SR-922

REPETITIVELY PULSED RUBY LASERS AS LIGHT SOURCES FOR HIGH-SPEED PHOTOGRAPHY

Jeffery M. Grace, Peter E. Nebolsine, Charles L. Goldey

Physical Sciences Inc.20 New England Business Center

Andover, MA 01810

Gurdaver Chahal, James Norby, and Jean-Marc Heritier

Continuum3150 Central ExpresswaySanta Clara, CA 95051

Optical Engineering 37(8) 2205-2212 (August 1998), pp. 2205-2205

Copyright © 1998 Society of Photo-Optical Instrumentation Engineers

This paper was published in Optical Engineering and is made avaialable as an electronic reprintwith permission of SPIE. Single print or electronic copies for personal use only are allowedSystematic or multiple reproduction, or distribution to multiple locations through an electroniclistserver or other electronic means, or duplication of any material in this paper for a fee forcommercial purposes is prohibited. By choosing to view or print this document, you agreeto all the provisions of the copyright law protecting it.

1

Repetitively Pulsed Ruby Lasers as Light Sources for High-Speed Photography

Jeffery M. Grace, Peter E. Nebolsine, Charles L. Goldey

Physical Sciences Inc.20 New England Business Center

Andover, MA 01810grace@psicorp.com

nebolsine@psicorp.com

Gurdaver Chahal, James Norby, Jean-Marc Heritier

Continuum3150 Central ExpresswaySanta Clara, CA 95051

Abstract

Two ruby laser systems employing multiple Q-switching technology have been

developed to provide ruby laser light for high-speed photography. Potential applications include

ballistics and flow visualization as well as non-destructive test evaluation using laser imaging

diagnostics such as photography, holography, and various interferometric techniques. The laser

systems produce more than 50 pulses at repetition rates up to 500 kHz with nearly constant

pulse-to-pulse energies. One system, based on a commercial laser, provides multiple pulses of

holographic quality light with individual pulse energies on the order of 10 mJ, pulse widths of 50

ns, and a pulse train length of more than 300 µs. A new ruby laser system has been developed to

provide higher pulse energies, on the order of 350 mJ per pulse, with 10 ns pulse widths and 140

µs pulse train length.

The method for multiple Q-switching by modulating the Pockels cell’s quarter wave

voltage and the formation of an individual Q-switched pulse has been investigated. The energy

within the individual pulses formed in the oscillator cavity has been successfully increased by

propagating through an amplification section, without degradation of the temporal or pulse-to-

2

pulse amplitude stability. Etalons for longitudinal mode selection and an iris for spatial mode

selection have been incorporated into the lower energy system and an image of a reconstructed

hologram is presented. Camera capabilities and the implications of higher pulse energies have

also been investigated.

Keywords: multiple Q-switching, Pockels cell, ruby laser, amplification, high-speed

photography, holography, high repetition rate laser

1. INTRODUCTION

Pulsed laser light is an ideal illumination source for use in the imaging of high-speed

events such as ballistics, material fractures and fluid combustion/flows. A pulsed light source

has been developed based on repetitively Q-switched ruby laser technology and soon will be

commercially available. The newly developed source is capable of providing illumination for

large area front lit photographic and cylindrical holographic applications with < 100 ns exposure

times. For these applications it is desired to have a pulsed source which emits tens of pulses,

each having a time duration on the order of tens of ns to freeze the motion of the image subject.

In addition a high pulse repetition rate of hundreds of kHz is desired with low pulse-to-pulse

timing jitter to synchronize with the camera and low energy variation to maintain proper

exposures. The imaging of such large photographic subject requires that the energy contained in

a single pulse be 100's of mJ when using high-speed framing cameras, for example, the Cordin

330A.

The technique for obtaining multiple pulses from a single laser rod by repetitive

Q-switching the cavity while the flashlamp pumps the rod has been previously demonstrated and

3

an extensive review of this technology is given by Huntley.1 This work demonstrated the largest

amount of pulse energy (~ 20 mJ) at the desired repetition rates (~ 500 kHz). In addition, use of

ruby lasers for holographic purposes is common in the imaging community. An alternative

approach of the chopping up of a cw beam and amplifying the pulse segments was not chosen

because the high energy pulse requirement would add an excessive amount of amplification

stages and result in a very complex system.

