A small, all solid state, regenerative ring amplifier designed as a prototype for space application is discussed. Novel features include dual side pumping of the Nd:YAG crystal and a triangular ring cavity design which minimizes the number of optical components and losses. The amplifier is relatively small (3 ns round trip time) even though standard optical elements are employed. The ring regeneratively amplifies a 100 ps single pulse by approximately 10(exp 5) at a repetition rate of 10 to 100 Hz. The amplifier is designed to be injection seeded with a pulsed, 100 ps laser diode at 1.06 microns, but another Nd:YAG laser system supplying higher pulse energies was employed for laboratory experiment. This system is a prototype laser oscillator for the Geoscience Laser Ranging System (GLRS) platform. Results on measurements of beam quality, astigmatism, and gain are given

Coyle, D. Barry

Degnan, John J.

Kay, Richard B.

Krebs, Danny J.

Seery, Bernard D.

English

NASA Technical Reports Server

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NASA-CR-19alTI
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InjectionSeeded,DiodePumpedRegenerativeRin_Nd:YAGAmplinerfor P--" /6_
Spaceborne Laser Ranging Technology Development
D. Barry Coyle and Richard B. Kay
The American University
Department of Physics
4400 Massachusetts Ave. N.W.
Washington, D.C. 20016
John J. Degnan
NASA-Goddard Space Flight Center
Crustal Dynamics Project, Code 901
Greenbelt, MD 20771
Danny J. K_rebs and Bernard D. Seery
NASA-Goddard Space Flight Center
Photonics Branch, Code 715
Greenbelt, MD 20771
Short Title: Injection Seeded Regenerative Ring Amplifier
Classification Number: 4260B
(NASA-CR-192171) INJECTION SEEDED,
DIODE PUMPED REGENERATIVE RING
Nd:YAG AMPLIFIER FOR SPACEBORNE
LASER RANGING TECHNOLOGY
DEVELOPMENT (American Univ.) 16 p
N93-20298
Unclas
G3/36 0145591
https://ntrs.nasa.gov/search.jsp?R=19930011109 2020-03-17T07:57:33+00:00Z
Abstract
We report on a small, all solid state, regenerative ring amplifier designed as a
prototype for space application. Novel features include dual side pumping of the
Nd:YAG crystal and a triangular ring cavity design which minimizes the number of
optical components and losses. The amplifier is relatively small (3 ns round trip time)
even though standard optical elements are employed. The ring regeneratively
amplifies a 100 ps single pulse by - l0 s at a repetition rate of 10 to 100 Hz. The
amplifier is designed to be injection seeded with a pulsed, I00 ps laser diode at 1.06
_tm, but another Nd:YAG laser system supplying higher pulse energies was employed
for laboratory experiment. This system is a prototype laser oscillator for the
Geoscience Laser Ranging System (GLRS) <'_ platform. Results on measurements of
beam quality, astigmatism and gain are given.
1. Introduction
The concept of regenerative amplificaiton of fast laser pulses has been around for some
timeJ 2>'C3)The work of Lowdermilk and Murray examined coupling problems and
charaterized the performance of a linear Nd:YAG regenerative amplifier. (4) Most
regenerative amplifier work has been directed at amplification of mode-locked pulses
from pulsed or cw oscillators. The more recent research has concentrated on increased
repetition rates, higher energies, faster pulses and the use of different gain
media, cs)'¢6)_7)'_8)We are ultimately interested in amplifying a single ~100 ps, 1.06 gm
diode laser generated pulse in the ring amplifier to effect an all solid state laser
"oscillator". This letter reports on the development of the regenerative ring amplifier
portion of this "oscillator", seeded by a 170 ps pulse from a mode locked linear laser.
2. Regnerative Amplifier Design
The ring configuration shown in figure 1 was chosen over conventional two mirror
cavities because fewer optical components and only one electro-optical polarization
switch are required. Other advantages of a ring configuration are reduced sensitivity to
feedback, reduced damage sensitivity due to no overlapping in the elements, and since
there are no standing waves, there is no spatial gain hole burning. The base side of the
