A high energy, single mode, all solid-state Nd:YAG laser primarily for pumping an UV converter is developed. Greater than 1 J/pulse at 50 HZ PRF and pulse widths around 22 ns have been demonstrated. Higher energy, greater efficiency may be possible. Refinements are known and practical to implement. Technology Demonstration of a highly efficient, high-pulse-energy, single mode UV wavelength generation using flash lamp pumped laser has been achieved. Greater than 90% pump depletion is observed. 190 mJ extra-cavity SFG; IR to UV efficiency > 21% (> 27% for 1 mJ seed). 160 mJ intra-cavity SFG; IR to UV efficiency up to 24% Fluence < 1 J/sq cm for most beams. The pump beam quality of the Nd:YAG pump laser is being refined to match or exceed the above UV converter results. Currently the Nd:YAG pump laser development is a technology demonstration. System can be engineered for compact packaging

Hovis, FLoyd

Prasad, Narasimha S.

Singh, Upendra N.

NASA Technical Reports Server

High energy, single-mode, all-solid-state 
and tunable UV laser transmitter
Narasimha S. Prasada, Upendra N. Singha, Floyd Hovisb
aNASA
 
Langley Research Center,
5 N. Dryden St., MS 468, Hampton, VA 23681
bFibertek, Inc.,
510 Herndon Parkway, Herndon, VA 20170
CLEO/Europe-IQEC Conference
June 17-22, 2007
World of Photonics Congress 2007 in the International 
Congress Centre Munich, Germany
Paper Session Code: CA5-4-TUE 15:30
https://ntrs.nasa.gov/search.jsp?R=20080013373 2019-08-30T04:10:48+00:00Z
2Laser Risk Reduction Program (LRRP)
• NASA began Laser Risk Reduction Program 
(LRRP) in 2002 to develop reliable, robust, and 
compact laser technologies for lidar applications 
from space based platforms
– Program: Joint operation of Langley Research 
Center and Goddard Space Flight Center
– Goal: 1 micron and 2 micron lasers and 
wavelength conversion technology
– Applications: Four Lidar Techniques-altimetry, 
Doppler, Differenctial
 
Absorption Lidar 
(DIAL), backscatter lidar
– Measurements: 6 priority Earth Science measurements:
(1) Surface and ice mapping, (b) Horizontal vector wind 
profiles (3) Carbon-di-oxide (CO2
 
) profiles (4) Ozone 
(O3
 
) profiles(5) Aerosol/clouds and (6) River currents
3UV Task Objectives
• The objective of the UV Task is to develop an efficient,  all-solid-state, diode 
pumped, conductively cooled, single longitudinal mode and high energy 1-
 micron to UV wavelength conversion technology
• The emphasis is to generate UV wavelengths of 308 nm and 320 nm for ozone 
sensing using DIfferential Absorption Lidar (DIAL) technique from space
• Performance Goals:
– Output energy at UV wavelengths:
 
≥
 
200 mJ
– Pulsewidth: 10 -
 
2
 
5 ns
– PRF:
 