Individual pulse energies as high as tens of mJ at repetition rates of hundreds of kHz have

been obtained by Huntley.1 This pulse energy is within a factor of ten for the desired pulse

energy but the laser systems used in Huntley’s study and in previous works were all custom built

devices of various sizes containing one-of-a-kind components. The first laser system (Section 2)

was then used to study scaling factors and operational parameters on an existing commercial

laser system that could then be applied to the higher energy version (Section 5). The formation

of an individual pulse in a pulse train has been investigated experimentally. This affects the

pulse-to-pulse amplitude and timing jitter stability. Such information is critical for high-speed

photography when the transmission function of a camera varies significantly over the exposure

time or for very short exposure times using a gated intensifier. Increasing the energy of the

individual pulses and the effect this amplification has on the system performance has been

explored. The low energy system has been operated with components for single mode selection

and the systems applicability to making shadowgraphic holograms has been demonstrated

(Section 7).

4

2. LOW ENERGY LASER SYSTEM

The ruby laser system for lower energy applications, e.g., low f/# CCD camera

applications, is a Model 22HD manufactured by Apollo Lasers Inc. The laser is constructed in

an oscillator/amplifier configuration along a single optical rail, Figure 1. When new it was rated

for a maximum energy output of 3 J in a single Q-switched pulse. The oscillator cavity is

composed of a rear reflector mirror, Pockels cell, etalon, 1/4 waveplate, polarizing beamsplitter

cube, aperture, rod head, and resonant reflector output coupler. The system also contains a rod

head for amplification and a HeNe laser for alignment purposes. All of the components are

original to the system except for the polarizing beamsplitter cube and 1/4 waveplate. The cube

was purchased from CVI Laser, Inc. for polarization selection and replaced the original brewster

stacks that were damaged. The 1/4 waveplate was introduced into the cavity to allow the laser to

be operated in a mode where the cavity has a high loss, low Q, when the voltage was not applied

to the Pockels cell. The Pockels cell is a KD*P type and was made by INRAD, Inc. The internal

cavity etalon is of the solid type. The output coupler to the system is an etalon of the double

plate air gapped type (resonant reflector) with a reflectivity of approximately 30%. The aperture

is a variable iris.

The oscillator rod has a diameter of 3/8 in. and a length of 3 in. The amplifier rod also

has a diameter of 3/8 in. but is 6 in. in length. Both the oscillator and the amplifier flashlamps

are of the helical style. The oscillator and amplifier sections were both operated at a flashlamp

energy of 2.2 kJ during the experiments. Figure 2 is a photodetector trace of the oscillator

flashlamp light and resulting non-Q-switched, free-running, laser emission from the oscillator

section of the system. The energy output from the oscillator section of the system when free-

running with the 1/4 waveplate removed is 2.65 J. The energy output from the oscillator section

5

in a single Q-switched pulse is 1.35 J. The amplification section of the laser increases the output

energy from the oscillator section by approximately x2.5.

3. FORMATION OF PULSES

Q-switching of a laser cavity is normally done once to obtain a single pulse. This is

usually done near the end of the flashlamp pump duration when the population inversion in the

rod is at its highest. To obtain multiple pulses the laser cavity is repetitively Q-switched over a

period of time for which the inversion, and resultant radiation, is greater than the losses

associated with the laser cavity. It has been found that the laser cavity can be repetitively

Q-switched with resulting pulses formed, for approximately the duration in which the laser will

emit when free-running as shown in Figure 2. However, for the system to yield multiple

Q-switched pulses, the laser’s Pockels cell has to be modulated at the desired pulse repetition

frequency.

The pulsing source, obtained from Medox Electro-Optics, Inc., is capable of rapidly

applying and removing 2.5 kV from the laser’s Pockels cell. The rise and fall times for the

voltage application are 10 ns or less. This voltage is the 1/4 wave voltage of the Pockels cell at

694.3 nm. The Medox system is capable of producing from 1 to 999 high voltage pulses of

between 50 and 300 ns in duration at a repetition rate up to 500 kHz. This system was integrated

into the laser system. A trigger output from the laser system was used to start the Medox unit’s

modulation of the Pockels cell. The Medox unit outputs a TTL logic voltage level signal that

rises with the leading edge of the first high voltage output pulse.

