAL DISCR,

1 OSCl LLOSCOPE + RANGE AND ;r 2 SLOW o THRESHOLD 0:

CONTROL OSCI LLO- SCOPE

MON 1 TOR VIDEO

t----- -1 I MINICOMPUTER I r - - 1 L , , , - , - I INTER- I

FACE y I FORTRAN f V

MICROPROCESSOR CONTROL,

DATA MANAGEMENT, AND DISPLAY

MONITOR

KEYBOARD

I DATA PROCESSING I L - , A

L,,,,,,J I TAPE - 3 LOCATED I N SDL COSTA MESA FACILITY) RECORDER

A E D C-T R -80-28

photomultiplier tubes, one for each spot*. The use of two detectors

provided compl.ctely unambiguous direction sensing capability and also

avoided a zero delay correlation which vould interfere with data

processing in high turbulence conditions. Perhaps the most subtle

advantage of two detectors is that the effective background light is

one-half for each detector over what would occur if both spots were

imaged on the same tube. Thus, the undesired background for the cross

correlation of two detector outputs is one-fourth that of the

autocorrelation of one detector output with both signals.

The pulse detection discriminators included several overlapping

ranges. Range 1 was optimized for pulses with duration 25-100 nsec;

Range 2 for 50-200 nsec; Range 3 for 100-400 nsec, and so forth for

seven ranges. Each range included separate optimum detection filters,

which were flat in response across the range so as not to bias the

detection probabilities towards higher or lower velocities, and pulse

center detection filters which determine the center of the pulse inde-

pendently of signal amplitude.

For signal pulses which contain only one, two, or a small num-

ber of single photoelectron pulses, the pulse center detection filter

measures a weighted average pulse center estimation. This means that

pulse center jitter errors can occur up to the transit time duration

as the number of detected photons goes down to one. This implies a

distinct precision advantage of working with three or more photo-

electron pulses per scatterer transit both to avoid processing so many

single photoelectron signals arising purely from background light and

to reduce the pulse-center estimation error. In order to use semi-

classical and classical pulse techniques, one raises the detection

threshold so that single photoelectron pulses are detected less often.

However, due to tile statistical distribution of single photoelectron

pulse heights fron the PHT electron multiplier, the transition from

single to multiple photoelectron events is not distinct, as the data

discussed later shows.

2 For these experiments Dl1 9816 and Dl1 D305 tubes were used. Both tubes are 2" diameter, 14 stage, S-20 tubes of high quality.

The discriminator detection range was both manually and micro-

computer selectab1.e iron the remote location where the operator

was located. The discimirator level and the R-IT high-voltage were

both manually controllable. The PEll"s were protected from maximum

anode current overload by high-voltage relzys following an anode DC

current monitor. These relays were remotely resetable (not shorm

in figure). The correlator data acquisition and prism rotator were

semi-automatically controlled by the microcomputer subsystem.

Measurements in the Slx-Foot Transonic Tunnel

l%e LTA optical head was set up on a table next to a two-inch

thick schlieren window. After one tunnel run duri~g which data were

collected, the tunnel was down for repairs for several days. When

the tunnel was again operational, only limited time was available;

but measurements were obtained at several subsonic and supersonic

conditions. No seeding was used. The measurements are reported in

detail in the NASA report(14). We will only use examples below in

our discussion of several points concerning LTA processing. The

correlograms presented below were obtained by recording then with

the LTA data management system microcomputer and then transferring

them to an HP System 1000 computer for additional processing and

plotting.

CONCEPTS CONCERNING SINGLE-PARTICLE LTA SIGNAL PROCESSING

Detection Filters

The following ideas are not necessarily original, but have been

overlooked by or disagreed with by some investigators. First,

single-particle LTA signals are often sparsely distributed as short

pulses in time while the background flare light is somewhat more

steady statistically. This implies that the optimum detection and

velocity estimation system should ideally shut itself off in between

particle scattering events and avoid processing single photon events

which arise out of background light since they contribute statistical

variability without contributing velocity information. This notion

was one of the reasons for the development of burst-counter processors

for classical fringe LV signals, and it is also applicable to "ideal"

photon correlation of either fringe LV or LTA signals. We have briefly

discussed this concept for photon correlation of fringe LV signals

before(15). An implication which follows immediately is that one must

statistically detect the presence of a particle while delaying the

signal waveform so that the velocity estimator can be noncausally

gated on at time with high probability of the presence of a particle.

This implies the use of a digital or analog detection filter in addi-

tion to the velocity estimation processing. Thus, we postulate that

pure photon correlation using all available photons is not optimum in

many applications.

The principle of gating of localized sections of a single-

photon impulse signal could be implemented in principle by a shift

register delay line with a digital filter which looked for high local

number of photon events. A more realizable system uses a classical

linear analog low-pass filter followed by threshold detection. The

disadvantage of such an approach is that the random variability of the

single photoelectron pulse height response of most photomultiplier

tubes makes it impossible to cleanly discriminate rnultiphoton events from

single-photon background u n l e s s t h e threshold is s e t up very high.

The use of c r o s s c o r r c l a t i o n is h e l p f u l i n t h i s regard , s i n c e it

a l lows us t o work i n t h e s i g n a l regime of a few photons pe r pu l se

even though s i g n i f i c a n t ni~mbers of l a r g e s i n g l e photon background

pu l se s a r e st i l l de t ec t ed .

It is important t h a t t h e d e t e c t i o n f i l t e r s have two cha rac t e r -

i s t i c s . They must have a smooth Gaussian-l ike non-ringing inpu l se

response, and they must have adequate bandwidth s o t h a t t h e h igher

speed p a r t i c l e s a r e no t d i scr imina ted a g a i n s t by t h e high-frequency

r o l l o f f . The bandwidth requirement i s n o t c o n s i s t e n t wi th t h e r e -

quirement of a pulse c e n t e r e s t ima t ion f i l t e r which must i n t e g r a t e

t h e t o t a l s i g n a l f o r an optimum es t ima te as d iscussed I n t e r .

F igure 3, 4, and 5 i l l u s t r a t e , by computer s imula t ion , what a

semi-Gaussian d e t e c t i o n f i l t e r does t o s e n i c l a s s i c a l s i g n a l s from

a photomul t ip l ie r tube. The s imula t ion was f o r 0.5 micrometer

d iameter p a r t i c l e s a t 400 meters p e r second as descr ibed i n d e t a i l

i n (13). F igure 3 assumes reasonable e f f icency f o r t h e o p t i c s (0.3)

whi le F igure 4 and 5 i nc lude an a d d i t i o n a l o p t i c a l e f f i c i e n c y f a c t o r

of 0.15 t o account f o r d i f f i c u l t i e s we were having wi th f i b e r

o p t i c s i n s e r t i o n l o s s . I n Figure 4 t he f o u r s c a t t e r i n g p a r t i c l e s

occur at t h e same t i m e as i n F igure 3, b u t t h e occurrence times a r e

obscured by t h e background s i n g l e photon events . Figure 5 shows t h e

f i l t e r e d ve r s ion of F igure 4 and we s e e t h a t a l l f o u r of t h e s i n g l e

p a r t i c l e s i g n a l s would have been de t ec t ed and none of t h e s i n g l e

photon background i f t h e d e t e c t i o n threshold were p laced j u s t so .

I n p r a c t i c e some d e s i r a b l e s i g n a l s must be l o s t i f t h e t h r e sho ld is

high enough t o m i s s most of t he background s i n g l e photon pulses .

F igures 6 and 7 i l l u s t r a t e t he d e t e c t i o n e f f e c t s which we

have been d iscuss ing . These f i g u r e s a r e reproduct ions of the r a w

c ros s correlograms obta ined i n a s e r i e s of s e q u e n t i a l five-second

d a t a c o l l e c t i o n experiments wi th t h e s ix- foot t unne l s t eady and

"on condit ion" a t Xach 0.9. The exper inent v a r i a b l e w a s t h e

de t ec t ion threshold vo l t age of t h e d iscr imina tors . The two PMT

Figure 3. Low-Pasa Filtered Signals from Four Equally-Timed 0.5 pm Scatterers.

l o o o .a02 o it104 o iooa a;oio TIME I SECONDS I x

-6 Figure 4. Simulated Signals, 0 . 5 ~ 1 0 m Particles,

Optical Efficiency 0.3x0.15.

nl a 9 a I I I I '0 .ooo

I 0 -002 0 a004 0 m00G 0.008 0 . O i U

TI ME [ SECONDS J x

Figure 5. Low-Pass Filtered Signals from Figure 4.

Example Threshold

RUN 71 PERK= 6107 50 mv

- -

Repeat, exactly on condition at M = 0 . 9 , RN = 3x10 6

RUN 73 PERK= 4662 70 rnv RUN 74 PERK= 2669 100 mv

Figure 6 . Raw Correlograms from Mach 0 . 9 Threshold Variation Series.

RUN 76 PERK= 1076 140 mv RUN 76 PERK= 364 200 mv

RUN 77 PERK= 238 280 m v RUN 78 PERK= 163 400 mv

Figure 7 . Additional Raw Correlograms from Mach 0 .9 Threshold Variation Series.

vol tages were "balanced" t o give s imi la r pulse r a t e s from the two

tubes. The displays a r e norml ized , but the value of the s t o r e

contents with the peak vlauc is pr in ted f o r each case. The r e s u l t s

show very c l e a r l y t h a t the v a r i a b i l i t y e r r o r of the "signal part"

of the correlograrns i s worse a t t h e 50 mv threshold l e v e l than

a t the 400 mv l eve l .

We have computed the "data p a i r ra te" a s the a rea under

the peak a f t e r subtract ion of the mean background l e v e l f o r the

s e r i e s of runs i l l u s t r a t e d i n Figures 6 and 7. These r e s u l t s a r e

shown i n Figure 8. The d a t a p a i r r a t e i s much higher wi th t h e

lower threshold, but the v a r i a b i l i t y e r r o r l e v e l is seen t o be

proportionally b e t t e r with respect t o the "signal" por t ion with a

200 mv threshold than with a 50 mv threshold. On the o the r hand,

the re doesn' t seem t o be much re tu rn i n going t o even higher

thresholds where the smaller p a r t i c l e s igna l s a r e l o s t .

We observe t h a t f o r run 76, the input mean pulse r a t e s

from the discr iminators were about 15,000 s-l while i n run 72 i t was

150,000 s-I. The data p a i r r a t e was only 100 and 240 respect ively

f o r these two threshold s e t t i n g s . We a l s o observe tha t the spot

t r a n s i t time was =50ns. Thus, run 72 r e f l e c t s approximately t h e same

behavior a s would be obtained with a f a s t e r , "photon" discr iminator

s ince the Malvern c o r r e l a t o r can only recognize one event per 50 ns.

Pulse Center Estimation

There is a well-know e lec t ron ic technique f o r f inding the

center (with known delay) of a smooth pulse of known width. The pulse

is delayed and subtracted from the underlayed waveform a s i l l u s t r a t e d i n

Figure 9. The zero crossing locat ion is thus a measure of the time

of the center of the pulse which is insens i t ive t o the pulse amplitude.

It i s easy t o see from Figure 4 t h a t t h i s technique would not

work f o r photon-limited s i g n a l s where the s i n g l e p a r t i c l e s i g n a l s

a r e "jagged". This occurs f o r the slower p a r t i c l e s on a given range

i f the detect ion f i l t e r is wideband. I f however, the s igna l is

addi t ional ly low-pass f i l t e r e d with a Gaussian-like f i l t e r impulse

Run No.

""i

Noisy Correlation

I / sea Run 72

6 Re. No. 3.0~10 / f t

5 f Re. NO. = 1 . 5 ~ 1 0 6 / f t A

Figure 8. Data Pair Rate Versus Threshold for Mach 0.9 Runs, October 17, 1978.

0 a ?

il m * P ;P

% .a00 50 :OOO ~oo'.aoo ~ S O ~ ~ O O O ~ a o l . o o u ZEO'.OOO ~ O O ' . O O O ~so' .oao ~ O O ! O O O THRESHOLD [MVI

A

\, Delayed P u l s e

\ \

'\ \. Dt

Figure 9 . P u l s e Center Est imation Concepts.

response which is as long o r longer than the c l a s s i c a l s i g n a l s be-

i n g de t ec t ed , then the f i l t e r ou tput response i s always smooth and

Gaussian-like i n shape r ega rd l e s s of t h e number of photons i n t he

s i g n a l pu lse . Furthermore, t h e du ra t ion of t he f i l t e r e d pu l se becomes

n e a r l y cons t an t and equa l t o t h e f i l t e r impulse response, s o t h e se-

l e c t i o n of t he optimum delay becomes independent of t h e p a r t i c l e ve l -

o c i t y over a use fu l ly l a r g e dynamic range. We have incorpora ted

these concepts i n t o s e p a r a t e p u l s e cen t e r e s t ima t ion f i l t e r s which

a r e only cond i t i ona l ly allowed t o produce a d i sc r imina to r ou tpu t

pu l se a f t e r be ing "armed" by t h e de t ec t ion c i r c u i t s .

The pu l se c e n t e r e s t ima to r j u s t descr ibed determines a

crude e s t ima te of t h e c e n t e r of mass of t h e s i n g l e - p a r t i c l e s i g n a l .

For sma l l photon nuinbers t h e r e w i l l be a random pu l se c e n t e r j i t t e r

e f f e c t due t o t he s ingle-photoe lec t ron pulse he igh t v a r i a t i o n of

t h e pho tomul t i p l i e r tube. Thus, t he u se of pho tomul t i p l i e r s w i th

good s ingle-photoe lec t ron r e s o l u t i o n w i l l improve the t iming accuracy

of t h e smal l s e m i c l a s s i c a l s i g n a l s .

When t h e threshold is high enough f o r s e v e r a l photoe lec t rons

t o be involved i n t he pu l se c e n t e r e s t ima t ion , t h e j i t t e r o r t iming

u n c e r t a i n t y introduced by t h e nonhomogeneous Poisson s t a t i s t i c s

of t h e photon events quickly becomes only a smal l f r a c t i o n of t h e

c l a s s i c a l s i g n a l dura t ion . Thus, t h e spot t r a n s i t time broadening

of t h e correlogram is e l imina ted from the measured da ta . This i s

analogous t o t h e f r i n g e LV b u r s t counter process ing e f f e c t which

avoids t h e " t r a n s i t t ime s p e c t r a l broadening" of t h e e a r l y LV

l i t e r a t u r e . The r e s u l t i s t h a t by us ing semic l a s s i ca l (multiphoton)

s i g n a l d e t e c t i o n , one source of "apparentf ' turbulence is removed,

and s i n g l e - p a r t i c l e v e l o c i t y e s t ima te s a r e meaningful. We r e i t e r a t e

t h a t d i g i t a l f i l t e r s or gated photon c o r r e l a t i o n would be b e t t e r

i n p r i n c i p l e due t o t h e p o t e n t i a l e l i m i n a t i o ~ i of PMT pu l se he igh t

v a r i a t i o n . But p r a c t i c a l high speed flows may produce 10-50 ns

du ra t ion t r a n s i t t imes, and d i g i t a l f i l t e r s wi th 1 ns r e s o l u t i o n

a r e n o t ava i l ab l e .

