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1.0 INTROOUCTION

Conven~onal poknzm 45NWWW hae" been used sinc last te f 1920s. The__einmanwtc of fte two-mape pommetauMMenumor, where one polahim is fixed and the

othe one is roowed. is bvown as MW*is eW:

f(S) /Tr(o) - (coes)3

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rs) / rto) - (coos) 1

To Owsn accuruw, Prrmee'Now"IM1. vwi OXIsnen sma of the Polw~tr OMd ebwev'gnceol 0"e idwf pownmae mum to kown wd aoio d Wo when the

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and was a high schhleren quality calcite, because the intensity variation was notsymmetric in the four quadrants. He suggested that a prism-type polarizer withadequate performance would have the advantage of a wider, more useful spectralrange than was possible with sheet polarizers (Ref. 3).

The current work was based on the same concept and used specially selected highquality optical Glan-Thompson prisms, extremely precise automated stages, and acombination of optical density fitters with a lock-in amplifier, to obtain accuratemeasurements and to deterrymne the leasibility of using a three-stage polarizerattenuator with laser beams in and near the visible range. This device should beeffective for indep calbrkang expermental signals and transmittance ofneutral density filters over a wavelength range from 350 to 2500 nm and over anoptical density range of nine orders of magnitude.

2

2.0 EXPERIMENTAL ARRANGEMENT

2.1 ATTENUATOR DESIGN

The three-stage attenuator was comprised of specially selected high quality opticalGlan-Thompson prisms manufactured by Karl Lambrecht Corporation (MGT25EIO-90),Chicago, Illinois. The calcite prisms are glued at the interface. The optical glue limitedthe capability of the attenuator at the ultraviolet end of the spectral range, but it alsoprovided better maximum attenuation. The prisms were selected so that one crossedpair would have an attenuation equal to or greater than 106:1. The prisms had a wave-length range of approximately 350 to 2500 nm and a tolerance on maximum beamdeviation of approximately 6 arc sec.

The prisms were each mounted in precision stages. Two of the stages, manufacturedby Klinger Scientific, Garden City, New York, were electronically driven. The first stagewas required to accurately return to the O-deg position and to have a repeatable 90-degrotation. These criteria were easily met with one of the Kiinger stages. The require-merits for the second stage (middle polarizer) were the most stringent since this stagewas the only one which would rotate after the attenuator was aligned. This stage wasresponsible for determining the angle "B" with great accuracy. Consequently, thesecond stage consisted of a Klinger Scientific, Garden City, New York, stage and anencoder. The encoder on the middle stage was accurate to approximately 1 arc sec.The third stage was a manual stage which incorp both fine and coarse adjust-meits. Figures la, lb. and 1c show the assembled three-stage polarizer attenuatortaken from the top (Fig. Ia), from the front (Fig. lb), and from the side (Fig. Ic).

(a). Top view.

Figure 1. Three-stage polarizer attenuator.

3

(b). Side view.

I

(c). Front view.

Figure 1. Concluded.

4

2.2 ALIGNMENT OF THE ATTENUATOR

The alignment for the stages was accomplished mechanically by using an optical postwhich was the same diameter as the prisms. The post was inserted through themounts (where the prisms would be mounted) and then the mounts were adjusted andtightened down. The post was removed.

Each prism was then individually mounted in each stage position and tested. A targetwas placed 6 to 8 ft away from the attenuator and then the stage was rotated through360 deg to ensure that the laser spot remained in the same location on the target. Allthree prisms performed properly.

An alignment procedure was developed which ensured the first and third polarizershad their optic axes in parallel positions and the second polarizer reached a minimumtransmittance when it was crossed with the other two polarizers. The proceduresused are described below and depicted in Figure 2. (Note: Prism 1 is mountedclosest to the laser source, prism 2 is mounted in the center of the attenuator, andprism 3 is mounted closest to the detector.)

Step 1. Set all of the mounts to their 0-deg position and insert all the prisms inthe same approximate orientation.

Step 2. Set prism 3 near the 90-deg or null position.

Step 3. Set prism 2 so that prisms 1 and 2 are exactly 90 deg off (crosspolarized). This location should provide an exact null and prism 2 will be inapproximately the 90-deg position.

Step 4. Set prism 1 in the same position as prism 2.

Step 5. Return prism 3 to the 0-deg position and adjust it until an exact null isreached (prisms 2 and 3 are crossed polarizers). Lock prism 3 in place.