We have successfully obtained data for repetition rates from 100 to 500 kHz. Figures 3

through 5 show plots of the photodetector signal data for the laser oscillator section emission

6

only for operation at the repetition rates 500 kHz. Figures 3 and 4 were taken with a photo-

detector which had its response slowed with a load resistor to act as an energy integrator. The

pulse-to-pulse energy, not necessarily the pulse form, is important in photographic/holographic

applications as long as the pulses are short in duration. Figure 5 shows a single pulse acquired

without the load resistor thereby using the full bandwidth of the detector.

The pulse train amplitude shown in Figure 3 follows the flashlamp discharge envelope

shown in Figure 2. In addition the duration over which pulses are formed is nearly the same as

the 400 us duration of free-running lasing in Figure 2. The pulse-to-pulse stability in these

figures is excellent. This is more apparent in Figure 4 which shows pulses plotted on an

expanded time scale. The individual pulse width in Figure 5 is about 50 ns FWHM. The width

of the individual pulses are consistent with those measured by both Huntley and Rowlands.1,2,3 In

addition it was found that the individual pulse widths did not vary greatly during a train of pulses

or for pulse trains formed at different repetition rates. This is also consistent with previous

measurements made by Huntley.1

The noise on the baseline in Figure 3 is due to the close proximity of the photodetector to

the Pockels cell and is electrical pickup from the high voltage switching. Huntley found in his

previous investigation that in order to prevent giant pulse formation the Pockels cell modulation

should start at or before when lasing threshold has been achieved.1 The noise can be more

clearly seen at the base of the start of the pulse in Figure 4 and corresponds to the application of

the high voltage pulse to the Pockels cell. Noise pickup by the photodetector due to the high

voltage switching is also evident in Figure 5. The ripple in the signal baseline and at the peak of

the pulse are considered RF pickup and not as a result of the detector sensing photons.

7

We calculated the energy contained in a single pulse from the total energy in a pulse

train. A computer program was written which takes a data file, as displayed in Figure 3, picks off

the peak values, equates this to the total energy measured, and determines what fraction of the

total energy was contained in an individual pulse. The bar graphs in Figures 6 and 7 are the

result of this calculation when applied to the data in Figure 3. Figure 6 is a bar graph for the

entire pulse train and Figure 7 is data from the same test but plotted on an expanded time scale.

The total energy measured for this test was 0.83 J. The first 100 pulses contain a nearly constant

individual pulse energy of about 5 mJ. The energy contained in an individual pulse was seen to

increase as the pulse repetition frequency for the tests was decreased from 500 to 100 kHz.

However, the total pulse train energy was measured to be nearly the same over this same range

of pulse repetition rates. This constant pulse train energy is approximately 30% of the energy

emitted by the laser oscillator when free-running and 60% of the energy contained in a single

Q-switched pulse.

Some of the trends in the data presented in Figures 3 through 5 can be better understood

through a closer examination of the formation of the individual pulses within a pulse train. The

output signal from the Medox unit which marks the rising edge of the first output pulse was fed

to a precision digital delay generator. A delay was set on the digital delay generator to

correspond with the application of the high voltage pulse to the Pockels cell within the pulse

train. The logic level output signal from the delay generator rises in time with the application of

the high voltage and decreases when the high voltage is removed. Figure 8 is a photodetector

trace of a single pulse from the oscillator section of the laser when operated at a pulse repetition

frequency of 500 kHz. Figure 9 is the output signal from the digital delay generator which

corresponds to the application of the high voltage to the laser’s Pockels cell for this single pulse.

8

The high voltage was applied to the Pockels cell for a duration of 250 ns during these

tests. It was found that varying the time the high voltage was applied to the Pockels cell, from

200 to 300 ns, had no effect on the laser’s ability to form stable pulse trains at repetition rates

from 100 to 500 kHz. This finding agrees with observations made by Rowlands2 however

Huntley1 in previous work found a dependence of shorter high voltage pulse widths at greater

repetition rates for good pulse formation. Huntley did vary the pump rate while it was held

constant during these tests. All three studies, Rowlands, Huntley and this one, did find that good

pulses were formed with a high voltage pulse duration in the range of 200 to 300 ns. This is an

interesting result because all three systems had large variations in cavity lengths, rod sizes and

pump energies.