STORE NUMBER

Figure 10. Curve Fit of Correlogram at M = 0.9, Threshold = 200 mv.

Ext rac t ion of Mean Veloci ty f o r Low Turbulence Flows

For i d e a l i z e d p e r f e c t e s t ima te s of t he s i n g l e p a r t i c l e

pulse c e n t e r s , t h e con t r ibu t ions t o t h e c r o s s c o r r e l a t i o n func t ion

under t h e peak bccose a histogram of p a r t i c l e two-spot t r a n s i t

t imes. This histogram is similar t o a his togram of t h e N f r i n g e

t r a n s i t t imes wi th f r i n g e systems, and many of the r e l a t e d concepts

apply d i r e c t l y . I n p a r t i c u l a r , we s e e t h a t by e s t ima t ing and

s u b t r a c t i n g t h e f l a t c r o s s c o r r e l a t i o n background, t h e histogram

of de lay times is an e s t ima te of t he p r o b a b i l i t y d e n s i t y f o r

i n v e r s e v e l o c i t i e s along t h e d i r e c t i o n of t h e two spo t s . For

smal l v e l o c i t y dev ia t i ons evenly d i s t r i b u t e d about t he mean t h e

p r o b a b i l i t y dens i ty f o r v e l o c i t i e s has a s i m i l a r shape t o t h a t

of t r a n s i t t imes according t o t h e approximation f o r sma l l E of :

For sma l l Gaussian turbulence then, i t is approximately c o r r e c t

t o f i t a Gaussian curve t o t h e t r a n s i t t ime histogram. We have

done t h i s f o r runs 72 through 78 i l l u s t r a t e d above i n F igures 6

and 7. Figure 1 0 shows t h e r e s u l t i n g f i t f o r run 76.

Examination of F igure 10 shows g raph ica l ly t h a t a paramet r ic

curve f i t does no t e s t ima te t h e mean t r a n s i t t ime a t t h e peak of

t h e raw correlogram. The f i g u r e makes it i n t u i t i v e l y obvious t h a t

t h e a v a i l a b l e p r e c i s i o n of t he mean v e l o c i t y e s t ima te i s much

f i n e r than the c o r r e l a t o r c lock increment. I t is a common mistake

t o t h ink t h a t t h i s i n t e r p o l a t i o n r e q u i r e s s u f f i c i e n t turbul.ence

i n t he f low t o broaden t h e peak t o i nc lude more than one s t o r e .

This i s not t rue . I n Reference 1 6 we showed t h a t t h e c l a s s i c a l

c o r r e l a t i o n func t ion is convolved wi th a t r i a n g u l a r weighting

func t ion whose base width i s two de lay increments. The e f f e c t of

t h i s f o r exac t ly s teady flow wi th no pu l se c e n t e r j i t t e r i s a s

fol lows: when t h e t r a n s i t de lay is exac t ly an i n t e g e r number of

s t o r e s , then only one s t o r e w i l l have any non-zero con ten t s . lu'llen

t he delay is something else, t h e f r a c t i o n a l time t h a t the f c lock

count e r r o r occurs d i s t r i b u t e s t he con ten t s i n two b ins i n such

a manner t h a t t h e c e n t e r of moment is t h e exac t delay value. This

concept i s i l l u s t r a t e d g raph ica l ly i n F igure 11.

A s evidence of t h e p rec i s ion which we a r e d i s c ~ s s i n g , w e

observe t h a t t h e curve f i t s f o r runs 75 through 78 (see Figure 7)

gave agreement on t h e mean v e l o c i t y e s t ima te of 0.15X.

Ext rac t ion of Turbulence I n t e n s i t y From Low Turbulence Flows

The c u r v e f i t i l l u s t r a t e d i n Figure 1 0 ind ica t ed 2.2% r . m . s .

turbulence. I n f a c t , however, we can observe t h a t t h e width is only

s l i g h t l y g r e a t e r than t h e p l u s and minus c lock increment which is auto-

ma t i ca l ly present . Thus w e conclude t h a t f o r t h i s s e t of d a t a , t h e

e f f e c t s of pu l se cen te r es t imat ion jitter and a c t u a l turbulence* were

both masked by a s i g n i f i c a n t l y l a r g e r c o r r e l a t o r time r e s o l u t i o n

e f f e c t .

It i s n o t unreasonable t o consider lqdeconvolution'l of such

e f f e c t s when they are the sane width a s t he e f f e c t s t o be measured

o r smal le r . It is no t w i s e t o t r y t o e l imina te a broadening e f f e c t

which is much wider because then smal l e r r o r s become s i g n i f i c a n t i n

t he s u b t r a c t i o n wi th uns table meaningless r e s u l t s . We conclude t h a t

f o r t r anson ic and supersonic flow measurement w i t h the small beam

spacing w e were using, a 10 ns c o r r e l a t o r would be needed t o r e so lve

0.5% turbulence accu ra t e ly , and a 5 n s r e s o l u t i o n would be p re fe rab le

f o r supersonic flows.

Data Rate and Background Level Equations

For s i m p l i c i t y assume t h a t a l l c o r r e l a t e d pulse p a i r s occurred

a t exac t ly the same t i n e delay. The r a t e of occurrence, on e i t h e r

channel, due t o these "signal" pu l se s i s then hs. The he ight of t h e

"signal" p a r t of the correlogram above the background l e v e l is thus

T where t h e sample dura t ion T i s given as S

* Which the tunnel personnel es t imated a t l e s s than 1.02 based on p r i o r experience.

Exact Transit Time

Delay T

4 - Longer Transit Time . .

i '. :

e m . . . . . . = *.a . a . . . a . - Delay T

Figure 11. Interpolation of Exact Hean Delay.

T = N A t

where N is the t o t a l number of A t c lock increments. For t u rbu len t

flow where t h e s i g n a l de lay spreads over nore than one de lay incre-

ment, t he he igh t of the s i g n a l p a r t of the correlogram i s d iv ided

by t h e number of de lay increments over which t h e s i g n a l de lay i s

spread.

Now assume t h a t t he r a t e s of a d d i t i o n a l uncorre la ted pulses

i n t o Channels A and B r e spec t ive ly a r e A and Ab. For l o c a t i o n s of a

t he c r o s s correlogram o t h e r than s i g n a l , t h e c o r r e l a t i o n func t ion is

a f l a t l e v e l wi th expected va lue given by NP(1,l) where P ( 1 , l ) i s t h e

p r o b a b i l i t y of both a 1 i n a present sample of Channel A and a l s o a 1

i n a delayed sample of Channel B, i.e., f o r the background loca t ions .

For c a s e s where X << X o r X as i n run 76 f o r example, Ph(l,l)-XaXb. s a b'

To demonstrate t he numbers p r a c t i c a l l y , we cons ider run 76 again:

A t = 50x10-~s

N = 10 8

3 3 Background = (15 .24~10 ) ( 1 2 . 7 ~ 1 0 ) (50xl0-')2 (10) * = 48.6.

This i s t o be compared wi th the run 76 c o r r e l a t o r mean value obtained -1 as Background = 50, Background Rate = 50/ (5 seconds) = 10 S .

I n t h i s example the correlogram background l e v e l is one t e n t h

the d a t a p a i r r a t e and the experiment agrees wi th theory. W e s e e t h a t

* Actual da t a measured by the monitor s t o r e s of the c o r r e l a t o r .

da ta r a t e cannot be p red ic t ed by measuring the inpu t pulse r a t e s h a

and A which a r e much higher. It must a c t u a l l y be measured wi th a b

c o r r e l a t o r as w a s done i n the experiments. I f c ros s c o r r e l a t i o n

process ing is no t used, then the form of t h e mean value of t he back-

ground is no t f l a t and may p resen t a d i f f i c u l t y i n t he e x t r a c t i o n of

d a t a from more tu rbu len t flows. I f s i n g l e s t a r t - s t o p time of f l i g h t

measurement i s used, d a t a is l o s t when the threshold is lowered

enough t o produce apprec iable background r a t e s . Autocorre la t ion of

s i g n a l s from a s i n g l e PMT k i t h both s p o t s inaged on the sane tube

produces a more d i f f i c u l t correlogram t o i n t e r p r e t and considerably

more background produces v a r i a b i l i t y .

DISCUSSION: CORRELATORS VS. BURST COUNTERS

There are some similarities between LTA s i g n a l processing and

f r i n g e LV s i g n a l processing. This i s not s t r ange s i n c e f r i n g e and

two-spot o p t i c s a r e simply two d i f f e r e n t forms of t r a n s i t time measure-

ment. F i r s t w e look a t two-spot o r LTA s i g n a l s and then cons ider t h e

analogous concepts f o r f r i n g e LV s i g n a l de t ec t ion .

LTA S igna l Process ing

For spa r se high-level c l a s s i c a l s i g n a l s a time-to-amplitude

conver ter , o r s t a r t - s t o p counter t i m e r , p l u s a histogram d i s p l a y

genera tor is a n e x c e l l e n t d a t a process ing technique. The d e t e c t i o n of

s i n g l e photoe lec t ron background pu l se s i s avoided and the background

l e v e l of t h e d i sp l ay r e s u l t s only when two sepa ra t e p a r t i c l e s each

c r o s s a spo t w i th in t h e d i sp l ay de lay time. This condi t ion is was te fu l

of t h e more abundant and more responsive smal ler p a r t i c l e s . t h e n t h e

de t ec t ion th re sho ld is lowered t o d e t e c t smal le r p a r t i c l e s , the l a r g e r

s ingle-photoelectron background pu l se s a r e a l s o de tec ted . Fa l se starts

by a counter timer on background l i g h t pu l se s al lows t r u e s i g n a l p u l s e s

on t h e start channel t o be missed u n t i l a f t e r counter r e s e t . D i g i t a l

c o r r e l a t i o n does no t m i s s t h i s da t a . Also, f o r c r o s s c o r r e l a t i o n , t h e

mean background remains f l a t s o t h a t it may be e a s i l y removed by post-

de t ec t ion c u r v e f i t processing.

The arguments t h a t some have given t o the e f f e c t t h a t co r r e l a -

t i o n does not work w e l l is t r u e f o r pure "photon" c o r r e l a t i o 2 o r

c l a s s i c a l c o r r e l a t i o n of l a r g e r s i g n a l s when l a r g e percentages of the

s i g n a l a r e c l a s s i c a l l y s teady s t a t e o r slowly varying. Such conclu-

s i o n s a r e n o t t r u e when appropr ia te s i g n a l f e a t u r e de t ec t ion is used

p r i o r t o co r re l a t ion , as we have shown.

Fringe LV Signal Process ing

There a r e d i r e c t p a r a l l e l s of t he above d iscuss ion f o r f r i nge

LV process ing , bu t they a r e harder t o implement and may not o f t en be

as practical-. For large classical signals, a burst counter is a superb

detection system. IF one could also use a matched tracking filter,

classical signals with signal to noise ratio adequate for one percent

accuracy could be obtained from 30 photoelectrons in the absence of

background light (see Refernce 17). Thus the fact that a "burst photon

correlator" should be capable of such a feat is not a surprise. It is,

however, not easy to obtain the proper tracking filter so in certain

situations, a single-burst correlation has an advantage if it is gated

off when the signal burst is not present.

We have implied before (see Reference 15) that filtering a

fringe LV signal and then correlating the zero crossing times conditioned

on a detection filter gate (or arming level for the filtered signal) could

be quite sensitive. This is more powerful than even the single-cycle

counter histogram approach developed by H. Kalb (Reference 18) because

it allows for using tha relationship between each zero-crossing and

every other one, and thus uses more of the information. The purpose of

the filtering and "arming" threshold would be the same as in our LTA

experiments, i.e., to avoid the variability added by correlating the back-

ground generated "noise" between signals. The sensitivity enhancement

reported by H. Kalb (Reference 18) over the conventional burst counter

seems to support these ideas.

Implications

When a separate receiver observes low-speed laser anemometer

signals with negligible background light, then single-particle photon

correlation is an efficient data processing technique. When background

light becomes an issue, semiclassical filtering and detection techniques

may enhance the system sensitivity over that of conventional photon

correlation. For high-speed flows, this filtering will be analog in

nature, but single bit digital correlation remains an effective data

extraction technique.

1. Pulse presence de t ec t ion and s e l e c t i v e s t g 2 a l process ing can

reduce t h e v a r i a b i l i t y e r r o r cont r ibu ted by background f l a r e

l i g h t when c ros s c o r r e l a t i o n d e t e c t i o n i s employed.

2. Pulse cen t e r e s t ima t ion based on s e v e r a l photons pe r p a r t i c l e

t r a n s i t can reduce t r a n s i t t ime broadening a s soc i a t ed wi th

convent iona l time average correlograms . 3. D i g i t a l c ros s c o r r e l a t i o n of s e m i c l a s s i c a l l y d i s c r imina t ed

pu l se s o f f e r s advantages ove r s i n p l e r t ima-of-f l ight s t a r t -

s t o p processing. It provides a well-behaved ( f l a t ) background

con t r ibu t ion which s i m p l i f i e s pos t -de tec t ion process ing , and

d a t a i s not missed when more than one " s t a r t " occurs before a II stop."

4. Mean flow p r e c i s i o n is n o t l i m i t e d by the clock i n t e r v a l even

f o r s t eady flow; b u t t h e c lock i n t e r v a l does produce an

apparent turbulence width which must be corzsidered.

5 . These concepts have been demonstrated i n t h e measurement of

unseeded subsonic and supersonic flow wi th a c o a x i a l b a c k s c a t t e r

LTA system employing a 135 mW l a s e r . The meari flow repea t ab i l -

i t y obta ined was 0.152 at Mach 0.9 f r o 2 s c a t t e r e r s computed t o

be 0.5 microneter i n diameter.

Discussions with and suggestions by A . E . Smart have been

numerous and invaluable. Mr. Tom Hunt was responsible for the

deta i led design and implementation o f the e l e c t r o n i c discriminators.

M s . Carolyn Greenwood provided simulation software implementation.

REFERENCES

1. Tanner, L. H., Jou rna l of S c i e n t i f i c Ins t ruments , Vol. 4, 1967, pp. 725-730.

2. Thompson, D. H., J o u r n a l of S c i e n t i f i c I n s t r u n 2 n t s , S e r i e s 2 , Vol. 1, 1968, pp. 929-932.