isosceles ring triangle is 45 cm long and the round trip time is 3 ns. The angles were
chosen so that the s polarization would be incident on the thin film polarizer (TFP) at
an angle ~57 ° for a reflective loss of about 1% per pass at this element.
The optically isolated, p polarized, seed pulse enters the ring through the TFP, passes
through the gain medium around to the pockels cell switch, SW1, which has a )d2
voltage holding off any lasing action. On the first pass SWI remains on and the p
polarized beam is rotated 90 °. Now an s polarized wave, the pulse is reflected by the
TFP and again passes through the gain medium and on to SWI which now has zero
potential. The wave now remains in the s polarization state until it has made the
required number of passes. When SW1 is turned on, the amplified pulse is rotated
back to the p state and escapes the cavity via the TFP. The cavity losses are calculated
to be 5-6% based on the measured individual losses of the components.
3. Diode Pumped Amplifier Head
A view of the pump head is shown in figure 2 A pair of 1 cm wide diode arrays pump
a maximum of 40W each of 809 nm light in 230 gts pulses into the sides of the laser
crystal. The highly divergent pump radiation is collimated towards the sides of the
Nd:YAG crystal by 2 mm diameter cylindrical rod lenses. The input sides of the
crystal are antireflection (AR) coated at the pump wavelength and the opposite sides of
the crystal are coated for maximum reflectivity. The ends of the crystal are AR coated
at 1.06 _rn.
The transfer efficiency (tit) from the pump diodes into the active medium is calculated
at 75% for this configuration, including the overlap of the 809 nm pump bandwidth
with the Nd:YAG absorption band. The stored energy in the upper level can be shown
to be:
(1) E,, = _ "l"lr'\l:D)
4wherethefirst termis thecolor efficiencyandthesecondis thetransferefficienc3/9_.
The timefactorsgive thefractionof thepumppowerat theendof thediodepump
pulse(ZD)whenthis pulseis finite relativeto thespontaneouslifetime ('_,v)of the
lasinglevel. A fill factor (riB) has been added for the fraction of excited medium
which can couple to the TEMoo mode. [In general, we follow the terminology of
reference 9. The D subscript will refer to the diode and L to the laser transition of
upper lasing level.] Our pump pulsewidth is equal to the spontaneous lifetime for the
upper lasing level (230 Its) and assuming a fill factor of 0.7, a stored energy efficiency
of 25% is obtained through equation 1. Due to the fact that the upper "level" in
Nd:YAG is a doublet at room temperature, the effective stored energy E'_, is but 40%
of the stored energy, as calculated using equation 1, or 2.84 rnJ per pump cycle. The
above value of stored energy yields a small signal gain coefficient go of 0.35 cm _.
Assuming a total round trip loss ~ 6% and active gain medium of 1 cm, a round trip
gain of F = 1.33 is projected.
The ring cavity was Q-switched to characterize its performance prior to injection
seeding. This was done by using the TFP in conjunction with a _4 pockels cell which
required a hold-off voltage of approximately 900 V. In order for the sliced pulse from
the FLP laser to be trapped in the ring, the electrical and optical timing associated with
Q-switching the ring laser must first be known. The unseeded Q-switched ring laser
revealed the energy build-up time to be a relatively long 1.5 Its as expected, due to the
low single-pass gain of the cavity. The pulsewidth varied between 400 ns and 500 ns
depending on the pockels cell alignment.
A flashlamppumped,Nd:YAG laserwith anactive-passivemode-lockedsystemwas
usedastheinjection seedsource. An electro-optic pulse slicer, consisting of a X/2
pockels ceil between a pair of prism polarizers, was positioned after the output coupler
to select the center pulse from the mode-locked train. The selected pulse was then sent
into a 50% beam splitter where half the energy was directed toward a photodiode for
the ring's Q-switch trigger and the other half reserved for injection seeding.