A critical result from Figures 8 and 9 is that the peak of the laser pulse is defined by the

falling edge of the high voltage. For this system, the falling edge of the high voltage is deter-

mined by a precision digital clock that controls the timing within the Medox unit, resulting in a

timing jitter for the laser pulses that is much less than a laser pulse width. The peak of the laser

pulse in Figure 8 coinciding with the falling edge of the trace in Figure 9 provides some

information on the pulse formation and characteristics of each pulse. When the pulse is formed

relative to the Q-switching of the system has been an issue of speculation in prior investigations.

From these figures it can be seen that the pulse builds up during the application of the high

voltage pulse, cavity at high Q, and then is cut off by the removal of the high voltage and the

resulting return of the cavity to a low Q state. This cut off of the pulse is a likely reason for the

decrease in energy seen from a pulse train as compared to a single Q-switched pulse. Every time

a pulse is formed the Pockels cell rotates the polarization of the light when the cavity is returning

to a low Q state and a portion of the pulse energy is rejected from the cavity by the polarizer.

9

A numerical model developed by Huntley (Ref. 4), based on the low energy laser system

here, i.e., cavity length, rod size, cavity losses, pump rate, has simulated this experiment and

found that the laser pulse occurs at the end of the Pockels cell pulse. Precision timing of the

laser pulse would prove to be beneficial if the laser system were to be used as a light source for a

digital camera employing a narrow imaging gate duration.

4. AMPLIFICATION OF PULSES

It is desired to uniformly amplify the energy of the individual pulses in the train without

distorting the temporal profile of the train. Neither Huntley nor Rowlands investigated the use of

an amplifier section with their repetitively Q-switched lasers. Although amplification of a train

of pulses formed by a laser has been done in the past it is not known if it was done with a ruby

laser for similar conditions.5 One concern with implementation of amplification is that the

amplification section would preferentially amplify the larger energy pulses emitted from the

oscillator cavity giving a non-uniform amplitude pulse train. An additional concern is the

possibility that the first pulse or pulses into the amplifier section would deplete the gain in the

section to the extent that the remaining pulses would pass through unamplified.

The output emission from the oscillator section was allowed to pass through the amplifier

section (Figure 1). Photodetectors were used to monitor the amplifier input and output

emissions. The amplification of pulses successfully produced a train of pulses with temporal

characteristics of the oscillator train. The energy per pulse for this low energy system was

increased x2.5 to about 12 mJ per pulse.4 Demonstration of successful operation with

amplification is covered in more detail in the following section on the high energy laser system.

10

5. HIGH ENERGY LASER SYSTEM

A new high energy laser system based on the technology described above has been

developed by Continuum in partnership with PSI. This system consists of an oscillator plus two

amplification stages. The oscillator section contains the same essential Q-switching elements as

described in Figure 1. The work reported here describes the illumination capability of this

advanced laser system; therefore, mode selection elements are not included in this discussion of

the oscillator cavity.

Figures 10 and 11 show the photodetector traces and energy per pulse results for pules

repetition frequencies (PRF) at 500 kHz and 250 kHz respectively. These figures exhibit nearly

ideal pulse-to-pulse stability. Figure 10a,b and Figure 11a,b were again acquired using a load

resistor to integrate the photodetector response. An active, optical element placed between the

oscillator cavity and first amplification stage artificially terminated the macro-pulse. This

element shaped the macro-pulse envelope to achieve even better stability. The macro-pulse

duration was designed to ensure the laser produces tens of pulses per shot. Figure 3 shows that

this macro-pulse time is not a fundamental limit to the laser. A modest modification has been

made to the experimental method described in Section 3 above to reduce the effects of EMI from

the Pockels cell switch and the impedance mismatch in the integration circuit.

Figures 10 and 11 show that the individual pulse energies are on the order of 350 mJ for

500 kHz and more than double that to a peak of nearly 1 J/pulse at 250 kHz. Pulse energy scales

inversely with pulse repetition frequency (PRF) for this operational envelope of the laser, as seen

in Figure 12. This figure shows the macro-pulse energy as a function of PRF for two pump

energies of amplifier stage 1 (2,900 and 3,600 J) and a fixed pump energy of the oscillator cavity

(3,600 J). This operational envelope is clearly below saturation, indicating that even higher

11

pulse energies can be obtained. The optical gain for these amplification stages is roughly x25 for

stage 1 and x2 for stage 2. Stage 1 is a double-passed amplifier and most of the gain (~x9)

occurs on the first pass while the second pass is about x2.5. Since the ruby rods in the oscillator

and the amplifier stage 1 are the same size, clearly the oscillator does not tax the ruby rod.