3. Tanner, L. H., Op t i c s and Laser Technology, June 1973, pp. 108-110.

4. Schodl, R. , ASME Paper No. 74-GT-15 7 (19 74) . 5. Eckardt , D., ASHE Paper No. 76-FE-13 (1976).

6. Lading, L., Paper 23, AGARD CP 193, 2-5 May 1976.

7. B a r t l e t t , K. G. and She, C. Y., Applied Opt ics , Vol. 15, No. 8, August 1976, pp. 1980-1983.

8. Smart, A. E. , Proc. Conf. on Photon Cor re l a t ion Techniques i n F lu id Mechanics, Church i l l College, Cainb r idge (England) 6-7 A p r i l 1977.

9. B a r t l e t t , K. G. and She, C. Y., Opt ics L e t t e r s , Vol. 1, Nov. 1977, pp. 175-177.

10. Lading, L., Jensen, A. S., Fog, C., Anderson, H., Applied Op t i c s , Vol. 17 , No. 10 , 15 Hay 1978, pp. 1486-1488.

11. Smart, A. E., "Data R e t r i e v a l i n Laser Anemorretry by D i g i t a l Correlat ion," Proc. Conf. Thi rd I n t e r n a t i o n a l Workshop on Laser Velocimetry, Purdue Un ive r s i ty , J u l y 11-13, 1978.

12. Smart, A. E., "Laser Anemonetry Close t o Walls," Proc. Dynamic Flow Conference 1978, Balt imore, Maryland, Sept . 18-21, 1978.

13. Mayo, W. T. , Jr. and Smart, A. E . , "Conparison of Data from T r a n s i t Time Velocimeter w i t h Other S y s t e m Now i n Use f o r Ve loc i ty Messureirent," F i n a l Report USAF Contract F40600-78-C-0002, Arnold Engineering Developm2nt Center , Tennessee (now i n p r i n t i n g ) .

14. Mayo, W. T., Jr. and Smart, A. E . , " F e a s i b i l i t y Study o f T r a n s i t Photon Cor re l a t ion Anemometer f o r h2s Research Center Unitary Wincl Tunnel Plan," NASA CR152238, F i n a l Report f o r Cont rac t NAS2-10072, NASA A m e s Research Center , ?:o f f e t t F i e l d , C a l i f o r n i a (now i n p r i n t i n g ) .

15. Smart, A. E. ant1 Mayo, V. T. , J r . , Proc- Conf. 011 Photon Cor re l a t ion Tech3Alucs i n F lu id I-lecl~anics, Stockilolm, Sweden, 14-16 June 1978.

16. Nayo, William T., J r . , "Study of Photon Cor re l a t i on Techniques f o r Process ing of Laser Velocimeter Signals" , NASA CR-2780, February 1977.

17. Fiayo, W i l l i a m T . , Jr . , "Ocean Laser Velocimetry Systems: S igna l Processing Accuracy by Simulation", presented a t t h e Third I n t e r n a t i o n a l Workshop on Laser Velocimetry, Purdue Univers i ty , 11-13 J u l y 1978.

18. Kalb, H. T. and Cl ine , V. A. , "New Technique i n t h e Processing and Handling of Laser Velocimeter Burs t Data", Rev. S c i . I n s t r u . , Vol. 47, pp 708-711.

APPENDIX 111

DISCRIMINATOR TESTS

The functioning of the LTA system is dependent upon noise

rejection by electronic as well as optical means and the principle

of single photon event rejection was discussed in References 20 and

21 of the main text. Under the present contract we have built new

discriminators and examined their performance. Several tests were

performed and limits of behavior quantified, the results of which

are presented here.

XEAN DELAY BIAS TESTS

The first test of systematic electronic delay difference,

before the diode test was introduced, was to rotate the spots 180°

in a jet flow and compute the velocity A+B and B-tA. Any difference

shows a systematic disparity in signal processing delay. To our

surprise, we found a disparity of 100 ns! This was finally traced

to a photomultiplier dynode, miswired on receipt from the manufacturer.

This was causing a charge cloud around one accelerating electrode

which thus retarded electron transit. The clue which led to the

discovery of the cause of the delay effect was reduced gain of one

P.M.T. which had required a compensatory increase in high voltage.

This was not suspected immediately because there is significant

difference in gain from one individual P.M.T. to another.

The photomultiplier/preamplifier/filter/discriminator com-

binations (Figure 1) were tested by inserting a light pulse from

a fast light emitting diode. The input electrical pulse is shown

as Figure 2. The final output pulse from the discriminator is shown

as Figure 3. The ttme delay between input pulse and output pulse

in a satisfactorily operating system was found to be about 70 ns and

is subject to both systematic mean and time dependent deviations.

The two channels may not be perfectly matched causing a systematic

difference between them. This was observed as no more than a few ns

i I, ight

Time Gate

Fi~otonultipl ier

Prcariplifier . - . - . - .BuEfcr Anplifier

- Signal i.lonitor

I One Shot

t Output Pu.lse

I.inc: Driver

FIGURE 1 . SCHIMATIC OF ONE CHANNEL OF ELECTRONICS

i I, ight

Time Gate

Fi~otonultipl ier

Prcariplifier . - . - . - .BuEfcr Anplifier

- Signal i.lonitor

I One Shot

t Output Pu.lse

I.inc: Driver

FIGURE 1 . SCHIMATIC OF ONE CHANNEL OF ELECTRONICS

A EDC-TR-80o28

Figure 2. Electrical Signal to Test LED.

Figure 3. Discriminator Output Pulse.

III

on most r a n g e s and its s i g n i f i c a n c e as a s y s t e m a t i c e r r o r depends on

t h e t i m e of f l i g h t o f a p a r t i c l e between s p o t s . On t h e h i g h speed

r a n g e s , 00 and 0 t h e mean d e v i a t i o n f o r a f i x e d ampl i tude i n p u t p u l s e

was on the o r d e r of 2 ns.

PULSE 1'11.1'I.!{ I:P'r'lSCTS AND 11!510V,21..

F i g u r e 4 shows t h e r e s p o n s e of t h e P.M.T. monitor c i r c u i t on

range 00 f o r a d i o d e i n p u t and a t t e n u a t i o n which y i e l d s a small number

of photons pe r p u l s e . There is a s i g n i f i c a n t p r o b a b i l i t y of g e t t i n g

NO photons from some of t h e p u l s e s . T h i s photograph, as a l l o t h e r s - i n t h e fo l lowing sequence is o b t a i n e d a s t h e s u p e r p o s i t i o n of about

h a l f a m i l l i o n e v e n t s . I'i.gure 3 s1;ows t h a t from such a s t i m u l u s , i t

is p o s s i b l e t o g e t o u t p u t p u l s e s of a wide range of a ~ p l i t u d e s . The

low t r i g g e r l e v e l (80 mV) makes i t p o s s i b l e t o s e e t h a t t h e r e is a

p r e f e r r e d h e i g h t o f t h e most p robab le p u l s e from one s i n g l e photon and

a s l i g h t l y reduced p r o b a b i l i t y of p u l s e s of l e s s e r h e i g h t . Th i s is,

t h e r e f o r e , f a i r l y good tube and cor responds t o a demons t ra t ion (of

s i n g l e p h o t o e l e c t r o n p u l s e h e i g h t s ) of t h e curve C i n F i g u r e 2 of

Reference 1 3 . T h i s is shorn a g a i n by F i g u r e 5 where t h e s i g n a l com-

p r i s e s a n average of l e s s than one photon (evinced by che b a s c l i n e )

t r i g g e r e d frorr! t i le d i o d e e x c i t i n g p u l s e . It shows ouer l .p i l~g peaks of

d i f f e r e n t h e i g h t s and a 'hole ' where t h e r e a r e n o t s i g n i f i c a n t numbers

of small p u l s e s from real photons . Two photon e v e n t s occur t o o i n f r e -

q u e n t l y t o be a s i g n i f i c a n t c o n t r i b u t i o n t o t h i s p i c t u r e . F i g u r e 6

i s a somewhat s i m i l a r p i c t u r e t o F i g u r e 4 b u t now h a s a t r i g g e r l e v e l

s e t t o 600 mV t o exc lude a l l p u l s e s which may n o t have a r i s e n from a

' r e a l ' i n c i d e n t photon.

r i g u r e s 4 and 6 show t h a t because of s i n g l e p h o t o e l e c t r o n p u l s e

h e i g h t d i f f e r e n c e s , t h e time of t h e peak r e l a t i v e t o t h e f i x e d t h r e s h o l d

d e t e c t i o n p o i n t shows a s c a t t e r of abou t 6 t o 8 n s - a q u i t e unaccept-

a b l e e q u i v a l e n t t r a n s i t t i m e broadening from nominally i d e n t i c a l

e v e n t s . F igure 7 shows how t h i s sp read is removed by t h e p u l s e c e n t e r

A E D C - T R - 8 0 - 2 8

Figure 4. Signal Triggered from a Low Threshold.

Figure 5. Signal Triggered from Diode Exciting Pulse.

113

A E DC-T R -80-28

Figure 6. Signal Triggered from a High Threshold.

Figure 7. Output Waveform from Pulse Center Estimation Filter.

114

f i nd ing c i r c u i t and shows the event t r iggered from t h e output pu lse .

The spread of t h e peaks of d i f f e r e n t he igh t s is now seen t o be sym-

me t r i c and involve minimal dependence of timing on pulse he ight .

PHOTON PROBABILITY JITTER

While t h e d i s c r imina to r s make good es t imates of t h e time

l o c a t i o n of a s i n g l e photon pulse , t h e r e a r e o the r e f f e c t s t o consider .

When only a few s i n g l e photon pulses occur during t h e c l a s s i c a l

s i g n a l p robab i l i t y envelope defined by c l a s s i c a l s c a t t e r i n g theory,

then j i t t e r occurs i n the pulse cen t e r es t imate due t o t h e inhomo-

geneous Poisson occurrence time s t a t i s t i c s of t h e s i n g l e photoelectron

pulses and due t o t h e random s i n g l e photoe lec t ron pulse he igh t d i s -

t r i b u t i o n of t h e P.X.T. The i n t e g r a t i n g pulse c e n t e r es t imat ion

f i l t e r s f i n d a swighted t r a n s i t mean cen te r , which ( p r a c t i c a l l y

speaking) e l imina tes t h e photon p r o b a b i l i t y j i t ter e f f e c t when

s e v e r a l s i n g l e photons con t r ibu t e t o a t r a n s i t event . However, no

a n a l y t i c a l theory f o r t h e ex t en t of time j i t t e r versus photon number

and P.H.T. p r o b a b i l i t y d i s t r i b u t i o n parameters has been developed t o

our knowledge.

In order t o f i n d the ex t en t of photon j i t t e r e f f e c t i n a t yp i ca l

high-speed measurement, the l i g h t emi t t ing diode was used aga in ,

bu t t h i s time the osc i l loscope w a s t r iggered from a sample of t he pulse

genera tor source pulses . The r e s u l t i n g mu l t i p l e t r a c e osci l lograms

of t he d iscr imina tor output pu lse leading edges a r e some measure of

t he o v e r a l l j i t t e r due to both t he previously discussed amplitude e f f e c t s

and the occurence p r o b a b i l i t y e f f e c t s .

Tes t s were conducted f o r d i f f e r e n t ranges with the o sc i l l o scope

t r iggered from the e l e c t r i c a l pu lse d r iv ing t h e diode. On an expanded

time base, the following i l l u s t r a t i o n s were obtained of the s t a r t i n g

edge of t h e d iscr imina tor output pulse. To simulate worst ca se opera-

t i o n , the average number of photons per pulse was s e t t o l e s s than one

a s is shown by t h e presence of a base l ine of t r iggered sweeps wi th

no d iscr imina ted event . The change of th reshold had almost no e f f e c t

on j i t t e r over a wide range, f o r a photon is a photon with e i t h e r a

discr i ininable event o r no t , but i t may be emit ted a t any time dur ing

the pulse of e l e c t r i c a l energy t o the diode (with a p robab i l i t y depend-

ing upon the wavef orrn and the device) . A sequence of j i t t e r p rope r t i e s i s shown on Figure 8 ( a through

e ) and t h e d e t a i l s a r e summarized on Table T. Each i s f o r about one

m i l l i o n events and t h i s sequence is a l l f o r range 00, t h e f a s t e s t .

A l l were taken a t the same threshold.

TABLE I

Figure 8

(Mean)

Many

Hany

1 - 2

1 - 2

< 1

High Voltage

(Volts)

1250

1400

1400

1600

1600

THRESHOLD/RANGE EFFECTS

Because t he re is a l i n k between threshold l e v e l and p rec i s ion

of timing, i t i s poss ib l e t o check t h a t s i m i l a r event j i t t e r charac-

t e r i s t i c s occur on each range. Figure 9 (a through d) a r e p i c t u r e s of

t he same type a s i n the previous s ec t ion . The scope was t r iggered from

the cu r r en t e x c i t i n g the diode and the d iscr imina tor pulse was observed

a t s u i t a b l e timebase d i l a t i o n . The ranges i nc rease i n f a c t o r s of two

with r e spec t t o t h e input f i l t e r except f o r range 0 which has the same

input f i l t e r a s range 1 with d i f f e r e n t pu l se cen t e r es t imat ion f i l t e r s .

A E DC-TR-80-28

(a) (b)

(c) (d)

Figure 8. Time Jitter Effects. (Table I)

(e)

117

AEDC-TR-80-28

(a) (b)

(c) (d)

Figure 9. Threshold Range Effects. (Table II)

l l 8

The same r e s u l t s would be expected i n terms of r a t e and j i t t e r when

changing the threshold 5y the inverse of t he e x t r a d i l a t i o n incur red

by using a lower frequency f i l t e r . The Figures a r e i d e n t i f i e d i n

Table 11.

TABLE 11

Figure 9 Range Threshold

FILTERS

Figure 10 (a t o f) a r e p i c t u r e s of t h e response of t h e ranges 00

through b t o t he same o p t i c a l input s i g n a l . T h e amplitudes reduce by

the same f a c t o r a s t he de t ec t ion f i l t e r and do not proceed p e r f e c t l y

i n t he r a t i o s 16:8:8:4:2:1 because of v a r i a t i o n s i n t r i g g e r l e v e l .

The tendency is i l l u s t r a t e d s a t i s f a c t o r i l y on the pulse lengths . There

i s a small amount of overshoot and some e f f e c t s of small r e s i d u a l

l i n e mismatch when observed From the monitor terminal b u t t h i s does

not a f f e c t t he nachine performance. Hore s e r ious ly , t he re i s a

g l i t c h some 6 squares a f t e r t he peak due t o t h e c r o s s channel i n t e r -

fe rence of t he d iscr imina tor output pu lse on the o the r channel. This

c r o s s t a l k has not p e t caused problems but i t may u l t ima te ly be

necessary t o e l imina te i t .

The plug-in d iscr imina tor cards have p rope r t i e s described by

Table 111 which r e l a t e s a l s o t o t h e f i g u r e s .