50 Hz
• High pulse energy allows enhanced performance during strong 
daylight conditions
• UV Task is a collaborative effort  among Sandia National Labs, Fibertek, 
and NASA LaRC
4Technical Approach to UV generation
• Basic Scheme comprises of a Nd:YAG laser pumped nonlinear 
optics based converter comprising of a second harmonic 
generation (SHG), optical parametric oscillator, (OPO) and sum 
frequency generation (SFG) processes
1064 nm SHG 532 nm 
532 nm OPO 803 nm +1576 nm;  731.5 nm + 1950 nm
SFG532 nm + 803 nm 320 nm 
532 nm + 731.5 nm SFG 308 nm 
5UV Wavelength Conversion 
-Experimental Results-
• The nonlinear optics based technology to efficiently generate UV 
wavelengths has been established using a flash lamp pumped 
Nd:YAG laser
• The scheme utilizes a novel (Rotated Image Singly Resonant 
Twisted RectAngle) RISTRA OPO to generate 803 and 731.5 nm 
wavelengths pumped using a 532 nm pump source
• A type-I BBO crystal is used in the RISTRA OPO and a LBO 
crystal is used for SFG 
• Single mode operation is obtained through pulsed seeding technique 
with temporally matched pump and idler pulse profile
• Pulse idler seeding is obtained by a tunable laser diode and RISTRA 
OPO in tandem as seed sources
• For 803 nm 
• A small or low energy RISTRA OPO that is locked by Pound-Drever-Hall 
(PDH)  technique and seeded by New Focus tunable diode laser operating at 
803 nm
• The 1.5x scaled big RISTRA OPO that is pulse seeded at 1576 nm from the 
small OPO and locked by energy stabilization technique
6Latest Results on the UV conversion
• State-of-the-art conversion efficiencies have been 
demonstrated using a flash lamp pumped 
Nd:YAG laser with a round top-hat profile
RISTRA OPO Module
¾ Greater than 90 % pump depletion obtained
¾ At 320 nm, >200 mJ extra cavity SFG with  good beam 
Quality
 IR to UV efficiency > 21% (27% for 1 mJ seed)
¾ At 320 nm , up to 160 mJ intra-cavity SFG
 IR to UV efficiency up to 24%
¾ Fluence ≥1 J/cm2 for most beams
7Solid-State Nd:YAG Pump laser
• For future space applications, an all solid-state, 
diode pumped Nd:YAG pump laser has been 
developed in collaboration with Fibertek, Inc.
– The pump laser is an upgrade of ~300 mJ/pulse 
Nd:YAG laser developed under NASA funded 
ATIP program
– Two amplifiers have been added to the NASA 
ATIP laser to achieve up to 1.2 J/pulse
8Nd:YAG Pump Laser 
-Summary of Technical Approach-
An all solid-state diode-pumped laser transmitter featuring:
y Injection seeded ring laser Improves emission brightness (M2)
y Diode-pumped zigzag slab amplifiers Robust and efficient design for use 
in space 
y Advanced E-O phase modulator material Allows high frequency cavity 
modulation for improved stability 
injection seeding 
y Alignment insensitive / boresight Stable and reliable operation over
stable 1.0 μm cavity and optical bench environment
y Conduction cooled Eliminates circulating liquids w/in 
cavity
y Space-qualifiable component designs Establishes a path to a space-based 
mission
9Single Frequency Laser Ring Laser Design
Optical Schematic
Design Features
y Near stable operation allows trading beam quality
against output energy by appropriate choice of
mode limiting aperture
−30 mJ TEM00 , M2 =1.2 at 50 Hz−30 mJ TEM00 , M2 =1.3 at 100 Hz−50 mJ square supergaussian, M2 = 1.4
at 50 Hz
y Injection seeding using an RTP phase modulator
provides reduced sensitivity to high frequency vibration
y PZT stabilization of cavity length reduces sensitivities to 
thermal fluctuations
y Zerodur optical bench results in high alignment and 
boresight stability
1.  Reverse wave suppressor
2.  Cube polarizer
3.  Odd bounce slab
4.  Steering wedge
5.  λ/2 waveplate
6.  Mode limiting aperture
7.  RTP phase modulator
8.   45° Dove prism
9.   Non-imaging telescope
10.  RTP q-switch
1         2                           3                        4          5        6          2
2           4                   9            5          8       5         7             2
5
10
Seed 
Final Zerodur Optical Bench (12cm x 32cm)
10
Amplifier Design Configuration
Dimensions 6.8 x 13.0 x 75.3 mm3 
Incident Angle Near Brewster (57˚)
Extraction 100% at full aperture
Aperture 11.5 x 6.8 mm2 (internal)
7.1 x 6.8 mm2 (external)
Doping Level 0.5 ± 0.1 % Nd3+ 
Pump Diodes 192 ea. 50 watt QCW bars
(12 ea. 16 bar arrays)
3 Bounces-Rectangular Shape-2 sided pumping in the TIR axis, 
2 sided conduction cooling, Pump faces uncoated (~10%loss)
11
Final System Configuration
Optical layout
Diode Bars and slabs are 
conductively coupled to the 
heat sink. 
For space applications, one 
can use heat pipes or 
radiators
Breadboard layout
12
Amplifier Upgrade 
2-Sided Pumped & Cooled Amplifier
Dual Stage Amplifier Modeling
Model is based  on Franz-Nodvic result for a 
amplifying a square (in time) pulse
Model includes all key parameters explicitly
y Number of pump diodes (192)
y Peak diode power (75 W)
y Diode pulse width
y Input oscillator pulse energy (60 mJ)
y Input beam diameter
y Gain path length in amp
y Slab volume
Accounts for reduced gain for second pass
1 J per pulse output is predicted for 210 µs 
diode pump pulses
Dual 2-sided pumped amplifiers meet  the requirements of most
space-based direct detection wind lidars designs
13
Pump Laser Performance
Oscillator Configuration
y 100 µs pump pulse
y 55 W/bar
y 100 bars
Oscillator Output
y 50 mJ/pulse
y PRF = 50 Hz
y 0.41 cm x 0.41 cm
square beam
y M2 = 1.2
Amplifier Configuration
y Vary pump pulse width
y 55 W/bar
y 112 bars/amp
Peak Dual Amplifier Output
y 350 mJ/pulse
y M2 = 1.6
Input = 280 mJ
First Stage Output > 700 mJ
PRF =  50 Hz
Pulsewidth = ~16 ns
Spatial Mode         =Rect. Super
Gaussian
M2 ~ 2 
Optical Eff.            >11%
Wall Plug Eff.        >7%
Input = 700 mJ
Second Stage O/P > 1000 mJ
PRF =  50 Hz
Pulsewidth TBD
Spatial Mode         TBD
M2 TBD
Optical Eff .            TBD
Wall Plug Eff .        TBD
1
22 ns
SG
2.5
11%
7%
• The laser is now operational at 50 Hz PRF with maximum 
pulsewidths around 22 ns
• The output beam profile is rectangular super gaussian
14
Nd:YAG Pump Laser 
- Typical Output Characteristics -
Typical pulsewidth = 22 ns.  Max. Pulse Energy achieved = 1.2 J.
Electrical to optical efficiency >7% was achieved with only 58 W 
peak power per diode bar pumping the amplifiers.  
Near field beam 
profile of final 
amplifier output
Average power at 
50 Hz of 51.0 W  
(1020 mJ/pulse)
Conductive to diodes and 
slabsNA
Conductive to 
diodes and slabsCooling
Separate custom moduleNASeparate custom moduleElectronics
Ramp & fire
Single breadboard in 
custom enclosure
2.5
1040
Design/Performance
Ramp & fire
Single breadboard
NA
900
Specification
NASeeding
NALaser head package
2M2
1200Pulse Energy (mJ)
GoalParameter
15
Temporal Characteristics
Oscillator Only: 16.5 ns Oscillator + Preamp 1 + Preamp 2 : 19.5 ns
Oscillator + Amp 1 
+ Amp 2 : 20.9 ns
Full System:  
Pulsewidth ~ 22 ns
16
50 Hz, Full Power Beam Quality Measurements
Mx2
 