The pump energies given in Figure 12 correspond to a reasonably cautious operational

envelope. It was not the goal to investigate the full operational envelope of the laser. However,

some of these conditions can be defined. The minimum PRF is not know for this laser system,

though PRFs of 167 kHz have been successfully tested in the current configuration while

100 kHz exhibited oscillation greater than 1 f-stop. A successful test is one where the individual

pulse energies vary by less than a factor of 2 (1 f-stop) over the macro-pulse envelope. The

maximum PRF reported here is 500 kHz due to the Pockels cell high-voltage pulser limit

(500 kHz).

Figure 13 shows a single laser pulse trace isolated from the center region of a macro-

pulse fired with the settings of Figure 11 (250 kHz). This method uses the full bandwidth of the

detector and oscilloscope (no load resistor). The FWHM of this laser pulse is approximately

10 ns. This fact coupled with the low jitter due to fact that the pulse forms at the close of the

Pockels cell yields a highly desirable combination for stop motion photography. In essence,

motion is frozen regardless of the open shutter time because the illumination is restricted to a

narrow time window (e.g. an object moving at 5 km/s travels 50 µm in 10 ns). This pulse shows

some interesting features that have not yet been understood. The central, large pulse contains

most of the energy.

Again, the features of the pulse are not critical for photographic applications as long as

the pulse is narrow enough.

12

6. CAMERA INTEGRATION

Many different types of high speed framing cameras are commercially available. In all

cases there is the effective f/# of the lens system to form the image, an optical transmission of the

camera system and a fluence(energy per unit area) requirement of the image recording medium.

We will take a broad definition of transmission of the optical system to include the effects of

gain of an image intensified camera systems. Hence, transmission can be greater than unity for

such cameras. Photographic film and CCD units are the two most common capture methods

leading to storage of the final image.

The optimum choice of which high speed framing camera is best suited for a specific

application depends on many factors, including type of illumination source (pulsed or con-

tinuous), number of frames desired, f/#, field of view, image format dimensions, transmission,

and fluence requirement of recording medium. When utilizing either laser system described in

this article, the effective exposure time to “stop” the action will be the laser pulse time (except

for some very specialized cameras). Thus, in some cases, this may relax the framing rate

requirement as compared to use with a continuous illumination source. A number of mechanical

framing cameras have an exposure time that is a set fraction of the interframe time. When using

mechanical framing cameras with CW sources, some operators intentionally go to higher

framing rates than necessary to record the dynamic motion of the object. This method reduces

the exposure time to an acceptable level in order to “stop” the action. Setting a framing rate

solely by this requirement is alleviated when using short laser pulses as described above.

Sample cases have been computed for several types of framing cameras including a

rotating mirror (Cordin 330A), gated intensified CCD (Imacon 468 by Haadland Photonics) and

a fast framing CCD (Silicon Mountain Design, Inc.), see Table 1. The Cordin 330A camera has

13

been chosen for illustrating a sample calculation. This 80-frame camera system uses conven-

tional 35 mm film. It has an effective f/# of 33 when taking into account the collection cone

angle and transmission of the optical system. Kodak High Speed Infrared 2481 film is an appro-

priate film to use in conjunction with ruby laser pulses. A fluence of 2.5 x 10-5 mJ/cm2 is

sufficient to expose the film based on nominal exposure numbers. The object plane fluence

requirements are then calculated as for any camera system. Given the format size, of 18 x

25 mm, a f/33 optical system, the illumination laser spot diameter in the object plane was

calculated. The results are shown in Figure 14 using the laser performance results described in

Section 5. This figure shows that the linear object dimension scales with the square root of the

laser pulse energy.