Step 6. Return prism 1 to its exact original position. At this point, prisms 1and 2 should be exactly 90 deg offset and prisms 2 and 3 should also be exactly 90deg offset.

Step 7. Return prism 2 to its 0-deg position. The attenuator should now beset for maximum transmission.

5

STEP1

2SEP 2

STEP 4-

SEP 5

STEP 6

SEP 7

21

Figure 2. Attenuation alignment procedures.

6

2.3 EXPERIMENTAL LAYOUT

The three-stage attenuator was then $ aW .,pwhEDet OR .range and accuracy. The expenmnwaw sc• ia *, Vh ,

The laser was a 5-mW helium-neon VOW , 43) *#expander. A quarter-wave pla was poaro oe SawA .0* ., ... **attenuator. Consequently, a u uO . , .aebeaI .ft,,W,-~source was normally incident upow te Af* aasw A .* , e -properly position the light for the Oslwo

A chopper with reflective blades we v AWo W ast tw 4• w t . w'integrating sphere which was cigoup, o a• aae, ..... -a*-input laser signal and was read on the w*p **#wow

The laser light then entered a uAtiky b -•s tt =1 - 4# .in its walls for both the incident &avd 'sebfua am. o* 1-,caused the laser light to scatte.,' an caro. ns• .-. -. in.. ......reflector (Labsphere, Inc.. North Su N-a , vitwn * V" 3W..4

center of the box. The laser beam oewn 0 t*.* .4, o% ,,..the sample's normal. A portion o0 nohawa,#," tanit w ,.w *. .the pnotometer. For alignment purpoe, * 1s,#4 ins vow .beam could exit when no samp4 vm se r _ #t•, , •,i.inner wall, were inserted throu~gh •i•e rail *o* o "s w .beam entered and exited the box A rrr e, am m e. i. sv.i,.,laser beam was adjusted until a s*i % to te. .0.t.m, en . ..the tubes. Figure 4 illustrates the Iboc o wk4 .. ,t swv

The photometer, filter wheel assent. ari #P.anwnmOi -, -'rseveral capabilities: (1) the a&bl•y o •in• cra•i. to. *" wk, othe ability to focus the beam. (3) the• e 1 • * - '. -f v... Veliminate most sources of stray ig. avd •i. S"o 4' o .*,neutral density filters in the opcav trm % rsi " o#g * vw.axis. Figure 5 provides a ray iagrra .me, owt ' 'Whousing (Products for Research. ft, %Camp%, ova ",-radiofrequency shielding. a dau•c s w•wi e * n

reduction of dark current.

The PMT was connected to a El'.to * retifuWo"t ,was used to obtain data over a s•ri ov•vV • •, ..n 4,orders of magnitude were otwaned br 9elr, a " n - a. •.*with a National Institute of Scienc arid *'rrhww 'e* a - .density filter. Consequently. e te tri 'r* -f ..limitation and within the linear rn" ' 0 o,

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16

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17

3.2 EXPERIMENTAL ERROR

3.2.1 Measurement Error

There were several sources of measurement and random error present throughout theexperiment. These errors include nonlinearity in the phase lock amplifier, noise fromthe photomultiplier, stray light in the optical train, variation in the lock-in amplifierreading, and drift in laser power. No attempt was made to control the laser power.

3.2.2 Systematic Error

Data collected around the 0-, 90-, 180-, and 270-deg angles indicated significantsystematic errors. The major errors include drift in the laser signal from the beginningto the end of the data run and slight alignment errors in the prisms. The drift wasnoticed in the signal from the beginning to the end of the data runs. In addition, theequipment was left for several 30-min intervals without altering the equipment settingand experimental data did change slightly with time. Alignment errors are noticed inthe lack of perfect symmetry in data around the 0- and 360-deg points. In addition,there was a small double hump pattern around the 90- and 270-deg points. Additionalsystematic errors would include the temperature change in the room affecting the laserdetector or the electronics and small inaccuracies in the mechanized center stage.

18

4.0 CONCLUSIONS

The three-stage polarizer attenuator, with the advanced Glan-Thompson opticalprisms, the extremely precise automated stages, and an experimental setup whichallowed accurate measurements of high extinction ratios, was capable of providingattenuation of a laser beam for nine orders of magnitude with uncertainties of 1 to 2percent. This attenuator was useful in providing practical accuracies of a few percentover a wide dynamic range with a simple technique. The data were symmetricthroughout all four quadrants. The attenuator can be used to calibrate neutral densityfilters over a wavelength range from 350 to 2500 nm and over an optical density rangeof nine orders of magnitude. These results were obtained with laser sources only, andno corrections were made for drift in the laser power or signal processing.