= 2.5, My2
 
= 2.5,
M2 data
Full System Results Beam Quality
17
Full Nd:YAG Laser Unit
• The dimensions of this laser unit, including a SHG module, is 34” x 22” x 8”
• With latest diode bars and modified opto-mechanical components, the above 
package can be reduced to less than a quarter of its size
18
Final System 
Control and Power Electronics
Custom power supplies and control electronics for the upgrade have been built
y Control electronics consists of two 19"rack mountable boxes
y All power supplies are contained in two 19" rack mountable power
supply modules
y Each amplifier can be individual set between high power and low power
operation to allow the user  to achieve a wide range of output powers at 50 Hz
Control electronicsSingle Power Supply Module
19
320 nm UV generation 
• Currently, we are generating a few mJ with limited pump 
energy of 280 mJ/pulse
– The elliptical beam allows reduced overlap inside the nonlinear crystal 
of RISTRA module hence reduces the conversion efficiency
20
Spatial fluence profile & RISTRA 
-
 
RISTRA OPO requires round, top-hat spatial pump profile -
OPO signal 
near-field 
spatial fluence 
profile, Fresnel 
Number > 450
Self-seeded 
oscillation 
in two-crystal 
RISTRA 
~85% pump 
depletion
Flat pump profiles have facilitated high pump depletion 
&hence high OPO conversion efficiency
Results Using refined Flash Lamp pump laser  Results Using Diode pumped Nd:YAG  laser  
Pump 
Beam 
at the 
Big 
OPO
Reduced 
Pump 
Depletion
21
On-Going Work
• Improve the Beam Quality of the Diode Pumped Nd:YAG 
Laser
– The goal is to achieve a Round, Top Hat spatial fluence profile with 
wavefront aberration less than 0.5
• Refinements to the ring oscillator cavity, pre amplifiers and amplifiers of the 
diode-pumped Nd:YAG laser to improve beam quality and reduce pulsewidth is 
nearing completion 
22
Summary and Conclusions
• A high energy, single mode, all solid-state Nd:YAG laser primarily for 
pumping an UV converter is developed
• Greater than 1 J/pulse at 50 HZ PRF and pulsewidths around 22 ns have 
been demonstrated
• Higher energy, greater efficiency may be possible
– Refinements are known and practical to implement
• Technology Demonstration of a highly efficient, high-pulse-energy, single 
mode UV wavelength generation using flash lamp pumped laser has been 
achieved
– Greater than 90% pump depletion is observed
– 190 mJ extra-cavity SFG; IR to UV efficiency > 21% (>
 
27% for 1 mJ seed)
– 160 mJ intra-cavity SFG; IR to UV efficiency up to 24%
– Fluence ≤
 
1 J/cm2 for most beams
• The pump beam quality of the Nd:YAG pump laser is being refined to 
match or exceed the above UV converter results
• Currently the Nd:YAG pump laser development is a technology 
demonstration
– System can be engineered for compact packaging


High Energy, Single-Mode, All-Solid-State and Tunable UV Laser Transmitter

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