In order for the ring pockels cell SWl to trigger with the arrival of the seed pulse, an
optical delay was required prior to injection through the TFP. The internal trigger
delay of SWI was measured to be more than 80 ns. An optical cavity similar to a
White Cell (1°)was constructed to create a total optical path length of N93 ft from 31
passes within the 3 mirrored cavity. This produced an optical delay of ~ 97 ns.
The spatial profile of the seed pulse required variability when it was injected into the
ring. This was adjusted by employing a pair of positive lenses on a 3-axis stage to
focus the beam in the ring and two 100% reflective fiats for beam steering. The 3-part,
injection seeded laser system, composed of the injection laser, optical delay, beam
steering optics and ring cavity is shown in figure 3.
4. Injection Seeded,Q-Switched Ring Laser Results
When the injected beam neared coalignment with the ring cavity, the Q-switch pulse
began to display defined temporal structure on the oscilloscope due to the reduction of
longitudinal modes, and the build-up time decreased. By monitoring these parameters,
the alignment was optimized. An oscilloscope plot in figure 4 shows 2 pair of
photodiode outputs; the top pair are the counter clockwise CCW and clockwise CW
6Q-switch pulses in the ring prior to injection seeding, and the bottom pair are after
injection seeding. The CW circulating energy was nearly extinguished when seeding
in the CCW direction. The initial injection energy was approximately 175 pJ in a 170
ps wide pulse. The beam diameter was about 33 _tm as it entered the TFP and had a
slight divergence, less than 1°, which was not precisely measured. By spreading the
injection beam into a cone, a solid angle section of the diverging beam mode-matched
with the ring's resonant single spatial mode. This helped decrease the coalignment
sensitivity requirements typical of a regenerative amplifier. Because the electronics
developed for the multi-purpose pockels cell induced a relatively slow fall time for the
Q-switch, timing was found not to be as critical as expected. If the seed pulse entered
the ring during the first 40 ns of the now longer 100 ns Q-switch voltage fall-time,
successful seeding was achieved.
Before attempting to dump the seeded Q-switched pulse, the injected pulse's temporal
shape had to match the circulating pulse in the ring. The seeded ring pulse had
parasitic pulses riding on the original sub-170 ps seed pulse from the injection laser.
These parasitics were approximately 200 ps apart and increased the overall pulse
envelope to more than 500 ps. An etalon effect was discovered within the pockels cell,
which proved to be the source of these pulses. Since the injection seed laser was not
assembled for sub-100 ps pulse production, the pockeIs ceil was rotated by 2° around
the horizontal axis, perpendicular to the ring's optical axis, which eliminated the etalon
effects. Although the pulse temporal purity was regained, we lost the optimum
electro-optical efficiency of the cell and some passive birefringence losses arose.
7Successful seeding was restored but with longer build-up times and greater Q-switch
pulsewidths. The injection seeded ring cavity's dumped output is shown in figure 5.
The threshold energy for injection seeding was measured as low as Est = 2 p J, but
there was an unseeded Q-switch pulse oscillating in the non-seeded CW direction of
almost equal intensity. Most of the amplification data was taken with seed pulse
energies of Es = 330 pJ to provide full CCW seeding with almost no detectable CW
oscillations. In the seeded but undumped case the energy in the cavity was measured
by the leakage of one the mirrors to be Eud = 4.0 gJ at the peak. This represented a
total gain of Gtot = 1.21 x 10 4 , or Gtot = 40.8 dB . When the injection seeded ring
laser is operated in its pulse dumping mode, the output energy in the 170 ps pulse is Ep
= 1.3 gJ, representing a gain of Gau,,p = 3.79 x 103 , or Ga_,,,p = 35.8 dB . The optical
efficiency was measured at Eff = 0.05% and Eff = 0.16% for the dumped and
undumped ring, respectively. These were much lower than expected but are attributed