7. APPLICATION TO HOLOGRAPHY

One type of imaging to which the multi-pulsed laser system is to be applied to is

holography. For good holograms, both temporal and spatial coherence of the individual pulses is

needed. Previous investigators of this rapid, repetitive Q-switching of a ruby laser, did not

include mode selection in their systems. To demonstrate that the laser system pulses contained

sufficient coherence, holograms were made. The experimental data previously presented was

generated with the manufacturer’s supplied etalons installed for longitudinal mode selection and

the oscillator cavity aperture set to the full rod diameter. To obtain a single transverse mode the

cavity aperture was reduced to a diameter of 2 mm. This method employed by Apollo Lasers,

Inc. for producing coherence in a ruby laser is discussed in depth by Pitlak.6 For exposure

control purposes, only a single Q-switched pulse emitted from the oscillator section of the laser

was used. This pulse contained approximately 10 mJ of energy. This single pulse is considered

14

to contain representative coherence properties of a train of pulses if the laser was repetitively

Q-switched.

The shadowgraphic hologram shown in Figure 15 was able to be made of the object in

Figure 16. The hologram was reconstructed using a HeNe laser and a photograph of the

reconstructed image was taken with a 35 mm camera. A shadowgraphic type hologram was

chosen as a test case because it is commonly applied to ballistics testing. The experimental

apparatus consisted of a PVC cylinder 5.75 in. in diameter with the target (Figure 16) placed at

its center. Holographic film (Agfa type 8E75) lined the interior of the cylinder from 180 to

360 deg for about 6 in. along the cylinder length. The other half of the cylinder interior (0 to

180 deg) was lined with a diffuse reflector for the same length. The laser beam was split into a

reference and object beam. These two beams then entered the cylinder on axis from opposite

ends and reflected off 2 ball bearings symmetrically located about 1 in. off the cylinder axis.

These ball bearings were chrome plated, “off-the-shelf” components of good optical quality. In

this configuration, the bearings acted like convex mirrors to diverge and reflect the two beams

onto the film/reflector lining.

8. CONCLUSIONS

A commercial holographic quality ruby laser system has been modified to provide a train

of pulses by repetitively Q-switching the oscillator cavity. The pulse trains have been formed at

repetition rates up to 500 kHz with tens of consecutive pulses having nearly constant energy.

The formation of an individual pulse within the train was investigated. It has been determined

that the pulse forms at the end of the high Q duration and is actually cut off by the transition of

the cavity to low Q by the rotation of the cavity polarization by the laser’s Pockels cell. A low

15

pulse-to-pulse timing jitter follows from this result. The energy contained within the individual

pulses of a pulse train emitted by the oscillator section have been successfully increased by an

amplification section without significant degradation of the pulse train temporal profile.

A high energy system was developed through a technology transfer based on the results

of the low energy system. This high energy laser has generated pulse trains with individual

pulse energies of nearly 1 J at 250 kHz and on the order of 350 mJ at 500 kHz for tens of pulses.

The precise timing, low jitter, short pulse duration and excellent pulse-to-pulse energy stability

meet the requirements for high-speed photographic applications.

9. ACKNOWLEDGMENTS

This work was performed under Contract No. F08630-96-C-0031 for the Air Force

Research Laboratory, US Air Force, Eglin AFB, FL.

10. REFERENCES

1. Huntley, J.M., “High-Speed Laser Speckle Photography. Part 1: Repetitively Q-switched

Ruby Laser Light Source,” Optical Engineering, Vol. 33, No. 5, May 1994.

2. Rowlands, R.E., Taylor, C.E., Daniel, I.M., “Ultrahigh-Speed Framing Photography

Employing a Multiply-Pulsed Ruby Laser and a ‘Smear-Type’ Camera: Application to

Dynamic Photoelasticity,” Proceedings of the 8th International Congress on High-Speed

Photography, June 1968.

3. Rowlands, R.E., Wentz, J.L., “A Low-Voltage Pockels Cell having High Repetition

Rates and Short Exposure Durations,” Proceedings of the 9th International Congress on

High-Speed Photography, August 1970.

16

4. Goldey, C.L., Grace, J.M., Nebolsine, P.E. and Huntley, J.H., “An Investigation of a

Ruby Laser Repetitively Pulsed at Rates up to 500 kHz Containing Etalons and

Amplification,” SPIE Proc. Solid State Lasers VII, January 1998.

5. Lempert, W.R., Wu, P-F., Zhang, B., Miles, R.B., “Pulse-Burst Laser System for High-

Speed Flow Diagnostics,” presented at 34th Aerospace Sciences Meeting and Exhibit,

AIAA 96-0179, January 1996.

