Power Amplifier System for High-Power, Passively Q-switched Micro laser

I. INTRODUCTION1.9%-doped Nd:YAG, a 0.75-mm layer of
Passively Q-switched microchip lasers are simple,Cr:YAG (6 cm-1), and an 80%R output
compact and reliable sources of high repetition rate (1facet. The pulse durations calculated for the three
to 100 kHz), near-infrared, sub-nanosecond pulses. Todesigns were 304, 204 and 200 ps, respectively. We
date, low-energy (0.3 to 3 mJ/pulse) and mid-energybelieve that pump-light-induced bleaching is one of the
(30 to 180 mJ/pulse) microchip lasers have beenreasons for increasing pulse durations [5].
reported [1] with pulse durations of 200 to 500 ps andThe 0.75-mm Cr:YAG microlaser was used to
650 to 2000 ps, respectively. For some applications, forconstruct the oscillator - double-pass-amplifier system.
instance, high precision ranging and imaging,The microchip laser was quasi-cw pumped with a
higher-energy pulses, up to 360-440 mJ/pulse [2, 3] are1.1-watt peak-power pulse, at a 2 kHz pulse rate. The
required with pulse durations approaching 200 ps. Thewidth of the diode pump pulse was adjusted to ensure
pathway to higher pulse energies requires the use ofsingle-pulse oscillation even when the double-pass
higher saturable absorption, which inevitably leads toamplifier was on. A typical oscilloscope trace of the
longer pulse durations. An alternative approach is theoscillator pulses is shown in Fig. 3.
use of a MOPA design with a microlaser oscillator andWith 6.4 mW of microchip laser power, double-pass
multipass amplifier [3]. In the only reported work to dateamplifier power was about 670 mW at an amplifier
on this approach, a MOPA system operated in thecurrent of 31A. The beam quality of the
10-m J range, producing 500-ps pulses [4].double-pass-amplified beam was measured (by the
We present here a MOPA system generating 335 mSpiricon M2 meter) using the "90/10 knife-edge"
J at 1064 nm with efficient harmonic conversion to themethod. M2 in the horizontal and vertical planes was
visible and UV. At all wavelengths, the pulse durationsmeasured to be 1.38 and 1.28, respectively. The pulse
were <400 ps. This source is a valuable tool fordurations decreased to 370 ps as compared to ~
applications that include ranging, LIDAR, micro-materials440-ps from the microlaser.
processing, and UV spectroscopy in chemistry and 
biochemistry. 
II. SYSTEM DESCRIPTION AND EXPERIMENTALFig. 3. Oscilloscope trace of 440-ps oscillator pulse.
RESULTS 
                Figure 1 shows a 
schematic layout of our MOPA system. A 1-watt, 
fiber-coupled diode laser operated at a 2 kHz pulse 
rate is used as a pump source for the Cr:YAG 
passively Q-switched Nd:YAG microlaser. Pump lightTable 1. Microlaser characteristics.
emerging from the 100-m m, 0.22 NA fiber is collectedMicrolaser parameters
and focused into the microchip using two AR-coatedMicrolaser # 1, 4:3 telescope
aspheric lenses. The fiber ispositioned at the front focalMicrolaser # 2, 2:1 telescope
plane of the first lens, and the microchip at the nominalMicrolaser # 3, 4:3 telescope
back focal plane of the second lens. Two differentAverage power, mW
"telescopes" were used to optimize the microlaser4.4
output: a 4:3 or a 2:1 reducing telescope. Approximately3.1
98&percnt; of the light emerging from the fiber is6.4
delivered to the microchip when using either telescope.Pulse energy, m J
The microchip laser output is collected by a spherical2.2
50-mm FL lens. This lens is typically positioned about1.55
63 mm from the microchip so the beam is gradually3.2
focused into the amplifier stage. The beam thenPulse width, FWHM, measured, ps
passes through a TGG Faraday isolator equipped with700
input and output Glan-laser polarizers.400-440
A half-wave plate positioned before the collimator lens400-440
adjusts the polarization angle of the microchip beamDelay, m sec
as it enters the first polarizer of the isolator. A second90
half-wave plate adjusts the polarization angle of the40
beam emerging from the second polarizer. The beam70
is turned 90° by the first 45° -incidence HR (45 HR),Pump pulse width, m sec
goes through a +150 mm FL cylindrical lens that120
focuses in the vertical plane, and bounces off the60
second 45 HR, before entering the 3-pass amplifier120
stage.Jitter, ns
The cylindrical lens is positioned about 150 mm from± 100
the center of the amplifier slab, taking into account the± 100
fact that the beam makes three passes through the± 100
slab (the separation between the slab assembly’sDrift, 5 min, ns
miniature fold mirrors is about 20 mm, and the slab is± 300
about 15 mm long). The beam is back-reflected± 200