to the small beam volume within the laser crystal and the necessary inefficient
alignment of the pockels cell.
5. Conclusions
We have demonstrated a diode array pumped, regenerative ring amplifier consisting of
only 5 optical elements, 2 mirrors, 1 TFP, 1 Nd:YAG crystal and 1 pockels cell. The
pockels cell performed both as a Q-switch and a cavity dumper for amplified pulse
ejection through the TFP. The total optical efficiency was low principally due to the
low gain provided by the 2-bar pumped laser head and the 6% loss per round trip.
After comparison with a computer model, a real seed threshld of-I 0 15 J was achieved
because only about 0.1% of the injected energy mode-matched with the ring. Much
improvement can be made upon the total performance by increasing the gain-per-pass
with higherpumppowers. This will alsoleadto betterextractionefficiencywhile
decreasingtheQ-switchbuilduptime. A shorterinjectionpulseor greateranglular
displacementon thepockel'scell windowscouldreducetheetaloneffectsanddecrease
thelossdueto birefringence.
Acknowledgements
This research was performed at the Photonics Branch - Code 715 at NASA's Goddard
Space Flight Center in Greenbelt, Maryland under government grant NSG-50333 in
association with The American University, Department of Physics. The authors wish
to thank Mr. Paul Weir for the use of his opto-mechanical design skills.
Figure Captions
Figure 1. Injection seeded ring cavity configuration: 2 mirrors, 1 thin film polarizer, 1
pockel's cell and diode pumped laser head. The pockel's cell is a two crystal design to
lower the required 1/2 wave voltage.
Figure 2. Optical axis view of the Nd:YAG laser head. A 2 mm x 2 mmx 10 mm
Nd:YAG slab is pumped by a pair of 40 W linear diode arrays which are collimated by
2 mm diameter fused silica glass rods. The slab is dieIectrically coated to provide a
double-pass pump geometry.
Figure 3. Schematic of the injection seeded laser system. The three sections from
right to left are: Q-switched mode-locked laser injection seed source, optical timing,
and the ring cavity.
Figure 4. Unseeded (lower trace pair) and seeded (top trace pair) operation of the
diode pumped ring cavity. Note: the Q-switch buildup time is significantly shortened
when the ring is seeded and the resulting longitudinal mode structure is noticable. This
is the single pulse circling the cavity.
Figure 5. Photodiode output of the ejected regeneratively amplified pulse from the
ring cavity. The small structure before and after the pulse shows the individual round
trips. It also shows that the amplified pulse was not completely ejected. This was due
to the necessary misalignment of the pockels cell which hindered its performance.
References
(') S.C. Cohen, J. J. Degnan, J. L. Bufton and J. B. Abshire 1987 1EEE Trans.
ErscL Remote Sensing GE-25 5. [See also J.L. Dallas, J.P. Czechanski, D.B. Coyle,
B.J. Zukowski and B.D. Seery 1990 1GARSS.]
(2) P.A. Belanquer and J. Boivin 1974 Phys. Can. 30 47.
(3) E.I. Moses, J. J. Turner and C. L. Tang 1976 Appl. Phys. Lett. 28 258.
(4) J.E. Murray and W. H. Lowdermilk 1980 J. Appl. Phys. 51 7 3548-3555.
(5) I.N. Duling III, T. Norris, T. Sizer II, P.I Bado and G. A. Mourou 1985 Jr. Opt.
Soc. Am. B. 2 616.
(6) P. Bodo, M. Bouvier and J. Scott Coe 1987 Opt. Lett. 12 319.
(7) Li. Yan, Jun-Da Ling, P.-T. Ho, Chi H. Lee and G. L. Burdge 1988 IEEEJ.
Quant. El. 24 41.
(s) Muhammad Saeed, Dalwoo Kim and Louis F. DiMauro 1990 Appl. Opt. 29
1752.
(9) See Sect. 3.4.2 "Laser Diode Pumped Oscillators" in W. Koechner 1988
Solid-State Laser Engineering, second edition, Springer Series in Optical Sciences
Springer-Verlag [Equation 1 is Koechner's equation 3.76 using his equation 3.75 for
P,_PD and modified for a diode pulse that is not long compared to the spontaneous
decay time of the upper laser level.]
(1o) J.U. White 1942 JOSA 32 285-288.
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Injection seeded, diode pumped regenerative ring Nd:YAG amplifier for spaceborne laser ranging technology development

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