19

REFERENCES

1. Mielenz, K. D. and Echerle, K. L., "Accuracy of Polarization Attenuators," A•DliedOptics, Vol. 11, No. 3, pp. 594-603, March 1972.

2. Dowell, J. H., J Sci Instrum, Vol. 8, p. 382, 1931.

3. Bennett, H.E., "Accurate Method for Determining Photometric Linearity," AUKiliedOptics. Vol. 5, No. 8, August 1966.

20

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30

APPENDIX B

REDUCED DATA

31

8 TRANSMITTANCE EXPERIMENTAL NORMALIZEDCOS, 8 VALUE EXPERIMENTAL

VALUE

-5.00 0.98 2.62E00 ± 0.01 0.95E00

-4.00 0.99 2.61 EOO ± 0.01 0.94E00

-3.00 0.99 2.69E00 ± 0.09 0.97E00

-2.00 1.00 2.70E00 ± 0.09 0.97E00

-1.00 1.00 2.70E00 ± 0.08 0.97E00

0.00 1.00 2.70E00 ± 0.11 0.97E00

1.00 1.00 2.69E00 ± 0.07 0.97E00

2.00 1.00 2.68E00 ± 0.06 0.96E00

3.00 0.99 2.67E00 ± 0.04 0.96E00

4.00 0.99 2.63E00 ± 0.01 0.95E00

5.00 0.98 2.67E00 ± 0.01 0.96E00

10.00 0.94 2.51 EOO ± 0.07 0.90E00

20.00 0.78 2.08E00 ± 0.02 0.75E00

30.00 0.56 1.51 E00 ± 0.05 0.54E00

40.00 0.34 0.92E00 ± 0.02 0.33E00

50.00 0.17 0.47E00 ± 0.02 0.17E00

60.00 0.06 1.72E-01 ± 0.05E-01 0.62E-01

70.00 0.01 3.77E-02 ± 0.08E-02 1.36E-02

80.00 9.09E-04 2.48E-03 ± 0.07E-03 8.93E-04

85.00 5.77E-05 1.57E-04 ± 0.06E-04 5.65E-05

86.00 2.37E-05 6.44E-05 ± 0.2E-05 2.32E-05

86.50 1.39E-05 3.75E-05 ± 0.04E-05 1.35E-05

87.00 7.5E-06 2.02E-05 ± 0.03E-05 7.27E-06

87.50 3.62E-06 9.67E-06 ± 0.16E-06 3.48E-06

88.00 1.48E-06 3.93E-06 ± 0.03E-06 1.41E-06

88.25 8.70E-07 2.32E-06 ± 0.01 E-06 8.35E-07

86.50 4.70E-07 1.26E-06 ± 0.01 E-07 4.54E-07

32

a TRANSMITTANCE EXPERIMENTAL NORMALIZEDCOS' B VALUE EXPERIMENTAL

VALUE

88.75 2.26E-07 6.06E-07 ± 0.04E-07 2.18E-07

89.00 9.28E-08 2.50E-07 ± 0.03E-07 9.OOE-08

89.25 2.94E-08 7.98E-08 ± 0.14E-08 2.87E-08

89.50 5.80E-09 1.65E-08 ± 0.06E-08 5.94E-09

89.60 2.38E-09 7.04E-09 ± 0.02E-09 2.53E-09

89.70 7.52E-10 2.75E-09 ± 0.02E-09 9.90E-10

89.75 3.62E-10 1.70E-09 ± 0.03E-09 6.12E-10

89.80 1.48E-10 1.16E-09 ± 0.03E-09 4.18E-10

89.90 9.28E-12 8.06E-10 ± 0.03E-10 2.90E-10

89.95 5.80E-13 7.70E-10 ± 0.01E-10 2.77E-10

89.96 2.38E-13 7.65E-10 ± 0.01E-10 2.75E-10

89.97 7.52E-14 7.59E-10 ± 0.01 E-10 2.73E-10

89.98 1.48E-14 7.62E-10 ± 0.01E-10 2.74E-10

89.99 9.28E-16 7.73E-10 ± 0.01E-10 2.78E-10

90.00 0 7.60E-10 ± 0.45E-10 2.74E-10

90.01 9.28E-16 7.70E-10 ± 0.01E-10 2.77E-10

90.02 1.48E-14 7.65E-10 ± 0.01E-10 2.75E-10

