We demonstrated up to 2 W average power, CW-pumped, passively- Q-switched, 1.5 ns monolithic MCL with single-longitudinal mode-operation. We discuss laser design issues to bring the average power to 5-1 OW and beyond

Konoplev, Oleg A.

Krainak, Michael A.

Li, Steven X.

Seas, Antonios A.

Shaw, George B.

Stephen, Mark A.

Vasilyev, Alexey A.

Yu, Anthony W.

NASA Technical Reports Server

..... 
Multi-Watt Average Power Nanosecond Microchip Laser 
and Power Scalability Estimates . 
Oleg A. Konoplev•, Alexey A. Vasilyev•, Antonios A. Seal, Anthony W. Yu\ Steven X. Lib,'George B. 
Shaw\ Mark A. Stephen b, Michael A. Krainakb 
a Sigma Space Corporalion, 4400 Lottsford Y/.Jta Road, Lanham, MD, 20706 
b NASA Goddard Space Flight Center, wer and Electro-Opt/c3 branch, MC 554, Greenbelt, MD 2077 J 
Email: oleg.a.konoplev@nasa.guv 
Abstract: We demonstrated up to 2 W average power, CW-pumped, passively- Q-switched, 1.5 
ns monolithic MCL with single-longitudinal mode-operation. We discuss laser design issues to 
bring the average power to 5-1 OW and beyond. 
OCIS codes: (140.3480) lAscrs, diode-pumped; (140.3540) Lasers, Q-switcbed; (010.3640) Lidar. 
1. Introduction 
High average power, -10-20W, compact, efficient and reliable nanosecond sources, with repetition rate of lOKHz 
and good beam quality,M2- 1-1.5, spectral width of-10 pm or narrower are needed for space ranging applications 
[1 ]. Passively Q-switched microchip' lasers (MCLs) are one of the best candidates for current and future space 
ranging applications. MCLs are compact, efficient, monolithic, cavity-alignment-free, short cavity, narrow 
frequency sources for generation ofns to ps pulses with multi- 100 uJ pulse energies multi-kW peak powers (2,3] 
MCLs are currently in maturation stage. Their scalability to MW peak power and multi-Watt average power have 
been experimentally demonstrated. A 1. 7 MW-peak power was demonstrated in QCW operation from 
Nd:YAG/Cr4+:YAG microlaser. Up to 4 pulses separated by 100 us, with.spatial beam quality ofM2-I.2, 3mJ 
each, with pulse duration of 12 ns were generated at 10 Hz QCW regime (4]. With optimization in laser cavity 
design, average powers of2.6-4.2W with pulse durations 16-48 ns were reached in CW pumping mode [5]. It is 
worth to note that pulse repetition rate can be stabilized with addition of pump modulation to a sub l % r.m.s. jitter 
level[ 6]. In this report, we demonstrate a medium-power version of monolithic Nd:YAG/Cr4+:YAG MCL with 
pulse duration of 1.5 ns and average power up exceeding 2W with relatively good, Gaussian beam. We also discuss 
possibility of scaling the design to 10W of average power while maintaining good beam quality. 
2. MCL description and experimental results 
Microchip Luer output power, CW pumping. 
T_mlcrochlp s20 C, OpticaMo-Optfcal Efficiency a28% 
• l'_oul(W) 
2.5 - 0 l'_...~_T_,,. _ Pol.(• 0.260"P _In - 0.874 - 24 • t, (-. FWHII) R' • OU!le 
-i • lJIIOOClfl) 22 D~T I g- 2.0 _,._ (l..,..oot,Q) 20 JI - ,._/1' _Gl.(W)) ···- 18 ICI. 
5"D 1.5 18 
e, 
-t1 
O il 
H 
14 . , 
1.0 12 i 10 J 0.5 8 l. 
8 
o.o ....... ~-~--~~---~ ........ ~--~-, 
o 2 , e a w n 
Pump Input Power (W@ 808 nm, 200 um, 0.14 NA) 
Figure 1. Output Power (W), pulse duration, FWHM (ns) and repetitioo rate (kHz) of monolithic, passively Q-switched MCL versus Pump Input 
Power (W) in CW pumping conditions. At lOW of pwnp power, the pump spectrum was adjusted by changing pump heatsink temperature by 
I.SC. This is shown by open circle and open square datapoints for power and PRR. The spectrum of pump shifted by 0.5 nm and pump was 
coupled more efficiently. For all other datapoints pump was only varied in forward bias current. 
A monolithic microchip slab with 2x2xl6 mm3 dimensions was made by diffusion bonding technique, similar to the 
described in (2,3,7]. The slab consisted of two 2-mm undoped YAG outer sections and two 3mm and 4 mm 1.1 wt°/o 
doped Nd:YAG inner sections. The 5 mm-long saturable absorber (SA) was sandwiched in the center of the cavity, 
between two doped sections in order to maximize the probability of single-longitudinal mode operation. This also 
offers us the possibility of end-pumping slab from both sides and ability to spread thermal load along cavity length. 
So far, we pumped the slab only from one side. All results reported in this work are from one-side pumping. The 
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pump-side end-facet that contained undoped Y AG section and followed by 3mm Nd-doped Y AG section was 