Figure I1 shows the d iscr imina tor wi th c o n t i n ~ o u s l i g h t and

too low a threshold s e t t i n g . There i s an undes i rab le bounce f e a t u r e

A E DC-TR o80-28

(a) (b)

(c) (d)

(e) (f)

Figure I0. Signal Monitor on Each Range. (Table III)

120

AEDC-TR-80-28

Figure Ii. Discriminator Output Pulse (With Bounce).

Figure 12. Discriminator Output Pulse (Corrected).

121

where s e v e r a l p u l s e s a r e produced r e l a t e d t o t h e i n i t i a l f i r i n g .

T h i s is h i g h l y u n s a t i s f a c t o r y f o r work where t h e o u t p u t p u l s e s a r e

c o r r e l a t e d and was cured by a s m a l l change i n t h e reset l i n e ti.ming

t o produce t h e c l e a n e r p i c t u r e of F i g u r e 12. T h i s change resul . ted i n

a s l i g h t i n c r e a s e i n d i s c r i n i n a t o r dead t i m e which i s n o t s o s e r i o u s

a p e n a l t y a s m u l t i p l e f i r i n g . Range 3, from which t h e s e photographs

were ob ta ined shows a dead t ime of 450 n s whereas t h e f a s t e r r anges

show p r o p o r t i o n a l l y s h o r t e r dead t i m e s .

TABLE I11

DISCRIMINATOR CARD PLUG-INS

D e t e c t i o n F i g u r e 1 0 Range r i l t e r

P u l s e Center E s t i n ~ a t i o n F i l t e r Card S l o t

APPENDIX I V

SECTION 7 .0 THROUGH 7 . 3 OF "LASER TRANSIT ANEMOMETER SYSTEM OPERATION MANUAL"

SDL R e p o r t No. 7 9 - 6 4 6 3

July 1 9 7 9

7 -0 OPTICAL CLEANING AND ALIGNMENT

There i s one important consideration t o be observed when

embarking upon the task o f alignment o r cleaning o f opt ics :

DON 'T

7.1 Gett ing D i r t y

There are several items which may get d i r t y a f t e r prolonged

o r careless use.

Slow and overa l l deposit ion o f dust on a1 1 surfaces

inc lud ing op t i ca l components may occur. This deposit

may be removable w i t h a b l a s t o f clean a i r . I f i t

becomes very bad, as i t may i f the u n i t i s f requent ly

opened i n d i r t y places, i t i s necessary t o disassemble

the machine, clean the components and reassemble and

a l i g n the u n i t . This i s not a recormended ac t ion f o r

the user -- we do, however, include s u f f i c i e n t informat ion

f o r you t o do th is .

7.1.2 Thermophoretic t ransport i n the close neighborhood o f

surfaces and dust deposi t ion on the output t r a i n

op t i ca l components may impair the performance. The

components most l i k e l y t o su f f e r from t h i s are the

fou r turn ing mirrors, the t ransceiver mi r ror , and the

focusing lens. The Wollaston prism i s qu i te we l l

protected by the a1 ignment stop and i n any case would

requi re disassembly f o r cleaning, although i t s reassembly

and alignment i s f a i r l y easy. The lower surface o f the

focusing lens i s prone t o thermophoretic deposi t ion

but cleaning should only be necessary a f t e r exposure

o f the naked systein t o a dusty environment (see

Section 7.2).

7.1.3 The f r o n t surface of the f i n a l outer t ransceiver lens

may get d i r t y more read i l y than any other component

simply because i t i s exposed t o the ambient condi t ions

more o f ten and f o r longer periods. Depending upon

the type o f contamination i t may be cleaned by blowing

w i t h clean dry a i r and/or cleaning w i th solvent as

i n l a t e r sections.

7.2 Cleaning Optical Components

I n general i f on ly one component a t a time i s removed f o r

cleaning i t i s not necessary t o r ea l i gn the e n t i r e system. It i s

however essent ia l t o replace each cleaned component i n p rec ise ly the

same place and o r ien ta t ion . It i s important t o f o l l ow these ins t ruc -

t i ons t o avoid unnecessary re-alignment tedium. I f a screw has red

sealant on i t you are advised not t o touch i t . Most can be recovered

w i t h s u f f i c i e n t patience but those adjustments on the ro ta to r prism

requi re great dedication. Just i n case you are tempted t o a l t e r

the system, we do g ive f u l l alignment de ta i l s i n Section 7.5, bu t

touch the red screws a t your own r i s k .

Any carelessness o r f a i l u r e t o observe the precise ins t ruc t ions

i n t h i s sect ion of course inva l idates the guarantee. P r i o r t o cleaning

o r op t i ca l maintenance o f any k ind set the op t i ca l head i n a clean,

warm, dry environment.

For the actual cleaning wear medical qua l i t y cap and mask and

use t h i n polythene gloves o r be su f f i c ien t l y pract iced never t o touch

a component on o r near an ac t i ve surface.

The ou te r p a r t o f t h e u n i t should be cleaned f r e e from dust

o r d i r t p r i o r t o the reinoval o f t h e s ide panels. Many components

may be adequately cleaned i n s i t u . For those which must be removed

observe the i n s t r u c t i o n s and do not become involved w i t h more than

one component a t a t ime.

7.2.1 A component should be i n i t i a l l y blown f r e e of deposited

dust o r p o t e n t i a l l y abrasive p a r t i c l e s w i t h a j e t o f

c lean d ry a i r o r n i t rogen. Most ly the s t i c k y p a r t i c l e s

a r e no t abrasive and t h e abrasive p a r t i c l e s a re no t

s t i c k y but t h i s i s no t guaranteed and i t should n o t

be assumed t h a t a l l g r i t has been o r can be blown off.

Obtain the purest methanol (methyl a1 coho1 , CH30H)

a v a i l a b l e and charge a hypodermic syr inge w i t h it.

Rinse the syr inge many t imes t o remove t h e so lub le

f r a c t i o n o f t he seal l u b r i c a n t . It i s usual t o favo r

an eye-dropper f o r supplying t h e alcohol bu t s ince

i n t h i s system most o f t he components t o be cleaned

a r e n o t ho r i zon ta l t h e syr inge i s more appropriate.

Since i t i s useful t o r e t a i n the hypodermic needle

t o permi t easy con t ro l o f a lcohol f l o w great care

must be exercised t o avoid touching t h e needle p o i n t

on the surface when supply ing alcohol t o the cdmponent

t o be cleaned.

Use good q u a l i t y o p t i c a l t i s s u e -- f a c i a l grade

Kleenex i s okay provided you always use a new pack

o r d iscard those pieces which have been exposed t o

dust c o l l e c t i o n . Break t h e t o p o f f t h e box so t h a t

a f resh t i s s u e i s accessib le w i thout exposing t h e

one beneath t o a s i t u a t i o n where i t no longer p ro tec ts

t he one beneath it. Keep t h e box covered a t a l l

t imes.

Lay the t i s s u e near the surface and supply s u f f i c i e n t

a lcohol t o a t tach the t i s s u e by c a p i l l a r y a t t r a c t i o n

a t one edge. Slowly and d e l i b e r a t e l y drag t h e t i s s u e

across t h e surface such t h a t no v i s i b l e l i q u i d res idue

i s l e f t . This procedure may be repeated several

times. Although a l l t he surfaces i n t h i s machine

have pro tec ted overcoat ing i t i s s t i 11 h i g h l y unde-

s i r a b l e t o 'scrub' t he surface. Under c e r t a i n types

o f greasy contamination the so lvent may be replaced

by chloroform (CHC1 3 ) o r even nethylene c h l o r i d e

(CH2C12) b u t these a r e more v o l a t i l e and p i c k up

contamination more e a s i l y r e q u i r i n g greater care

i n use.

7.2.3 I f a component cannot be cleaned adequately o r becomes

v i s i b l y scratched t h e damage must be considered i n t h e

l i g h t o f i t s f u n c t i o n i n the system. Some components

a r e r e l a t i v e l y t o l e r a n t of c e r t a i n types o f damage;

o thers must be replaced. I t i s usua l l y poss ib le t o

make t h i s dec is ion by care fu l observat ion and d e t a i l e d

thought. I f you are unsure seek pro fess iona l advice.

7.3 Cleaning Spec i f i c Components

Proceeding through the system i n t h e same way as the l i g h t

we observe a sequence o f components which have d i f f e r e n t p roper t i es

and requirements. (Refer t o Figure 1 -1.1-A)

Fiber Wecdles (12) Laser ,- Wollns ton Prism (3)

Focusing Lens Laser Front Cover

Turning Kirrors

Transceiver Lenses

Xain Frme Structurc Transceiver ~ i r r o r ' ~ )

Enlarging Objective

'Piold Stop (9)

PICURE 1.1.1-A MAIN COMPONENTS OF OPTICAL IIEAD

7.3-1 Laser Output Window

It i s u n l i k e l y t ha t t h i s component w i l l become d i r t y .

I t i s both f a r recessed and protected w i t h i n the laser

top cover enclosure. I f t h i s component causes bad

beam p r o f i l e or reduced in tens i t y , remove and clean

according t o the manufacturer's ins t ruct ions.

(See the Laser Handbook.)

7.3.2 Four Turning Mi r ro rs

Clean only when necessary using method o f 7.2.1 and

7.2.2. Do not remove any f i x i n g screws. Clean one

m i r r o r a t a time and rea l i gn before moving on t o the

next one. For realignment, use the method ou t l i ned

i n 4.1.13 and 4.1.14.

7.3.3 Wollaston Prism

This i s we l l protected and should no t requi re cleaning.

If i t does, remove the mounting block: t h i s i s i r revo-

cable and should not be considered casually. Remove the

a1 ignment stop, and s l i d e the prism out not ing i t s

approximate or ienta t ion. If you damage e i t he r surface

i n d i sassmbly o r cleaning, rep1 ace the component.

Realignment w i l l be necessary a f t e r the cleaning

operat i on.

I f the e n t i r e system i s being cleaned, replace the

Wollaston prism bu t not the a1 ignment stop a t t h i s

stage. (See Section 7.4 on system alignment.)

7.3.4 Focusing Lens

The most l i k e l y contamination o f t h i s lens w i l l be

on t h e lower sur face which i s e a s i l y cleaned by method

7.2.2 w i thout removal. If the upper sur face i s d i r t y ,

then i t must be cleaned a f t e r removal. I t i s poss ib le

w i t h s u f f i c i e n t care t o mark the depth - and o r i e n t a t i o n

o f t h i s lens and t o replace i t wi thout need f o r r e a l i g n -

ment b u t t h i s i s n o t espec ia l l y easy. With some care,

i t i s poss ib le t o make very small adjustment t o t h i s

lens both i n depth i n t o the mount and r o t a t i o n o f the

lens . The r o t a t i o n does n o t always have any s i g n i f i -

cant e f f e c t : i t depends on the a x i a l p e r f e c t i o n o f

t he s p e c i f i c focusing lens and they vary a l i t t l e from

u n i t t o u n i t .

7.3.5 Transceiver M i r r o r

This i s a small r i g h t angled pr ism whose hypotenuse

i s the on ly a c t i v e surface. It i s coated w i t h s i l v e r

and protected, b u t i s s t i l l f r a g i l e . We do n o t recommend

changing the adjustment o f t h i s component. I t may be

cleaned i n s i t u by the method o f 7.2.1 and 7.2.2. I n

the event o f damage i t must be replaced and readjusted

(See Sect ion 7.4).

7.3.6 Rotator Prism

This component may be cleaned i n s i t u on l y w i t h great

care and some d i f f i c u l t y . I f i t i s removed f o r clean-

i n g (7.2.1 and 7.2.2), see real ignment sequences (7.4).

I f the pr ism as a subuni t i s damaged o r i t s e l f mis- a1 igned, see Sect ion 7.5.

7.3.7 Inner Transceiver Lens

Th is lens may be removed f o r c leaning (7.2.1 and

7.2.2) as necessary, bu t nus t be replaced i n the

same pos i t i on , depth o f i n s e r t i o n i n t o the ho lder

and square t o the beam. It must, o f course, be r e -

f i t t e d the same way round! It i s wise t o check t h a t

t he output l i g h t w i t h the ou te r t ransce iver lens

removed i s p e r f e c t l y c o l l imated.

7.3.8 Outer Transceiver Lens

This lens has no c r i t i c a l pos i t i on ing . Removal i ncu rs

no problems except the need t o r e c a l i b r a t e the throw

and spot spacing. I t i s q u i t e poss ib le w i t h care t o

replace i t so exac t l y t h a t even t h i s i s n o t essent ia l .

7.3.9 F i e l d Stop

It i s u n l i k e l y t h a t the f i e l d s top w i l l become clogged

w i t h deb r i s unless the r e s t o f the instrument i s

v i r t u a l l y destroyed. Any c leaning should be performed

as i n Sect ion 7.2.1.

7.3.10 Cross Stop

It i s n o t necessary t o do anything t o main ta in t h i s

component unless i t has acquired b i t s o f f l u f f o r wool

( o r pieces o f f l e s h i f you have caught your hand on it,

i n which case, i t i s probably bent and w i l l need f l a t t e n -

i n g and repos i t i on ing ) . I n a complete c leaning and r e -

al ignment procedure i t may need s l i g h t adjustment.

7.3.11 Enlarging Object ive

This should never need c leaning o r touching i n any

way except under the complete realignment procedure

(Sect ion 7.4).

7.3.12 F iber Need1 es

These may be very c a r e f u l l y swabbed w i t h alcohol

(7.2.2) i f essent ia l .

No o ther items should requ i re c leaning t o improve the func t ion ing

o f the instrument nor t o res to re i t t o i t s former operat ing spec i f i ca t i on .

APPENDIX V

SECTION 7.4 OF "LASER TRANSIT ANEMO~IETER SYSTEM OPERATION ~ A L ' ~

SDL R e p o r t No . 79-6463

July 1979

7.4 A l ignmentof System

The op t i ca l system shoul d never need complete rea l i gnment

unless t e r r i b l e th ings have happened t o the instrument, i .e., ex-

treme heating (from r i g malfunction!) t o the po in t o f warpage o r

v i o l en t physical shock t o the po in t o f case d i s t o r t i on . I f compo-

nents are exchanged i n groups o r s ing ly i t may be f e l t tha t the

alignments w i l l improve the system, and t h i s sect ion includes se-

quence and de ta i l s o f such an a l ignnent. There are p a r t i a l a l i gn -

ments which may be performed i n the event o f d i f f e r e n t component

changes. I n most cases the realignment may be made by se lect ing

the appropriate sect ion from the fo l lowing sequence. This presup-

poses t h a t only one component i s out o f i t s ' co r rec t ' posi t ion.