90.03 7.52E-14 7.65E-10 ± 0.01E-10 2.75E-10

90.04 2.38E-13 7.62E-10 ± 0.01E-10 2.74E-10

90.05 5.80E-13 7.56E-10 ± 0.01E-10 2.72E-10

90.10 9.28E-12 7.89E-10 ± 0.01E-10 2.84E-10

90.20 1.48E-10 1.17E-09 ± 0.01E-09 4.21 E-10

90.25 3.62E-10 1.72E-09 ± 0.06E-09 6.19E-10

90.30 7.52E-10 2.74E-09 ± 0.01 E-09 9.86E-10

90.40 2.38E-09 7.12E-09 ± 0.01E-09 2.56E-09

90.50 5.80E-09 1.63E-08 ± 0.01E-08 5.87E-09

90.75 2.94E-08 7.84E-08 ± 0.05E-08 2.82E-08

33

a TRANSMITrANCE EXPERIMENTAL NORMALIZEDCOS4 a VALUE EXPERIMENTAL

VALUE

91.00 9.28E-08 2.49E-07 ± 0.02E-07 8.96E-08

91.25 2.26E-07 6.08E-07 ± 0.03E-07 2.19E-07

91.50 4.70E-07 1.27E-06 ± 0.01 E-07 4.57E-07

91.75 8.70E-07 2.36E-06 ± 0.04E-07 8.50E-07

92.00 1.48E-06 3.99E-06 ± 0.03E-06 1.44E-06

92.50 3.62E-06 9.75E-06 ± 0.07E-06 3.51E-06

93.00 7.50E-06 2.02E-05 ± 0.01 E-05 7.27E-06

93.50 1.39E-05 3.74E-05 ± 0.03E-05 1.35E-05

94.00 2.37E-05 6.41 E-05 ± 0.01 E-05 2.31E-05

95.00 5.77E-05 1.56E-04 ± 0.01 E-04 5.62E-05

100.00 9.09E-04 2.48E-03 ± 0.02E-03 8.93E-04

110.00 0.01 3.78E-02 ± 0.05E-02 1.36E-02

120.00 0.06 1.71E-01 ± 0.03E-01 6.16E-02

130.00 0.17 0.46E00 + 0.07E-01 1.66E-01

140.00 0.34 0.96E00 ± 0.02E00 3.46E-01

150.00 0.56 1.53E00 + 0.01E00 0.55E00

160.00 0.78 2.12E00 + 0.05E00 0.76E00

170.00 0.94 2.59E00 ± 0.06E00 0.93E00

175.00 0.98 2.68E00 ± 0.06E00 0.96E00

176.00 0.99 2.71 E00 ± 0.03E00 0.98E00

177.00 0.99 2.72E00 ± 0.01E0O 0.98E00

178.00 1.00 2.73E00 ± 0.05E-01 0.98E00

179.00 1.00 2.75E00 ± 0.01EOO 0.99E00

180.00 1.00 2.77E00 + 0.03E00 1.00E00

181.00 1.00 2.76E00 ± 0.01 E00 0.99E00

182.00 1.00 2.75E00 ± 0.01 E00 0.99E00

183.00 0.99 2.74E00 ± 0.03E00 0.99E00

34

B TRANSMITTANCE (~AtE

184.00 0.99 27rEaiz5til-

185.00 0.96 2 7"

187.00 0.97 2 ?*to

188.00 0.96 z r2mI~i'~ A4wt

189.00 0.95 2 Mu z~

190.00 0.94 ZtI U

200.00 0.78 aI 1460 it -41

210.00 0.56 ~ 1M lim s WIMI

220.00 0.34 0W m Ma i C11

230.00 0.17 _"m____qa

240.00 0.06 W'@~* 4

250.00 0.01 1* m I R 0

260.00 9.09E-04 I_____ a ýK

265.00 5.77E-05 )4at.W 4S1 am s

266.00 2.37E-0 tanS 40 4

266.50 1.39E-M oe 4stW. 4w.

267.00 7.50E-06 a194

267.50 3.62E-06 Iat~h44 do t1-*

268.00 1.48E-06 4

268.25 8.70E-07 ON4 tS.$ I UN

268.50 4.70E-07 t ~ a t_10 g

268.75 2.26E-O? i~t a4r, I x*14.)

269.00 9.2BE-WSI4a.u~

269.10 6.09E-0S V 1, 4? j . r

269.20 3.SOE-O & ? w

269.25 2.94E-06

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