6. Pitlak, R.T., Page, R., “Pulsed Lasers for Holographic Interferometry,” Optical

Engineering, Vol. 24, No. 4, July 1985.

17

Table 1. Camera Specifications

CameraFrame Rate

(MHz)Number of

Frames

Minimum ImageFluence(mJ/cm2)

Cordin 330AImacon 468SMD-64k1M

1100

1

808

16

a2.5 x 10-5 b70

b350

aFor an OD=1 using Kodak high-speed infrared filmbAssuming a 22 µm2 pixel, f/11 and 1000 cm2 object.

18

Resonant

Reflector

Ruby

Amplifier

Head

Ruby

Oscillator

Head

Polarizing

Beamsplitter

Cube

Aperture

Pockels

Cell

1/4

Wave

Plate

Solid

Etalon Mirror

Alignment

HeNe

Laser

Figure 1. Ruby laser configuration.

Figure 2. Oscillator flashlamp envelope and laser emission.

19

Figure 3. Q-switched laser pulse train at 500 kHz.

Figure 4. Expanded time scale for Figure 3.

20

Figure 5. Temporal profile of a single pulse formed at 500 kHz.

Figure 6. Pulse energy for Q-switched laser pulses at 500 kHz (data from Figure 3).

21

Figure 7. Expanded time scale for Figure 3.

Figure 8. Single pulse within a 500 kHz pulse train.

22

Figure 9. Delay generator signal corresponding to the high voltage applied to the Pockels cell.

23

Osc/Amp1/Amp2 = 2.0/2.0/1.95 kV, PS = 2 µs, PW = 180 ns (sc2.083)

Time [µs]25 50 75 100 125 150 175 200

Pho

tode

tect

orV

olta

ge [V

]

0

2

4

6

8

10

90 92 94 96 98 100

Pho

tode

tect

orV

olta

ge [V

]

0

2

4

6

8

10

Pulse Number0 20 40 60 80

Pul

se E

nerg

y [J

]

0.0

0.1

0.2

0.3

0.4

0.5

(a)

(b)

(c)

D-8129

Time [µs]

Figure 10. 500 kHz pulse train, total energy = 24.87 J. (a) macro-pulse envelope; (b) expandedtime scale of (a); (c) energy per pulse in (a).

24

Osc/Amp1/Amp2 = 2.0/2.0/1.95 kV, PS = 4 µs, PW = 180 ns (Sc2.096) E = 26.04 J

Time [µs]25 50 75 100 125 150 175 200

Pho

tode

tect

orV

olta

ge [V

]

0

1

2

3

100 102 104 106 108 110 112 114 116 118 120

Pho

tode

tect

orV

olta

ge [V

]

0

1

2

3

Pulse No.0 10 20 30 40

0.0

0.2

0.4

0.6

0.8

1.0

Pul

se E

nerg

y [J

]

(a)

(b)

(c)

D-8130

Time [µs]

Figure 11. 250 kHz pulse train, total energy = 26.04 J. (a) macro-pulse envelope; (b) expandedtime scale of (a); (c) energy per pulse in (a).

Amplifier Stage2 Pump Voltage [kV]

1.65 1.70 1.75 1.80 1.85 1.90 1.95 2.00 2.05

Mac

ro-P

ulse

Ene

rgy

[J]

8

10

12

14

16

18

20

22

24

26

28

Amp1=1.8 kV, 500 kHzAmp1=1.8 kV, 250 kHzAmp1=2.0 kV, 500 kHzAmp1=2.0 kV, 250 kHz

D-8128

OSC = 2.0 kV

Figure 12. Total macro-pulse energy for 500, 250 kHz at a fixed oscillator pump energy and twoseparate stage 1 amplifier pump energies.

25

Time [ns]0 50 100 150 200 250

Pho

tode

tect

or V

olta

ge [V

]

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

D-8260

Figure 13. Fast time response trace of a single pulse from 250 kHz data.

200

100

10100 1000

PRF (kHz) D-8133

Illum

inat

ed C

ircle

Dia

m (

cm) Phase II Scale

Performance

Phase IBaseline Design

f/33High Speed

Infrared 2481 Film

ER = 350 mJ 500 kHzPRF

40

Figure 14. Object dimension scaling with individual pulse energy.

26

Figure 15. Photograph of a reconstructed shadowgraphic hologram.

Figure 16. Holographic subject.