NOTE: The r o t a t o r prism i s preal igned a t the fac tory and - it i s extremely un l i ke l y t ha t t h i s i s the cause o f any spot o r b i t

o r other output problems (see Section 7.5). It i s easy to t e l l i f

the prism i s a t f a u l t as the spot o r b i t becomes card io ida l . Under

a l l condi t ions o f c i r c u l a r o r b i t s i t i s not the in te rna l a1 ignment

i n t e g r i t y o f the r o ta to r prism which can be a t f a u l t .

An ou t l i ne o f the major alignment sequence and tasks i s

given as Table 7.4 which fu r ther re fe rs t o the sections where i n -

d iv idua l de ta i l s and ins t ruc t ions are t o be found.

PROJECTION SYSTEM ALIGNMENT

7.4.1 Reduce the laser output power t o CLASS 1 operation

by con t ro l l i ng the tube current. The ' l i g h t cont ro l ' may be the more stable and reduce the chance o f the

laser going out. Do not use an at tenuator as the

thickness o f mater ia l and re f l ec t i ons from i t s sur-

face may impair the a1 ignment. An at tenuator may

TABLE 7.4. ALIGNMENT SEQUENCE AND TASKS

Section(s)

PROJECTION SYSTEM ALIGNMENT

1. Laser In te rna l A1 ignment

2. Beam Posi t ion and Poin t ing a t Box Entrance

3. Beam Posi t ion and Point ing t o Transceiver M i r r o r

4. Beam Posi t ion and Point ing through Image Rotator Prism t o Final Output

5. Centration o f Transceiver M i r r o r

6. Set Up Screen

7. Focusing Lens and Wollaston Prism Mounting

8. Inner Transcei ver Lens Col 1 imat ion

9. Outer Transceiver Lens Placement

10. Orb i t Minimisation

11. Wollaston Rotation

12. Beam Para1 l e l ism

13. Final Orb i t Alignment a t Outer Transceiver Lens and Sampl i ng Volume

14. Place Alignment Stop and C r i t e r i a o f A1 ignment

RECEIVER SYSTEM ALIGNMENT

15. Location o f Sampling Volume

16. Mounting o f Fiber Optic Needles

17. Posi t ion o f Enlarging Objective

18. Focus o f Enlarging Objective

19. Fiber Optics Alignment

20. Cross Stop Alignment

21. F i e l d Stop Pos i t ion and Alignment

22. Review Checks

be used i n the parallel section of the input laser

beam i f i t i s aligned with sufficient care to avoid

any change in beam steering or mu1 t ip le wal k-off

effects. I t i s necessary to allow several minutes warmup time.

7.4.2 Remove the top and both side covers, observing the

clean1 iness precautions out1 ined in Section 7.2.

Str ip the spine plate of components 4 through 10

except 6, the image rotator prim, which may be l e f t in s i tu unless i t requires attention. Identify

components on Figure 1 . l . 1 . A . Cl ean , repair, or renew as necessary.

7.4.3 Make sure that the laser i s bolted down t ightly, has

been on for some time, and i s in a s table operating condition. By use of the "Vertical and Wavelength" knob (top control) peak the laser power. I t i s sel-

dom necessary to touch the "Horizontal" knob (lower

control). The laser should be tuned to the 514.5 nm l ine unless there i s some special reason why,for the contemplated experiment, i t should be otherwise. The

system, once aligned, should not be sensitive to

change of wavelength. I f these controls seem espec-

ia l ly free, you may tighten the clamps onto the ad-

justment screws, an action performed with the laser cover removed (see laser handbook).

This power maximisation, most easily performed by observing either of the pokter monitors supplied with

the laser , in the current control mode, also guaran- tees that the output will be TEMoOq o f clean Gaussian

profile about 0.9 mm diameter.

C~

f~

~0

O

~D

FIGURE 7.4.5 FRONT OF LASER ADJUSTMENTS

• t ~ a G 1

7.4 .4 Tape a microscope cover s l i p t o the top p l a t e across

the ho le where the l ase r beam i s t o en te r t he box.

Use e a s i l y removable masking tape and l i g h t l y smear

the cover s l i p w i t h a f i nge r . Make sure the s l i p i s

taped down a l l t he way round and i s thus as f l u s h

as poss ib le w i t h t h e p l a t e which supports t h e l ase r .

Confirm t h a t t he ex terna l m i r r o r mount post and the

two m i r r o r s a r e t i g h t l y clamped.

7.4.5 Using t h e f o u r screw adjustments on the two ex terna l

m i r r o r s (screws A t o D i n F igure 7.4.5), arrange t h a t

t h e i n p u t beam s h a l l be i n the middle o f t he e n t r y

ho le and perpendicular t o the surface. This i s guar-

anteed when the mu1 t i p l e r e f l e c t i o n s between the cover

s l i p and the l a s e r output m i r r o r a re co inc ident . Th is

i s a four-dimensional search, b u t can be rendered as

two two-dimensional searches by us ing A and D as a

p a i r and B and C as a pa i r . Do no t touch these m i r r o r s

again, y e t . I t i s a good idea t o replace the top

cover as a precaut ion aga ins t acc identa l contac t w i t h

the adjustment screws.

7.4.6 Unless the m i r r o r s 3 and 4 have been removed, the

mounting pos i t i ons w i l l be co r rec t . I f they have

no t been removed, ignore the mounting i n s t r u c t i o n s

which fo l low, merely per forn ing the adjustments of

screws E through H. I f they have been removed, pro-

ceed.

Arrange m i r r o r 3 t o be square and p a r a l l e l t o i t s

ad jus tab le mount and b o l t i t i n p o s i t i o n such t h a t

the po in t o f contact o f the l i g h t i s near the center

o f the m i r r o r and 1.2 inches below the underside o f

the top p la te . It w i l l be 1.5 inches from the spine

p la te (which car r ies a l l components i n t h i s p ro jec t ion system). B o l t the m i r ro r and support r i g i d l y t o the

spine plate.

7.4.7 Adjust t h i s m i r ro r t o make the beam pa ra l l e l t o the

spine and top plates a t the above distances. Use

screws E and F on Figure 7.4.7.

7.4.8 Bo l t m i r r o r 4 i n pos i t i on so t h a t the beam may be

made p a r a l l e l t o the spine p l a te (1.5 inches away)

and the b a f f l e p l a te (2.70 inches away) and may pass

d i r e c t l y over the center o f the ho le which supports/

w i l l support the t ransceiver mi r ror . Tighten the

m i r ro r support.

7.4.9 Using screws G and H i n Figure 7.4.7, arrange t h a t

the beam s a t i s f i e s the dimensional spec i f i ca t ions o f

7.4.8.

7.4.10 F i x the t ransceiver m i r r o r t o i t s stub. It can be

t ightened enough t o prevent casual movement. Arrange

t h a t i t s mount stub sha l l be centered i n i t s oversize

mounting hole. Do not forget the washer. If the

cross stop i s not already removed, remove i t now.

Put i t somewhere safe.

7.4.11 Make translucent targets which look l i k e Figure 7.4.11

o r s im i la r . Center one on the inpu t face o f the image

r o t a t o r prism and one on the output face o f the op t i ca l

head. (The one on the prism must be cu t down t o a

su i tab le size. )

b - -

t ~

FIGURE 7.4.7

- % -

0

" ' 3

INTERNAL ALIGNMENTS I

i~ ~ •

0

m O 0

-n

0

O0

FIGURE 7.4.11 TARGET

7.4.12 I f f o r any reason the image r o t a t o r was removed as

under 7.4.2 f o r c leaning o r other a t ten t ion , i t may

now be replaced; otherwise, continue w i t h 7.4.1 3.

Mount the r o t a t o r prism assembly and f i x w i t h three

screws only.

7.4.13 By jud ic ious manipulat ion o f the t ransce iver m i r r o r

assembly w i t h the screws I through K s u f f i c i e n t l y

s lack t o a l low movement w i t h st i g h t s t i c t i o n ,

arrange t h a t the beam s h a l l pass through the exact

center o f both ' t a rge ts ' .

7.4.14 By f u r t h e r manipulat ion o f L, K, and J, make the

inc iden t beam f a l l i n the cent re o f the t ransce iver

m i r ro r , both i n he ight from the spine p l a t e and

p o s i t i o n using the nylon faced jacking screw and

the p lay i n the mounting hole.

Do no t derange the alignment which was co r rec t

under 7.4.13, o r i f you do, make sure i t i s obtained

again before you move on from t h i s sect ion.

The l a s e r beam w i l l now p o i n t ou t o f the f r o n t o f

the instrument. There w i l l be some sca t te r from the

t rans lucent ta rgets which are s t i l l i n place, bu t

ignore t h i s . Arrange f o r the beam t o s t r i k e a surface

between 3 m and 5 m away. Closer than 3 m gives too

1 i t t l e magni f icat ion, and more than 5 m becomes a

tedious walk. This surface may be u s e f u l l y covered

w i t h squared paper w i t h the l i n e s hor izonta l and

v e r t i c a l - use a s p i r i t l e v e l t o l e v e l both the

o p t i c a l head and the screen. This screen should be

approximately normal t o the beam t o minimize l a t e r

d i f f i c u l t i e s (See Section 4.5).

I f the image ro ta to r prism i s ro ta ted through f u l l

t rave l , the spot should not o r b i t by more than a

few mm's . Tape a smaller p l a i n sheet o f paper t o

the squared screen using removable masking tape.

You w i l l use many such pieces o f paper and wish

t o discard o l d ca l i b ra t i on points t o avoid confusion.

Mark the centre o f the beam o r b i t .

7.4.16 Assesble the Wollaston prism and focusing lens

support block. For outer t ransceiver lens throws o f

400 nim t o 700 r m and where the machine i s t o be

used w i t h turbulence leve ls i n excess o f 0.5% t o

I%, mount the f r o n t face o f the focusing lens leve l

w i t h the end face o f the mounting block. This ax ia l

pos i t i on i s a f i n e tune adjustment on the para1 l e l i sm

o f the two spots near the foca l region and may be

important f o r low turbulence leve ls . I t a lso d i f f e r s

s l i g h t l y f o r d i f f e r e n t foca l lengths o f output trans-

ce iver lens. It may be changed w i t h only minor

readjustment o f the f i n a l system should i t be

necessary t o work w i t h very low turbulence leve ls

and hence very good pa ra l l e l ism o f the spots over

the length o f the sens i t ive volume.

Mount the Wollaston prism w i t h the black paper d isc

between i t and the focusing lens. S l ide i t as f a r

towards the lens as i t w i l l go. Arrange i t t o be

a t approximately 45" t o the sides o f the block.

There are fou r possible posi t ions. Only two w i l l

be sa t i s fac to ry t o al low the f i n a l spot o r ien ta t ion

t o correspond t o tha t o f the f i b e r op t i cs . There

w i l l be a l a t e r oppor tun i ty t o co r rec t t h i s i f you

get i t inco r rec t a t t h i s stage. It does not mat ter

which way the l i g h t goes through t h i s prism and

hence i t has no pre fer red ' r i g h t way up'.

Do no t mount the aperture p la te .

7.4.17 Arrange the p o s i t i o n of the Wollaston mount b lock

t o be p a r a l l e l t o the sides of the box and t o a l l ow

l i g h t t o pass square and c e n t r a l l y through the focus-

i n g lens t o centre the f i n a l output image. If the

incidence i s no t square and cent ra l , there w i l l be

res idua l astigmatism which w i l l impai r the f i n a l

performance. Tighten the support b o l t s f o r t h e

mounting block and make sure t h a t the focusing lens

lock screw i s t i g h t . A demonstration o f co r rec t

placement o f t h i s component i s the recen t ra t i on of

the spot on t h e t ransce iver m i r r o r and the uniform

spread of the b r i g h t patch o f l i g h t on the screen

about the prev ious ly marked po in t .

7.4.18 Mount the inboard t ransce iver lens i n the inboard

lens mount the co r rec t way round (see Figure 7.4.18) leav ing the yoke clatrping screws a l i t t l e loose.

Place an opaque mask w i t h a 1 inch ho le i n i t sym-

m e t r i c a l l y over the output beam. Measure the beam

diameter a t the screen. Adjust the p o s i t i o n o f the

t ransce iver lens u n t i l the diameter o f t h i s c i r c l e

( t o the appropr iate p a r t of i t s d i f f r a c t i o n r i ngs ,

see standard t e x t s on physical o p t i c s ) i s about 1

inch.

FIGURE 7.4.18 TROPEL LENS SENSE

Make sure there i s NO INTERMEDIATE FOCUS, by running

a card along the length of the beam. Tighten t h e

lens mount yoke.

7.4.19 Mount the ou te r t ransce iver lens chosen f o r t he

present app l i ca t i on . I t s a x i a l p o s i t i o n i s n o t

s i g n i f i c a n t and may be used t o ad jus t t he exact

throw over a small range. Normally a t t h i s stage

i t should n o t be pushed hard aga ins t t he i nne r t rans-

ce i ve r lens t o a l l o w movement under t h e repeat of

7.4.18 which may be necessary a f t e r 7.4.22. Af te r

t h i s f i n e tune operat ion, i t may be good t o push

the ou te r t ransce iver l ens hard aga ins t t he i nne r t o

ensure squareness and maximize r i g i d i t y . It i s nec-

essary t o t i g h t e n the yoke t o prevent casual move-

ment even though i t may be moved l a t e r .

7.4.20 Remove both t rans1 ucent ta rge ts so t h a t s c a t t e r i n g

through them w i l l n o t impa i r t he beam q u a l i t y a t t he

spots. Mount a x20 microscope o b j e c t i v e w i t h a

l a r g e entrance face so as t o p r o j e c t t he reg ion of

best focus o f t he spots on t o t h e screen. Tape a clean sheet of blank paper t o the screen so t h a t t h e

spot o r b i t stays w i t h i n i t s bounds.

On r o t a t i o n o f t h e r o t a t o r prism, the spots w i l l

o r b i t t h e common centre of g r a v i t y o f t he two spots.

There w i l l be a dim spot of l i g h t a t t h i s cen t ra l

po in t . This cen t ra l p o i n t w i l l most probably t r a c e

a c i r c l e tw ice f o r each r o t a t i o n o f t he prism.

C i r c u l a r o r b i t t i n g i s caused e i t h e r by the entrance

d i r e c t i o n o r the entrance p o s i t i o n a t t he image ro- t a t o r pr ism being s l i g h t l y away from the mechanical

r o t a t i o n ax i s o f t he prism assembly.

Using on ly tu rn ing m i r r o r 4, screws G and H, and the

t ransce iver m i r r o r angular adjustmsnt screws I and J

slackened, walk the beam about the entrance face o f

t he pr ism t o minimize the s i z e o f the o r b i t made

by the spot on r o t a t i o n o f the pr ism assembly. It

i s necessary t o g e t both the entrance p o s i t i o n and

angle exac t l y co r rec t . Necessary adjustments should

be very small and should n o t take t h e beam percep t ib l y

from the cent re o f t he focusing lens. I f i t does, i t

w i l l be necessary t o move the focusing lens mount

b lock t o re-cent re the u n i t and repeat from t h a t p o i n t

i n the al ignment (7.4.17). A t t h i s time, i t i s n o t

necessary t o arrange f o r the c i r c u l a r cent re o r b i t

t o be much l e s s than the spot separation. Fur ther

p rec i s ion w i l l be requ i red i n sec t ion 7.4.23 i n due

course.