through the amplifier with a flat HR , and makes± 200
another 3 passes through the amplifier slab.We have also conducted experiments on nonlinear
The back-reflected, double-pass amplified beamconversion of the amplifier beam. For second harmonic
passes back through the optical system and into thegeneration (SHG) we used a
Faraday isolator. The plane of polarization at the firstnon-critically-phase-matched, Type I LBO crystal, with
polarizer is now rotated 90° relative to the microchipdimensions of 3 x 3 x 15 mm, mounted in a 1700 C,
laser polarization. The double-pass-amplified beam istemperature-stabilized oven. Third harmonic generation
coupled out the system at the first polarizer, and(THG) at 355 nm was accomplished with a
emerges with a polarization vertical to the plane of theroom-temperature, 3 x 3 x 12 mm, Type II
paper.critically-phase-matched LBO crystal (q = 42.7° , f =
 90° ). And, finally, for fourth harmonic generation
 (4HG) at 266 nm, a Type I critically-phase-matched,
Fig. 1. Schematic layout of the Microlaser-Amplifierroom-temperature BBO crystal (q = 47.6° , f = 0° ),
system.3 x 3 x 7 mm crystal was used. The beams were
Our amplifier gain material, Nd:YVO4, is particularly wellseparated using a Pellin-Broca prism. At input power of
suited for amplifying pulses with energies below 100 m670 mW, the output power of SHG, THG and 4HG
J because of its extremely high gain. This iswas 400, 240 and 86 mW, respectively, which
demonstrated by Fig. 2, where we present calculatedcorresponds to ~ 60&percnt;, 36&percnt;, and
double-pass gain curves for cw-pumped multi-pass13&percnt; conversion efficiency.
slab amplifiers based on different Nd-doped materials.III. CONCLUSION
The amplifier design employs a slab-geometry gainWe have designed and constructed a highly efficient
module with transverse pumping. The gain modulediode-pumped, short-pulse, energetic, compact and
consists of an a-axis-cut, 2-mm high by 3-mm wide byreliable microlaser-amplifier system. This design
15-mm long Nd:YVO4 slab. The slab was cwapproach, we believe, will allow us to achieve even
side-pumped by two 20-W diode laser bars emitting atshorter (~ 200 ps) and higher-energy pulses, increase
808 nm, with top and bottom heatsinking. The sidethe conversion efficiencies of harmonic generation, and
faces of the slab are polished and antireflectionimprove the compactness of the system.
coated at 808 nm for maximum coupling of the pump 
light. The outputs of the diode laser arrays are 
collimated, in the highly diverging direction, by a drawn 
aspheric cylinder lens to produce a nearly rectangular 
excitation region in the laser crystal. The laser mode isREFERENCES
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region, using a pair of miniature external mirrors,lasers and applications," Rev. Laser Eng., v. 26, pp.
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 lasers," in OSA Trends in Optics and Photonics on
Fig. 2. Calculated double-pass gain curves forAdvanced Solid-State Lasers, v. 26, M. M. Fejer, H.
cw-pumped Nd-doped multi-pass slab amplifiers.Injean, and U. Keller (eds), (Optical Society of America,
Three different monolithic microchip oscillator designsWashington DC, 2007) pp. 178-186.
were evaluated, each having a different Cr:YAG layer3. J. J. Degnan, "Optimal design of passively
thickness. The first two designs were made byQ-switched microlaser transmitters for satellite laser
Synoptics according to Q-Peak specifications. Bothranging," in Proceeding of 10-th International Workshop
employ a 0.5 mm thick layer of 3&percnt;-dopedon Laser Ranging, Shanghai, PRC, 2006, pp. 334-343.
Nd:YAG. One chip design has a 0.25-mm-thick layer ofF. Druon, F. Balembois, P. Georges, A. Brun, "Compact
Cr:YAG with a nominal unsaturated absorption of 6high-repetition-rate pulsed UV sources using
cm-1. The output facet of the chip is coated fordiode-pumped microchip laser and multipass amplifier,"
80&percnt;R. The other chip design is the same,in Advanced Solid-State Lasers, OSA Technical Digest
except that the Cr:YAG layer thickness is 0.5 mm.(Optical Society of America, Washington DC, 2008),
Originally, we intended to try a similar third design, butpp. 329-331.
with a 0.75 mm Cr:YAG layer and a 60&percnt;RM. A. Jaspan, J. A. Russell, D. Welford, "Degradation of
output facet. However, this chip was not coatedpassively Q-switched microlaser performance due to
properly and was replaced with an off-the-shelf chippump-light induced bleaching of the saturable absorber,"
designed by Synoptics, which had features close toto be submitted to Opt. Letters.
what we desired. This chip has a 1.25 mm layer of