HR1064/AR808 coated. Another facet was coated with reflectivity of 40% at 1064 run. We placed the slab 
symmetrically in 14-mm-long copper heatsink similar to [2]. The pump was 808 n!ll fiber-coupled diode laser. The 
fiber had diameter 200 um and NA=0.14. We focused pump power to approximately the center of 3-mm doped 
section with 1: 1 imaging optics. 
The experimental results for average power, Q-switched pulse duration and pulse repetition rate at CW pumping 
conditions are shown in Figure I. The temperature of the MCL heatsink was maintained at 20C±2C throughout all 
pump conditions. The 808 nm pump heatsink temperature was controlled with 0.25C accuracy. We observed 26% 
of optical-to-optical slope efficiency and overall optical-to-optical efficiency at or near 15% for I W output power 
with about 2% jitter. Operating MCL at 30C reduced power for about 10-15% or less. As thermal resistance of 
junction-to-heatsink of the pump laser was finite, when pump power was increased from 4W to IOW, the pump 
output was slowly drifted off the optimal wavelength. We made one re-adjustment at the 10W pump level to 
maximize MCL average power and pulse repetition rate (PRR). Pump temperature adjustment by I.SC improved 
MCL average output power from 1.9W to 2.IW, PRR increased from 16.5 kHz to 17.6 kHz. Pulse duration did not 
change. The pump-adjusted average power and PRR are shown with corresponding open circle and open square 
marks. The laser is capable of operating in CW mode up to 2.7W of average output power. 
The spatial and temporal profiles of the MCL at 1.1 W and at 2.1 W are shown on Figure 2. 
1.20 
'i 1.00 C ,. 
i 0.80 
f O.IIO i 
1i 
t 0 .-40 
r 0.20 ~ 
-+-t T+--t&•tt--~: 
- -~---- -----~---- ~---i j f l l i ! : 
--~---~ -~--~---~ ~---
1 ! ! I 2.~WO..C @l7.,l<Hz ; 1 
--r--
1
- -r--~-- 2'WX•7.9mm.2-wy.6.2lffll --: 
0.00 
5 W g ~ " ~ " ~ ~ ~ 5 ~ g ~ ~ ~ " -40 ~ ~ 
Tlme (ns) Tlme (ns) 
Figure 2. Spatial and temporal profile oflhe beam out ofMCL. Temporal profile was recorded with 12 GHz bandwidth system and represent an 
average of about 20 measurements. We have not observed any beating within the main pulse. Spatial (far.field) profile was recorded at 47 cm 
from the output facet of MCL. The temporal profile was recorded by detecting diffilsed (i.e. spatially integrated) reflection of the beam. 
We observed substantial afterpulse at the distance - 13-14 ns after the main pulse similar to the described in [7]. The 
afterpulse is believed due to the excitation of a higher transverse mode in the resonator cavity [7]. It is considered 
objectionable feature for LIDAR applications. It can be minimized or removed completely by careful pump-to-MCL 
mode matching [7]. It also can be further reduced through amplification of the MCL pulse in the gain-saturated 
amplifier and/or through nonlinear frequency conversion. Although overall conversion efficiency slope did not 
change when we ramp pump power to lOW, the main pulse energy grew much slower, from 65 uJ to 69.5 uJ when 
average output power increased from 0.58W @ 6.35kHz to 1.1 W @ 11.~ KHz. When output power increased from 
1.1 W to 2.1 W, the main pulse energy remained approximately constant (less than 1 % increase observed) at 70 
uJ/pulse level, while afterpulse contribution to total energy increased from 27% to 40%. We also observed 
shortening of main pulse length from 1.65 ns to 1.Sns and increase in far field divergence similar to [2,3J. At 1.1 W 
spatial quality of the beam, M2 estimated to be near - 1.1-1.3. Due to the observed temporal profile it is believed that 
radiation out of this MCL (main pulse) is single-longitudinal mode. Relevant tests are underway. More details about 
this MCL behavior will be published elsewhere. 
3. Scaling monolithic MCL to higher average and peak powers 
We limit our discussion to Nd;Cr4+;YAG MCLs. Although Yb:YAG -based MCLs are scalable, especially in peak 
powers, but not so easily in average power, this is outside the scope of this short presentation. The design of the 
considered MCLs can be further optimized. In current design we deposited about 75-85% of the pump energy within 
3mm of the 16 mm cavity. The sizeable part of the residual energy was deposited in 5mm long SA section which 
was positioned in the middle of the cavity. It is estimated that optimization of the cavity design and better thermal 