Some h i n t s t o ease t h i s tedious adjustment a r e t o

use a r e s t i n g p o s i t i o n o f t he r o t a t o r pr ism such

t h a t i t s plane o f r e f l e c t i o n symmetry i n c l u d i n g i t s

r o t a t i o n a x i s i s p a r a l l e l t o the s ide o r t op o f t he

o p t i c a l head box, and t o use t h e screws G and J

together and a l t e r n a t e l y w i t h H and I together.

I f the beam now s t r i k e s v i s i b l y away from the cent re

o f the t ransce iver m i r ro r , repeat 7.4.14.

7.4.21 Slacken the Wollaston pr ism r e t a i n e r screw (N) and

r o t a t e the pr ism u n t i l the two spots on the screen

a r e o f equal br ightness. There a re four angular

p o s i t i o n s where t h i s w i l l be t rue . Only two o f them

a r e acceptable, those where the plane of t he spots

i s u l t i m a t e l y imaged on the plane o f the f i b e r o p t i c

needles. Choose one o f these. I t i s convenient,

and s u f f i c i e n t l y accurate, t o use a l a s e r power

meter f o r t h i s func t ion .

7.4.22 Fol lowing the i n i t i a l remarks on beam para1 l e l ism

i n Sect ion 7.4.16 i s inc luded a more prec ise d i s -

cussion. Th is sec t ion i s o n l y re levan t f o r ve ry

low turbulence s i t ua t i ons .

I f you need very h igh p a r a l l e l i s m -- say t o b e t t e r

than 0.1% -- i t w i l l be necessary t o proceed as

fo l lows. For v i s u a l i z a t i o n , arrange t h a t t h e sampling

volume i s suf fused w i t h a l i g h t mist . Suggestions

are d r y i c e i n warm water, c i g a r e t t e smoke, incense,

o r a foggy day. Observe the beams t o look l i k e the

middle diagram (0) i n Figure 7.4.22. Th is w i t 1 a l -

most c e r t a i n l y be t r u e f o r t he e a r l i e r procedure w i l l

have reduced any divergence t o a (probably s i g n i f i c a n t )

f r a c t i o n o f 1%. To improve the system f u r t h e r , i t i s

necessary t o measure the divergence w i t h h igh p rec i s -

i o n and t h i s i s best done by using a x20 microscope

o b j e c t i v e as i l l u s t r a t e d e l sewhere and rack ing i t

along t h e beams w i t h a c a l i b r a t e d l i n e a r stage. I t i s n o t very useful t o move the o b j e c t i v e more than

about 2 mm (total ) because it w i l l not be easy t o

p lace t h e cent re o f t he spot w i t h s u f f i c i e n t accur-

acy outs ide t h i s range. By measuring t h e spot sep-

a r a t i o n as pro jec ted on the screen, i t i s poss ib le

t o ad jus t t h e a x i a l p o s i t i o n o f the focusing l ens

u n t i l i t i s reduced t o zero. Ac tua l ly , about

O.l%/mm i s the bes t i t i s reasonable t o achieve.

A t t h i s value, there would be no s i g n i f i c a n t e r r o r

FIGURE 7 . 4 . 2 2 BEAM PARALLELISM

from t h i s source above .05% turbulence. It i s nec-

essary t o repeat t h i s procedure f o r each output

t ransceiver 1 ens.

7.4.23 Check t ha t the spots o r b i t no t too f a r from t h e i r

centre o f grav i ty . Repeat the work o f 7.4.20.

This time, the adjustments w i l l be very small bu t

the c r i t e r i a t o be met are t i g h t e r . This i s a f i n a l

output system opt imi sation.

r * NOTE ALL CLASS I I I B LASER SAFETY REQUIREMENTS AND

PRECAUTIONS !

Turn the 1 aser t o maximum power under ' 1 i g h t con t ro l I .

Wait 20 minutes for thermal s t a b i l i sa t ion . Use the

ins t ruc t ions i n 7.4.20 t o reduce the maynif ied spot

centre o r b i t t o a small f r a c t i o n o f the spot separ-

at ion. The e r ro rs remaining accrue from three sources:

a) C i rcu la r o r b i t s der ive from er ro rs o f d i r e c t i o n

o r pos i t i on o r a combination o f both a t the en t r y

face t o the image ro ta to r prism. Theoret ical ly, t h i s

may be el iminated t o give a pure cardioid.

b) Cardioidal o r b i t s der ive from in te rna l prism

alignment which may be r e c t i f i e d i f s u f f i c i e n t l y

large by the sequence, o r i n t e l l i g e n t l y selected

par ts thereof, i n Section 7.5. The ea r l y p a r t o f t h i s

sequence may be omitted since the residual er rors are

very small and on ly minute changes o f pos i t i on of

screws 7 through 13 (Figure 7.5) should be attempted.

c ) Gl i tches der ive from play, espec ia l ly end f l o a t , i n the prism bearings. These w i l l be very small, bu t

may never be reducible t o zero. Errors o f up t o

0.1 m i l 1 i rad ian may occur i n extremely unfortunate

combi nations o f circumstances, but 10 microradians

i s more l i k e l y -- corresponding very roughly t o 0.1

o f the spot separation.

Simultaneous w i t h the o r b i t c r i t e r i o n a t the screen,

i t i s necessary t h a t the i l l um ina t i on o f the outer

t ransceiver 1 ens sha l l be cent ra l and syrrmetric.

This i s not a mutually exclusive c r i t e r i on , bu t MUST be simultaneously achieved. Again, t h i s i s cor rect

when the input d i r ec t i on and pos i t i on t o the r o t a t o r

prism i s correct . There are necessary and s u f f i c i e n t

adjustments f o r these sirnul taneous c r i t e r i a . (Screws

G, H, I, and J -- i f a l l these, espec ia l ly G and K,

are used a t t h i s stage i n the game, i t may be nec-

essary t o repeat par ts o f the e a r l i e r sequence.)

7.4.24 Remove the x20 object ive and note t h a t the pat tern

on the screen i s a roughly Gaussian patch w i t h square

terminator and some d i f f r a c t i o n patterns, especial ly

near the edge.

This w i l l be qu i t e symmetric if 7.4.14 was performed

w i t h care. Arrange tha t the alignment stop plate,

f i xed w i t h the screws and washers 0 and P, j u s t cuts

o f f the corners o f t h i s square w i t h per fec t symnetry.

This w i 1 I be a subsequent datum making realignment i n

the f i e l d very simple if i t s cause does no t der ive

from physical damage t o the instrument.

Be careful not t o touch any op t i ca l surfaces a t any

t ime you have your f ingers ins ide the instrument t o

perform adjustments.

7.4.25 Turn the lase r back t o ' cu r ren t con t ro l ' and reduce

the power t o Class I operation.

7.4.26 You have now completed the adjustment o f the output

system fo r p ro jec t ion of the two spots i n t o the t e s t

space. Now t h e alignment stop i s c o r r e c t l y placed,

pe r fec t alignment may be restored by use o f m i r r o r 1

on ly under the fo l lowing causes of misalignment:

a) l a s e r aging, \ b) thermal changes, 1 up t o c e r t a i n

1 i m i t s c ) box d i s t o r t i o n through

m i shandl i n g , d ) change o f wavelength ( d i f f e r e n t l a s e r l i n e ) .

I n general, t h i s m i r r o r 1 cor rec t ion w i l l be a com-

p l e t e co r rec t ion inc lud ing the rece iv ing system a1 i g n -

ment, which i s f i x e d conjugate w i t h the pro jec ted

system and w i l l no t change w i t h the above proper t ies .

Outside the l i m i t s o f m i r r o r 1 cor rec t ion using t h e

alignment stop (and i f necessary the normal cen t ra l

i npu t c r i t e r i a o f 7.4.5 simultaneously) , the quest ion

must be asked, " I s the misal ignment f a r enough away

from what I requ i re t o m e r i t realignment of i t s system?"

The c r i t e r i a o f p r o j e c t i o n system a1 ignment a re sum-

marized i n Table 7.4.26.

RECEIVER SYSTEM ALIGNMENT

7.4.27 Obtain a f i n e ground s tee l surface (a machine r u l e r i s i dea l ) . S l i d e the surface along the beams through

the crossing po in t u n t i l the fa r f i e l d speckle pa t te rn

TABLE 7.4.26. FINAL ALIGNMENT CRITERIA FOR PROJECTION SYSTEM

Possible Achievable w i t h This Reasonable Standard

Property Des i gn for Extended Use Remarks

Spot O rb i t a ) Prism 0 1011 rad. Cardioidal b) Input 0 1011 rad. Two c i r c l e s c ) G l i t c h 0 -100~ rad. Fixed Depends on i ns t r u -

ment

Output Centration 0 1-2 mm

Spot Qua1 i t y D i f f rac t ion Limited 10% El i p t i c i t y Focusing lens not from Astigmatism square t o beam

Laser Tube Maximi se power by m i r r o r a1 ignment. Mode s t ructure i s then OK ( T E M ~ ~ ~ ) .

Spot I n t ens i t y Equal Within 1% o r Bet ter

Spot Para l le l ism Para1 1 e l Low Turbulence Depends on the 0.5 m rad. needs o f the

user and the High Turbulence present appl i - 1 t o 5 m rad. ca t ion

has s t ruc tu re o f the la rges t size. A t t h i s po in t ,

the speckle p z t t e r n diwension w i l l be o f order 4" t o

7" of arc depending on the surface o f the sca t te re r

and w i l l contain c lose ly spaced e r r a t i c f r i nges .

You can observe these on the f r o n t o f the instrument

o r on any surface ava i l ab le i n the room. Since the

l a s e r i s running a t low power, i t may be usefu l t o

darken the room.

7.4.28 With none of the cross and f i e l d stops and the en-

l a r g i n g lens mounted, b o l t the c a r r i e r o f the f i b e r

needles t o the spine p la te . Arrange w i t h the screws

X through Z (Figure 7.4.28) t h a t the ends o f the

f ibers s h a l l be c lose together and near the cent re

o f the ho le i n the mounting block.

7.4.29 S l i d e the enlarging lens ( f o r most systems, a x7 microscope ob jec t i ve on a 10 m mounting tube) i n t o

i t s rece iv ing ho le and l i g h t l y n i p i t i n p o s i t i o n

w i t h screw Q.

m

L~ Oo

FIGURE 7.4.28 INTERNAL ALIGNMENTS II

i~il ii ii~ii ~i i ~I~I i~i~ ii~ I i ~

i iii~iii!Lil ¸ .... i!i~i ~i~ ii ~ i~

m

O

O

7.4.30 Slacken the four screws (bJ) on the back o f the

b a f f l e p la te and centre the enlarging ob jec t i ve

t o place the re tu rn spot images as near the needle

ends as possible; ce r t a i n l y w i t h i n 0.5 mm (.020

inches). The image may not be i n focus -- do

not worry.

Tighten the four screws ( W ) .

7.4.31 Loosen the set screw (Q) holding the shank o f the en-

la rg ing lens holder. S l ide the ob ject ive very

gent ly t o obta in the best focus on the f i b e r needle

ends o r on a piece o f t h i n whi te card f i xed i n the

plane o f the f i b e r needle ends. The card should

not be more than a few thousandths of an inch away

from the plane o f the f i b e r needle ends. Use a low

power microscope t o observe the imag~s . x20 t o x30

i s idea l . The spots w i l l change as you search

through the planes f o r the 'best focus'. Choose a

po in t where the 'd i sc o f l eas t confusion' i s as

small as i t may be, ce r t a i n l y no la rger than the

s ize o f the f i b e r which w i l l be c l e a r l y v i s i b l e i n

the microscope and should be clean and f ree from v i s -

i b l e damage. The lenses concerned here are very

we1 1 corrected and the best pos i t i on w i 11 be very

sharply defined. Even so, there w i l l m s t l i k e l y

be some residual spherical aberrat ion. There may

a lso appear traces o f astigmatism and a f a i r amount

o f d i f f r a c t i o n . This d i f f r a c t i o n w i l l look even

more ccmplex a f t e r the cross stop i s f i t t e d , so do

not worry about it. There w i 11 always be a c l ea r

po in t o f b r igh tes t centre. You may f ind t h a t the

adjustment o f t h i s lens i s f a c i l i t a t e d by using

a tube w i t h a female microscope ob ject ive thread

on one end. These are standard and read i l y ava i l -

able.

Tighten the se t screw. It has a nylon face, so

w i l l no t feel very t i g h t . Do not overt ighten o r

the nylon w i l l cold f low and make the adjustment

very d i f f i c u l t f o r the next occasion.

7.4.32 Slacken the screws X and use what l i t t l e r o ta t i ona l

p lay there i s t o l i n e up the ends of the f ibers w i t h

the mean l i n e connecting the two spots. This i s t o

a l low f o r the Wollaston prism no t being a t exact ly

45" because the equa l i t y c r i t e r i o n i s dominant. The

f i b e r faces must l i e somewhere along t h i s datum l i n e .

See the sketch 7.4.32. Tighten both the screws X.

7.4.33 Slacken two set screws Y. These have nylon faces

and w i l l again co ld f low i f overtightened. S l ide

the upper needle t o make i t coincide w i t h i t s spot.

Do the equivalent w i t h screws Z. Whi ls t these are

no t f u l l y t i g h t , i t i s possible t o lever the block

which holds the needle away from the plane face o f

the mounting block. This enables a very small amount

o f side play i n the d i r ec t i on +x on Figure 7.4.32.

The f i n a l alignment i s best performed by observing

the back o f the f i b e r ( i n the mount which couples i t

t o the photomul t ip l ie r tube housing) w i t h a power

meter and performing small adjustments o f pos i t i on as

above w i t h the screws not qu i t e t i g h t . F ina l l y ,

t i gh ten screws Y and Z.

Y e s Y e s

0 N e e d l e s

0 B e s t Image

F I G U R E 7 .4 .32 RETURN IMAGE ON N E E D L E S

7.4.34 Look a t the PMT ends of both f i be r s a t the same

time. I t i s bene f i c ia l t o tape some f a i r l y t h i n

white paper t o these mount plates because the emission

from the f i b e r i s i n t o a t i g h t angle and i t i s awkward

t o l i n e one's eye up w i t h both of these simultaneously

Note now tha t i f the scat ter p l a te i s s l i d along the

op t i ca l ax is , both beams ext inguish together and

over a few mm movement of the plate. I f you a re

very observant, you w i l l observe t ha t a t one po in t

i n the excursion t o ext inc t ion, you w i l l detect a

minute brightening. Do no t worry about it. It i s

cured by l a t e r operations.

The beams should appear qu i t e s im i l a r -- a b r i g h t

centre region surrounded by a d i s t i n c t but dimmer

halo. If any other appearance i s present, the

f i be r s may be d i r t y or damaged. I f the appearance

i s corrected by cleaning, t h i s w i l l su f f i ce . I f

not, the f i be r s should be replaced. The damage

w i l l mainly lead t o loss o f op t i ca l e f f i c i ency or unbalance between the two channels, but can a lso

lead t o loss o f precise r e s t r i c t i o n o f the depth

o f the sens i t ive volume.

7.4.35 Retrieve the cross stop from the 'safe place' o f

7.4.10 and mount i t w i t h a washer behind the trans-

ce iver mi r ror . I t s o r ien ta t ion should be such t ha t

the 'semaphore' arm which i s pointed towards the

base defined by the mounting ho le i s a t the s tar -

board s ide o f the system.

Arrange a plane f ron t surface m i r r o r between the en- la rg ing lens and the f i b e r op t i c needles -- be very

careful not t o touch the need1 es -- t o r e f 1 ec t the

image o f the spots on t o the back of a sheet o f

t ranslucent paper f i xed t o the starboard s ide of the

box where the s ide cover normally f i t s . Note t ha t

as the scat ter p la te i n the sample volume i s moved

along the op t i ca l axis t h a t the cross stop in te r rup ts

a diametral sector of the now defocused image. Use the small amount o f r o t a t i on ava i lab le i n the cross

stop t o prevent the c i r c u l a r inage o f one spot

passing over the place corresponding t o where the

other spot has i t s focus. See Figure 7.4.35.

The rea l image w i l l never be as clean as the idea l ized

diagram. The pat tern w i l l be covered w i t h speckles

and i r r e g u l a r i t i e s which der ive from the minute i m -

perfect ions and s t ructure o f t he scat ter screen.

Nevertheless, there must be a blank ho1 e i n t he an-

nu lar image where the focal spot centres (dotted)

were when they were a t best focus. The dark por t ion

(not qu i t e a sector) which i s no t al igned w i t h the

spots w i l l i n ve r t when the sca t te r p l a te i s moved

from the sens i t ive volume i n the opposite d i r ec t i on

from i t s former movement.

Tighten the cross stop screw.

7.4.36 F i t the f i e l d stop by means o f screws S through V.

Slacken the nylon faced set screw R.

Observe the i l luminated face of the f i e l d stop p l a te

w i t h the low power microscope which you used formerly.

FIGURE 7.4.35 CENTER AND CROSS STOP IHAGES

Screw the f i e l d stop p l a t e u n t i l the spots are i n

best focus -- a repeat o f the c r i t e r i o n o f 7.4.31,

but now the spots w i l l be smaller. However, the screw

adjustment w i l l make i t q u i t e easy. Measure and record

the gap through which you can see threads and note the

angular pos i t i on o f the p l a t e which re ta ins the screws

f o r the f i e l d stop adjustment. Rotate t h i s p l a t e

through 180" and replace the two set-screws w i t h

spr ing plungers (nylon faced set-screws w i t h spr ing

shanks). Two o f these are provided i n the accessory

k i t . Screw these up loosely when the f i e l d stop p l a t e

i s centered. Rotate the p l a t e 180" t o g i ve access t o

the pointed set-screws. Observing through the micro-

scope, r o t a t e the f i e l d stop u n t i l i t s holes l i n e up

i n d i r e c t i o n on ly w i t h the b r i g h t images. Now, using

the pointed set-screws and observing the ends o f the

f i b e r s f o r maximum brightness and the stop p l a t e f o r

darkness because the b r i g h t images now go through the

holes, move the stop p l a t e i n i t s own plane. When

t h i s c r i t e r i o n i s achieved, r o t a t e the p l a t e 180°,

replace the spr ing plungers w i t h r i g i d set-screws,

and t u r n the p l a t e back through 180". Tighten screw R and note t h a t the above condi t ions are met ( t h a t the

images o f the two holes i n the stop p l a t e are symmetric

about the b r i g h t points which i l l u m i n a t e the f i b e r

ends). Check a l l screws f o r t ightness whi le observing

t h a t the alignment i s per fec t .

7.4.37 Check t h a t s l i d i n g the scat ter p l a t e along the o p t i c a l

ax is o f the spots now produces much sharper e x t i n c t i o n

w i t h no t race o f re tu rn br ightening as the sca t te r

p l a t e moves away from the volume o f maximum s e n s i t i v i t y .

I f you measure the change i n r e t u r n l i g h t from an

approximately La~cbert ian sca t te re r as i t i s

racked through the sampling volume, i t w i l l have

the approximate proper t ies o f Table 7.4.37 f o r a

400 mm h igh r e s o l u t i o n (Tropel) ou ter t ransce iver

1 ens.

Other lenses w i l l have d i f f e r e n t c r i t e r i a and i t i s

up t o the user t o decide whether the p rec i s ion o f

adjustment he has achieved i s adequate f o r h i s ?resent

purposes.

There i s an even more accurate method t o cent re these

f i e l d stops and a l so assure t h a t t h e i r plane o f focus

i s co r rec t . Using the f r o n t sur face m i r r o r o f Sect ion

7.4.35 and the x20 microscope ob jec t ive , reimage the

f i e l d stop a meter o r two from the instrument. You

w i l l observe two c i r c l e s corresponding t o the f i e l d

s top holes. When these a re i n sharp focus, t h e c e n t r a l

p o r t i o n o f each ho le should e x h i b i t the d i s c o f l e a s t

confusion o f t he r e t u r n images. The f i e l d stop ad jus t -

ment may now be rendered near perfect by minute movement

o f screws S through V.

7.4.38 Review a l l t he checks i n t h i s sec t ion and screw the

f i b e r o p t i c guides i n t o t h e i r respect ive photomul t i -

p l i e r s . Be c a r e f u l t o get t he sense cor rec t . The

top f i b e r guide goes t o the top PMT and i s c a l l e d

Channel A. I f desired, fit the narrow band l a s e r

t ransmission f i l t e r s f o r t he suppression o f broad

band l i g h t , e.g., from sun l i gh t , self- luminous

phenomena, f luorescent media, e tc .

TABLE 7.4.37

FINAL ALIGNMENT CRITERIA FOR RECEIVING SYSTEM

Defined here w i t h 400 mm f / 4 TROPEL l e n s .

Property S p e c i f i c a t i o n

Ax ia l Reject ion F u l l Width t o I n t e n s i t y

Below Peak

F iber Opt ic Output Cone % 10" H a l f Angle

APPENDIX VI

THICK WINDOW ABERRATION EFFECTS ON A LASER TRANSIT ANEMOMETER

INTRODUCTION

This appendix p re sen t s t he r e s u l t s of a "quick look" s tudy of

t h e ~ e g r e e t o which a t h i c k o p t i c a l l y f l a t window w i l l degrade the

performance o f an otherwise d i f f r ac t ion - l imi t ed laser t r a n s i t anemom-

eter (LTA) system. The s tudy is r e s t r i c t e d t o a concent r ic coannular

t ransmi t te r - rece iver system wi th the t ransmi t ted beams assumed t o

occupy the c e n t r a l po r t ion of t h e t r a n s m i t t e r lens . The l e n s is

assumed t o b e i d e a l and geometrical ray o p t i c s and S n e l l ' s law a r e

adequate t o j u s t i f y the po in t s we in t end t o make. The t ransmi t ted

beam is assumed t o focus t o a p o i n t i n t h e absence of t h e window; the

r e t u r n s c a t t e r e d l i g h t is assumed t o o r i g i n a t e a t a p o i n t s c a t t e r e r

l oca t ed at t h e p a r a x i a l focus of t he t ransmi t ted beam. The e f f e c t s

of d i f f r a c t i o n a re considered separa te ly . Only one beam is considered

a s though i t were a long the l e n s a x i s f o r s i m p l i c i t y , although i n a

normal LTA system t h e r e are two beams symmetrically i n c l i n e d at a

small angle t o t he ax i s . This angle is s u f f i c i e n t l y smal l t o produce

no e f f e c t s which could b e s i g n i f i c a n t here.

A s a f i g u r e of mer i t we c a l c u l a t e a d i a z e t e r of t h e t r ansmi t t ed

and received s p o t s and compare these diameters wi th the 1 / e2 i n t e n s i t y

diameter of t h e t ransmi t ted beam which i s predic ted by Fresnel

d i f f r a c t i o n theory. Since d i f f r ac t ion - l imi t ed o p t i c s a r e assumed,

the window induced spher ica l aberra t ion can be neglected when the

geometrically conputed spot diameters a r e much l e s s than the trans-

mitted spot diameter (o r i t s image, i n the case of the receiver) .

FOCAL SHIFT RAY EQUATIONS

Figure 1 i l l u s t r a t e s the s h i f t of a point image produced when

the transmitted l i g h t is not collimated. The dotted l i n e ind ica tes

the foca l pos i t ion t h a t would obta in i n the absence of the g l a s s

block, and i n t e r s e c t s the o p t i c a l ax i s (2) at the point A, which is

invar ian t wi th 0 When the g lass block is present , the focus is i'

s h i f t e d by a distance bMb which is now a function of Oi and which

f o r pa rax ia l conditions ( i .e . , Oi+O) tends t o b and i s i l l u s t r a t e d a s

point B. The r e f r a c t i v e index of the g lass n is r e l a t e d t o t h a t of g

the surrounding medium n a s 0

and is typ ica l ly 1.5 f o r o p t i c a l g lass i n a i r . Using Sne l l ' s law

s i n Oi = n s i n Or

and the r u l e s of geometrical op t i c s , i t i s easy to wr i t e the t o t a l

s h i f t of focus a s a function of incidence angle a s

Optical A x i s

'Ab-

Figure 1. SIiiEt of Focus by a Flat Window i n a Converging Penci l .

For pa rax ia l condition Ab = 0 and

tan 0 b =

Observing t h a t point C (Figure 1) i s the additionat s h i f t of focus from

the pa rax ia l condition and hence corresponds t o the e f f e c t s of aberra-

t i o n caused by the f l a t p l a t e we may rearrange Equations (3) and (4)

t o y i e l d

('. tan (s in-1 r?))) Bb(Oi,n,t) = t - -

t an (5)

Figure 2 is a p l o t of t h i s function f o r n = 1.5 and a range of values

of t from 0.25" t o 4.0". The range of values of 0 correspond t o i

F/D r a t i o s ( foca l numbers) from f /4 t o f/50.

Figures 3 through 6 i l l u s t r a t e the way i n which the wlndow-

induced aberra t ion a f f e c t s the focus when the window is normal t o the

o p t i c a l axis. The rays from each d i f f e r e n t incident angle focus with

d i f f e r e n t slope t o d i f f e r e n t locat ions . The distance from the o p t i c a l

a x i s to each ray cone is a function y which depends on both loca t ion

Refractive Index:

11 = 1.5

T = 101.6 rnm (4 i n . )

T = 50 .8 mm (2 i n . )

T = 25.4 mm ( 1 i n . )

0

0 a 9 m 1 I I I 1 I 1 '0 .DO0 0.020 0 .U40 0.060 0 mO80 0.100 0 .I20 0.140

INCIDENCE f3NGLE [ RRD IRNS I

sr r - 0

O

+'? LN- W I x CD

J E a=?- % L L

Figure 2. Geometric Focal S h i f t from Paraxial Focus for a Plat Window.

,, T = 12.7 mm (0.5 i n . )

T = 6.35 mm (0.25 i n . )

z (measured from the p a r a x i a l focus) and the inc idence angle. Hence,

t h e equat ion f o r the t ransmi t ted rays becomes

y(z,Oi,n,t) [ Z - Ab(eianat)1 t an Oi ( 6 )

where 8 ( i n rad ians) i s p o s i t i v e i n t h e f i g u r e f o r l i n e s wi th p o s i t i v e i

s lope . Figures 3 through 6 are p l o t s of Equation (6) wi th t h e param-

eters t and n chosen t o i l l u s t r a t e q u a n t i t a t i v e l y t h e abe r ra t ion f o r

p r a c t i c a l cases of i n t e r e s t . At t h e p a r a x i a l focus, t he i n t e n s i t y

p a t t e r n i s b r i g h t a t t h e cen te r b u t h a s a l a r g e ou te r area wi th energy

no t w e l l focused. A d i s k of l e a s t confusion occurs a t a p o s i t i v e

va lue of z where the outermost r ays a r e contained i n a sma l l e r r ad ius

than they a r e a t t he p a r a x i a l focus.

Figures 3 through 6 i l l u s t r a t e g raph ica l ly t h a t t h e sma l l e r t he

window thickness and the sma l l e r t h e angle of inc idence of the o u t e r

edge r ays , the smaller w i l l be t h e d i s k of least confusion. On the

o t h e r hand, the d i f f r a c t i o n l i m i t e d radius inc reases wi th decreas ing

edge r a y angle . Figure 7 is a p l o t of t h e d i f f r a c t i o n l i m i t e d l / e 2

f o c a l waist r ad ius f o r t h r e e wavelengths of l i g h t wi th the focus ing

angle def ined by the l / e 2 i n t e n s i t y cones i n t h e po r t ion of t h e rad ia-

t i o n away from t h e focus. The equat ion f o r t he beam waist r a d i u s i s

From Figures 3 through 6 i t i s poss ib l e t o ob ta in va lues f o r t h e

r ad ius of the d i sk of least confusion f o r d i f f e r e n t l i m i t i n g values

of 8 where the re i s no lower l i m i t t o 8 , i .e. , a s o l i d cone. i

I V E R T I C A L - 0 -02 M M / D I V HORIZONTAL = 0-10 MM/DIV

€Ii (radians) I

A

Paraxial Focal Plane

Figure 3. Focal Spot Ray Diagram: t = 0.5"

I VERTICAL - 0 SO2 MM/DIV HORIZONTA~ = 0.10 MM/DIV

1 f.25-4 MM

Paraxial Focal Plane

Figure 4. Focal Spot Ray Diagram: t = 1.0"

Bi (radians) I VERTICAL - 0.02 MM/DIV H O R I Z O N T A ~ = 0.10 MM/DIV

Paraxial Focal Plane

Figure 5. Focal Spot Ray Diagram: t = 2.0"

ei (radians) CC

I Paraxial

Focal Plane

0*12 \

Figure 6 . Focal Spot Ray Diagram: t = 4.0"

VERTICflL - 0 .I32 MM/DIV H O R I Z O N T A ~ 0.10 MM/DIV

X W = - cotan 8 o T i

I I I I I I I

0 -020 0 -040 0 .a00 0 aOa0 0.100 0.120 0 140 INCIDENCE flNGLE (RRDIANSI

Figure 7 . Gaussian Beam Waist Radius as a Function of l / c 2

Incidence Ray Angle.