management can ramp up peak and average power by at least factor of two to three in assumption of maintaining 
material quality and low absorption and.scattering losses inside the cavity. We have been able to observe 10-20% · 
pulse energy increase with pump waist increase from 200 to 250 um. The laser was operated up to 7 kHz in CW 
mode with approximately the same - 1 W output power. When waist is increased, the threshold pump power is 
increased as well. As the result, MCL may start lasing at distorted conditions even near threshold. For any working 
MCL design there is a range of pumping conditions when lasing is stable and spatial and temporal quality of the 
... 
pulse are not compromised: The design presented here is capable of dissipating about 5-6W of pump power without 
thennal and nonlinear distortions but is capable of producing CW train of the Q-switched pulses up to - 10-I2W of 
CW pump power. 5-lOW output power laser must manage - 50-lOOW of pump power. This is considered feasible 
for mulit- KHz, I ns operation. Corresponding QCW operation was already demonstrated 
To be able to scale power of a MCL, the simple set of equations for pulse duration and energy of the pulse should be 
established. The ability to support CW pump operation should be verified through thennal modeling and thermal 
degradation measurement ofMCL components. 
The I-dimensional model ofMCL dynamics, including excited state absorption is well developed. (3,5]. Although 
not rigorous, the simplest scaling equation for MCL pulse duration that explicitly includes the most impo~t cavity 
parameters and matches well with experimental data was noted in [2] and re-emphasized in (8], 
3.52T" (l) tPWIIM•---
/!TSA 
where, T 11 is round-trip cavity time and ATsA is the modulation depth of the SA or differenc.e in single-pass 
transmission of the bleached state and closed state. The formula assumes that all non-SA intracavity losses are 
minimal. The Eq. (I) was extensively verified with semiconductor saturable absorbers [8]. We compared our design 
and 7 similar molonlithic Nd/Cr4+:Y AG designs (2,3] and found 30% or better agreement with experimental data 
for all 0.5-2 ns cavities. Only for 0.3 ns HPMCL-4 laser Eq. (I) underestimated the pulse duration by 45%. For our 
cavity, the calculated value is about 1.45 ns. With practically feasible SA modulation depth, the cavity made with 
various Y AG undoped or Y AG-doped materials can be as long as 20-25 mm to maintain the pulse within - 1-1.5 ns 
pulse duration. The gain medium can be distributed along- 15-18 mm length. This is about 5x larger than pump 
absorption length reported in this work. 
The second most important scaling parameter of the passively Q-switched MCLs is the energy of the pulse. The 
experimentally confirmed pulse energy matched well to [3] 
E • F.,..,Aef1*ln(Go) (2) 
P 0.623 
where Fm is the linewidth reduction adjusted saturation fluence in standing-wave resonator, A& is the effective 
beam area and Go is the single-pass gain at the onset of Q-switching. With saturation fluence ofNd:Y AG of -0.29 
J/cm2 and linewidth reduction factor of0.41, we get Fsat- 0.35 J/cm2. With l/e2 waist of the MCL of70 um [M2= 
1.14] and ln(Go) -1.2, we get estimated 70 uJ/pulse, which is in excellent agreement with experimentally observed 
values. The 2-3x larger pump waist an~ MCL waist are probably feasible in QCW regime as lasing threshold scales 
with square of the pump waist Spreading pump power deposition over 10-15 mm distance and using 400-500 um 
pumping we expect to bring single pulse power to 0.4-0.5 mJ and average power to 3-5W while maintaining pulse 
duration and beam quality. Further optimization of the cavity and thermal design improvements may create another 
3dB boost in average power scaling. 
In conclusion we demonstrated 1.5 ns monolithic MCL capable of emitting powers in excess of2W in CW 
pumping conditions at PRR-10-15 kHz. We matched our theoretical predictions with experimental data and 
discussed possibility of increasing average pump power to 5-IOW level from a single monolithic MCL chip. 
4. Acknowledgement 
This work was supported under NASA contra~t NNG09CP09C for ICESat-Il ATLAS instrument. 
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Multi-Watt Average Power Nanosecond Microchip Laser and Power Scalability Estimates

Multi-Watt Average Power Nanosecond Microchip Laser and Power Scalability Estimates

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