These a r e p l o t t e d on Figure 8 f o r a range of th icknesses from t = 0.25"

t o t = 4.0" and f o r gi (rnax) from 0.02 t o 0.12 radians .

The geometrical d e f i n i t i o n of the d i sk of l e a s t confusion is

i n e r r o r by the in f luence of d i f f r a c t i o n and the c r i t e r i o n t h a t t h e

e f f e c t of the g l a s s block s h a l l no t be s e r i o u s l y de t r imen ta l i s equiv-

a l e n t t o t h e q u a n t i t a t i v e s ta tement t h a t t h e convolution of t h e geo-

m e t r i c a l r ad ius wi th the Gaussian radius s h a l l not exceed t h e va lue

of t h e Gaussian r ad ius by more than a c e r t a i n f i g u r e , chosen by the

app l i ca t ion .

It is c l e a r from Figure 8 t h a t wi th 8. (max) of 0.062, equ iva l en t 1

t o an £18 t r a n s m i t t e r , t he re i s almost no s i g n i f i c a n t spo t expansion

even f o r g l a s s th icknesses up t o 4.0". For t h inne r p i eces of g l a s s ,

t h i s e f f e c t becomes even less s i g n i f i c a n t .

Thus, t he s p h e r i c a l abe r ra t ion induced i n t o t h e t r a n s m i t t e r

beam is not s i g n i f i c a n t , e s p e c i a l l y when compared wi th a b e r r a t i o n s

which are introduced i n t o t h e system by the r ece iv ing o p t i c s wi th

its g r e a t e r va lues of 8 These e f f e c t s are discussed i n t h e nex t i'

s e c t i o n .

SPHERICAL ABERRATIOX OF THE RECEIVER

Figure 9 i l l u s t r a t e s schemat ica l ly both the t ransmit . ter and t h e

r ece ive r rays f o r normal incidence of t h e o p t i c a l a x i s on t:he f l a t window.

The e f f e c t s a r e , of course, g ros s ly exaggerated i n t h i s diagram.

I n Figure 9(a) t h e d e t e r i o r a t i o n nea r focus descr ibed i n t h e pre-

ceding s e c t i o n i s ind ica t ed by the edge rays (poin t source over ape r tu re )

\ Gaussian Radius

Outermost Limiting Ray - oi (max) Radius

-

3

I

I

.I

n

0

Plgure 8. Extreme Disk of Least Confusion R a d i i .

t = 4.0" Geometrical Radii (Disk of Least Confusion)

I I I I I I I 1 -02 .04 .06 .08 1.0 1.2 1.4 1.6

F l a t G l a s s

(a) Tr;msmi t t er

r Flat Class

Focal A Window

\-

Scatterer Point Source

Figure 9 . Axial Focus Distortion for Transmitter and Receiver.

focusing f u r t h e r away from the l e n s than the p a r a x i a l focus. Figure

9(b) i l l u s t r a t e s t h a t i f a s c a t t e r i n g po in t source is loca t ed a t the

p a r a x i a l focus of t h e t r ansmi t t e r beam, then the l i g h t i n tens icy

d i s t o r t i o n i n the real image w i l l be of a form s i m i l a r t o t h a t of the

t r a n s d t t e r . I n f a c t , f o r a p e r f e c t l ens wi th s s a l l flat-window

abe r ra t ions , w e can argue by pe r tu rba t ion theory t h a t the r ece ive r

l i g h t d i s t r i b u t i o n nea r t he focus has the same ma the ra t i ca l form a s

was previous ly derived f o r t he t r ansmi t t e r . (For s i m p l i c i t y we assume

t h a t the l e n s is imaging one-to-one. A change of magnif icat ion a f f e c t s

the d i f f r ac t ion - l imi t ed spot s i z e and the s c a l i n g of the g e o n e t r i c a l

image i s t h e same way.)

The d i r e c t i o n of propagation i n a geone t r i ca l ray diagram may

be reversed without change. We w i l l state, but not prove he re , t h a t

t he e f f e c t s of interchanging the p o s i t i o n s (order ) of l e n s and window

w i l l change n e i t h e r t he q u a l i t a t i v e nor q u a n t i t a t i v e d e s c r i p t o r s of

the abe r ra t ion pa t t e rn . Hence, t he abe r ra t ions may be predic ted by

use of t h e former equat ions and the diagrams of Figures 3 through 6

nay be used once more.

By t h e preceding s l i g h t of hand w e a r e ab le t o examine r e l a t i v e

magnitude of the e f f e c t s of t r ansmi t t e r and r ece ive r s p h e r i c a l aber-

r a t i o n on the same f igu re . Figure 10 i s an e d i t e d s e c t i o n of Figure 6

wi th a t y p i c a l choice of t r ansmi t t e r and r ece ive r edge rays a s shown.

We s e e by examining Figure 10 t h a t while t he t r ansmi t t e r bean geomet-

r i c a l l y focuses t o a spo t much smal ler than the d i f f r a c t i o n l i m i t

(which means the t ransmi t ted beam is d i f f r a c t i o n l imi t ed ) the r ece ive r

s p o t image i s now s i g n i f i c a n t l y l a r g e r than what would be e i t h e r i t s

VERTICRL - 13-02 MM/DIV H O R I E O N T R ~ = 0-10 HM/OIV

Projected RECEIVER

/ M M n = 1 .5

Figure 10. Worst Case I l lustrat ion of £14 Receiver and 4-Inch Thick Window.

own dif f rac t ion- l imi ted image o r the d i f f rac t ion- l in i t ed image of the

inner ray cone which would have been po ten t i a l ly l a rge r .

Note tha t from Figure 10 with f / 4 receiver op t i c s and a 4.0"

th ick window the image covers about twice the radius which i t would i f

the window were absent.

RECEIVLK FIELD STOP SIZE AND IMAGE SPOT SIZE REQUIIWEKTS

The question of how la rge t o make the receiver f i e l d s top pin-

hole aper tures is not such an easy one i n some cases. For example, i f

the a c t u a l r e tu rn image of the point s c a t t e r e r were a s l a rge o r

l a rge r than the i d e a l pe r fec t image of the transmitted focused beams,

then we would be forced to enlarge the pinholes t o avoid los ing s i g n a l

radia t ion. However, such l o s s of focus and pinhole enlargement would

degrade the system's a b i l i t y t o r e j e c t wa l l f l a r e .

Idea l ly , we wish f o r the aberrated receiver spot image t o be

smaller than the d i f f rac t ion- l imi ted t ransmit ter beam focus pa t t e rn .

I f the receiver had no aberra t ions , the d i f f raction-l imited image of

the s c a t t e r e r s would be on the order of hal f the s i z e of the i d e a l

image of the t ransmit ter beam. This i s because the receiver l e n s

diameter i s approximately twice t h a t of the transmitted beam. I n order

t o maintain near i d e a l wall f l a r e r e jec t ion then, we may say t h a t the

receiver point s c a t t e r e r image diameter must be l e s s than o r equal t o

the receiver Airy disk ( the diameter of the c e n t r a l b r i g h t por t ion of

the foca l spot pat tern of a f u l l y i l luminated di f f rac t ion- l imi ted l e n s ) .

The Airy d i sk r ad ius ro i s given i n terms of t he edge ray angle by a

formula similar t o t h a t f o r tile Gaussian bean given e a r l i e r

This equat ion is p l o t t e d i n Figure 12. We now have a r r i v e d a t the con-

c lus ion t h a t the requirement concerning t h e acceptable d i s k of l e a s t

confusion i s about the same f o r t he t r a n s n i t t e r and t h e r e c e i v e r , i .e.,

equa l t o about ha l f of t h e d i f f r ac t ion - l imi t ed t r a n s m i t t e r beam

diameter; b u t t h e abe r ra t ion e f f e c t s a t e much worse f o r t he r ece ive r

a s seen from Figures 6 and 10 of t he previous sec t ion .

SOLUTION FOR THE RECEIVER DISK OF LUST CONFUSION

The diameter of t h e d i s k of least confusion is h e r e defined a s

t he diameter of t he ray diagrams where the converging edge r ays

i n t e r s e c t the outermost d iverg ing inne r ray. Close in spec t ion o f

Figure 6 r e v e a l s t h a t f o r a fu l ly- i l luminated ape r tu re wi th edge rays

having 9 = 0.12, t he disk of l e a s t confusion is formed by i n t e r s e c t i o n

with t he 8 = -0.06 r a d i u s ray. I n order t o s tudy the way these i n t e r -

s e c t i o n s behave, w e may choose an inne r and an o u t e r ray and so lve f o r

the rad ius of i n t e r s e c t i o n . Figure 10 i l l u s t r a t e s the fol lowing

no ta t ion :

y = p t an 0 = q t an 8 1 2 (9 )

From which w e so lve t o o b t a i n

t a n 0 t a n e2 [Ab(8*) - Ab(Ol) ] 1

Y = t a n 0 y T E G 91 (11)

The l o c a t i o n of t h e i n t e r s e c t i o n along t h e o p t i c a x i s i s given by

Ab(02) t a n 62 + bb(3 ) t a a el - bbdn - 1 t a n O 2 + t an 8 (12)

1

Figure 11 is a p l o t of Equation (12) wi th the edge r ay 0 a s a param- 2

eter and the i n n e r ray angle 9 a s a va r i ab l e . We see t h a t f o r each 1

case t h e r e i s a va lue of i n n e r ray angle above which t h e diameter of

the d i s k of l e a s t confusion decreases ( c r i t i c a l angle) . The f u n c t i o n a l

behavior of Equation (12) s c a l e s l i n e a r l y w i th window th ickness t ,

s o t is no t of concern i n determining the va lue of t h e c r i t i c a l i nne r

angle. Graphica l ly , w e s ee t h a t f o r a t y p i c a l index of r e f r a c t i o n o f

1.5, t h i s angle is gene ra l ly about h a l f tire a ~ g l e 02. The imp l i ca t ion

of t h i s is t h a t , f o r p r a c t i c a l LTA systems, t h e d i s k of l e a s t confusion

may be obta ined a s t h e i n t e r s e c t i o n of t he o u t e r ray and t h e minimum

angle ray def ined by the annular r ece ive r ape r tu re s t o p s . This minimum

ray angle i s gene ra l ly g r e a t e r than h a l f t h e maximum ray angle.

I n o rde r t o eva lua t e t h e t y p i c a l diameter of t he d i s k of l e a s t

confusion f o r LTA design purposes, we now eva lua t e Equation (12)

pa rame t r i ca l l y wi th 0 a r b i t r a r i l y chosen equal t o 0.5 02. The r e s u l t s 1

a r e p l o t t e d i n Figure 12, which while appearing s u p e r f i c i a l l y s i m i l a r

t o Figure 8 has r a t h e r d i f f e r e n t numerical va lues and, n a t u r a l l y , 4 -

d i f f e r e n t c r i t e r i a .

rn 0

S- -?

a

ol

1

0 . INNER RRY flNGLE [THETA 11

Figure 11. Ray Intersection Radius vs. Receiver Inner and Outer Ray Angles for a 2-Inch Thick Window.

EXPERIMENTAL

For t h e conf igura t ion of t he £14 o u t e r l e n s -- worst case --

some observa t ions were made by p r o j e c t i n g the h ighly magnified f o c a l

reg ion on t o a d i s t a n t screen. The f i r s t observa t ions were made, us ing

a magnif ica t ion of about 500x, of the s p o t from the t r ansmi t t e r . Using

g l a s s blocks of up t o 4.0" th i ck , the e f f e c t s on the spot s i z e were

observed t o be n e g l i g i b l e as was p red ic t ed by Figure 8.

When the r e t u r n s p o t s were imaged a t a s i m i l a r magnif icat ion

where t h e source was a f i n e e tched s t a i n l e s s s t e e l su r f ace placed

a t t h e b e s t focus of t he t r a n s m i t t e r , the spo t s i z e was measured t o

inc rease i ts diameter by about a f a c t o r of two with the in t roduc t ion

of a 4.0" b lock of o p t i c a l g l a s s . This , too , accords with the d a t a

from Figure 12.

A f u r t h e r observat ion was made of the way the image changed as

the a x i a l p o s i t i o n w a s scanned. I n a l l cases the change i n p a t t e r n w a s

c lo se ly s i m i l a r t o t h a t which is p red ic t ed by Figure 10.

CONCLUSIONS

There i s much p r a c t i c a l information which can be deduced from

the preceding graphs. It is immediately apparent from Figure 12 t h a t

f o r windows a s t h i ck a s 4 inches abs r r a t ion co r rec t ion of the r ece ive r

would be requi red f o r a high-performance £14 system. I n more s p e c i f i c

terms, a 400 mm f / 4 Trope1 col l imator may be used uncorrected with

only minor l o s s of e f f i c i e n c y f o r windows up to 2 inches th ick . I n

N=1.5 Window Thickness

E3 0

3-? 9 .ROO

1'

0 4 2 0 ,

o .040 o .a60 O.OOO 0.100 o . i20 C!*i40 OUTER RRY flNGLE [THETA 21

Figure 12. Diffractlon-Limited Receiver Radius Compared with Disk of Least Confusion Radius.

addit ion, w e see t h a t even 11 inch thick windows a r e not a problem f o r

£16 opt ics . Experiments with 4 inch windows ve r i f i ed the a n a l y t i c a l

r e s u l t s .

Incidenta l experimental observations indicated a marked de te r i -

o ra t ion of the images i f the g lass block wers incl ined t o the mean

o p t i c a l axis . This ind ica tes t h a t the thinner windows and smaller

co l l ec t ion angle o p t i c s may be advisable f o r appl ica t ion which require

observations through windows a t o the r than n o r m 1 incidence, while

scanning f o r ins tance , i f p a r t i c u l a r points of i n t e r e s t a r e only

access ible by an angular scan. It w i l l be necessary t o attempt some

quan t i f i ca t ion of these e f f e c t s a t a l a t e r ti= but w i l l not be a

simple exercise.

There e x i s t methods of correct ing the spher ica l aberra t ions from

which t h i s system can s u f f e r under the extreme window thicknesses c i t e d .

The simplest is t o use a low-power meniscus l ens with pos i t ive s p h e r i c a l

aberra t ions . The curvature and power may both be chosen allowing a

simple l ens to perform adequate correction. It should be located a f t e r

the t ranceiver turning mirror because a t o the r posi t ions i n the

system i t would introduce more surfaces i n t o the t ransmit ter beam and

run the r i s k of increas ing the i n t e r n a l f l a r e , which so much design nas

gone t o remove.

For most appl ica t ions , e i t h e r an £16 co l l ec t ing l ens o r a window

2 inches o r l e s s i n thickness w i l l be avai lable . Since these c r i t e r i a

a l so reduce incidence angle e f f e c t s , we do not an t i c ipa te many s i t u a -

t i o n s i n which spher ica l aberra t ion compensation would be required.

i 0.140

OUTER RflY RNGLE [THETR 21