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CryoYb:YAG-1DJR 04/21/23
MIT Lincoln Laboratory
Cryogenically cooled solid-state lasers: Recent developments and
future prospects *
T. Y. Fan, D. J. Ripin, J. D. Hybl, J. T. Gopinath, A. K. Goyal, D. A. Rand, S. J. Augst, and J. R. Ochoa
MIT Lincoln Laboratory
* This work is sponsored by the Missile Defense Agency’s Airborne Laser Directorate, DARPA, and HEL-JTO under Air Force contract number FA8721-05-C-0002. Opinions, interpretations, conclusions, and recommendations are those of the authors, and are not necessarily endorsed by the United States Government.
MIT Lincoln LaboratoryCryoYb:YAG-2DJR 04/21/23
Outline
• Cryogenic laser background
• The case for power scalability and high efficiency in Yb lasers
• Laser demonstration results
• Summary
MIT Lincoln LaboratoryCryoYb:YAG-3DJR 04/21/23
Motivation
• Goal: Many laser applications require:– High average power
– Near-diffraction-limited beam quality
– Low weight and volume
– Low cost
• Challenge # 1: Average power and beam quality of solid-state lasers is generally limited by thermo-optic effects
– Thermo-optic distortion
– Thermally induced birefringence
• Challenge # 2: Cost, size, and weight of solid-state laser systems are generally limited by low efficiency
– Lower efficiency systems require more pump lasers, larger power supplies, and larger cooling systems
Cryogenic solid-state lasers can effectively address these challenges
MIT Lincoln LaboratoryCryoYb:YAG-4DJR 04/21/23
Approaches to Generate High-Brightness from Solid-State Lasers
• Optimize gain-element geometry for low thermo-optic distortion– Thin-disk, slab lasers
• Compensate for thermo-optic distortion outside of gain element– Deformable mirror driven by feedback loop
– Phase-conjugate mirror to reverse phase distortions
• Guide beam to maintain beam quality while spreading heat– Fiber, waveguide lasers
• Combine multiple lower-power lasers– Coherent or wavelength beam combining
• Ceramic materials to scale size, provide spatially varying properties
Cryogenic cooling is complementary to many other solid-state-laser power-scaling approaches
MIT Lincoln LaboratoryCryoYb:YAG-5DJR 04/21/23
Outline
• Cryogenic laser background
• The case for power scalability and high efficiency in Yb lasers
• Laser demonstration results
• Summary
MIT Lincoln LaboratoryCryoYb:YAG-6DJR 04/21/23
Materials Properties
• Values of thermo-optic properties of dielectric crystals substantially improve at lower temperatures for higher-power laser operation
– Higher thermal conductivity and diffusivity (scales like 1/T)
– Generally smaller coefficient of thermal expansion (CTE) (goes to 0 at T = 0)
– Generally smaller dn/dT
dn/dT is affected by CTE and bandgap changes with temperature
• Cryogenic materials properties are needed in order to perform modeling and simulation and assess power scalability but only limited properties data exists below 300 K
MIT Lincoln LaboratoryCryoYb:YAG-7DJR 04/21/23
Distortion (OPD) Depolarization
h fractional thermal load thermal conductivity thermal expansiondn/dT change in refractive index with temperature
FOMd = / [hdn/dT] FOMb = / h
Thermo-Optics Improve with Cooling
• Larger material FOM’s give less OPD and less stress-induced birefringence
• Key material properties (, , dn/dT) scale favorably at lower temperature in bulk single crystals
Properties of Undoped YAG
10
15
20
25
30
35
40
45
50
0
1
2
3
4
5
6
7
8
100 150 200 250 300
TH
ER
MA
L C
ON
DU
CT
IVIT
Y (
W/m
K)
CT
E(p
pm
/K), d
n/d
T (p
pm
/K)
TEMPERATURE (K)
UNDOPED YAG
Th
erm
al
Co
nd
uc
tiv
ity
(W
/m K
) CT
E (p
pm
/K), d
n/d
T (p
pm
/K)
Temperature (K)
100 K 300 K
(in W/mK) 47 11
dn/dT(ppm/K) 0.9 7.9
(ppm/K) 2.0 6.2
Relative FOMd
(300-K Nd:YAG = 1)
87(Yb:YAG)
1(Nd:YAG)
Relative FOMb
(300-K Nd:YAG = 1)
31(Yb:YAG)
1(Nd:YAG)
Un-doped YAG Figures of Merit
MIT Lincoln LaboratoryCryoYb:YAG-8DJR 04/21/23
Thermo-Optic Properties of Host Crystals
• Thermo-optic properties of single-crystal laser hosts generally improve at cryogenic temperatures
• Improvement in thermal conductivity is present but reduced for high-doping levels
Thermal Conductivity Yb:YAG Thermal Conductivity
Undoped Hosts
Aggarwal et al, JAP (2005)Fan et al, JSTQE (2007)
MIT Lincoln LaboratoryCryoYb:YAG-9DJR 04/21/23
Efficiency Improves at Cryogenic Temperatures
• Cryo-cooling allows efficient use of gain media– Yb:YAG has high intrinsic efficiency (quantum defect ~ 9%)
– Yb:YAG is four-level system at low temperatures
• Broad absorption band maintained at low temperature– Efficient diode pumping possible
– Reliable temperature-tune-free operation
Yb:YAG Absorption Spectrum
Ab
sorp
tio
n C
oef
fici
ent
(cm
–1)
900Wavelength (nm)
0
2
4
6
8
10
920 940 960 980 1000 1020 1040
LaserWavelength
77 K
300 K
PumpArray
Energy Levels in Yb:YAG
Laser:1030 nm
Pump:940 nm
En
erg
y
3kBT @ 300K, 9kBT @ 100K
MIT Lincoln LaboratoryCryoYb:YAG-10
DJR 04/21/23
Thermal Sources for Yb:YAG Lasers
• Typical measured heat load is 0.3 W dissipated per W output
– 9% of absorbed pump power dissipated in Yb:YAG by quantum defect
– Additional contribution to cold-tip thermal load from trapped fluorescence
• Modest amounts of liquid nitrogen are required
– A 10-kW laser (3000 W of heat) will consume 1 LPM of L N2
Fluorescence
LaserOutput
Quantum Defect
UnabsorbedPump
Untrapped
Trapped
PumpPhotons
Cooled Yb:YAG
AbsorbedPump
MIT Lincoln LaboratoryCryoYb:YAG-11
DJR 04/21/23
Outline
• Cryogenic laser background
• The case for power scalability and high efficiency in Yb lasers
• Laser demonstration results
• Summary
MIT Lincoln LaboratoryCryoYb:YAG-12
DJR 04/21/23
Typical Laser Breadboard Layout
Pump Lasers
Polarizers
• Yb:YAG cryogenically cooled in LN2 cryostat
• Efficient end-pumping with high-brightness diode pump lasers
• Yb:YAG crystal mounted to copper for heat-sinking
Laser Output
Beam Profile
OutputCoupler
LN2 Dewar
Yb:YAG Crystal
MIT Lincoln LaboratoryCryoYb:YAG-13
DJR 04/21/23
494-W CW Power Oscillator
• 494-W CW power
• 71% optical-optical efficiency
• M2 ~ 1.4 at 455 W
• OC reflectivity = 25%, L = 1 m, Near-flat-flat resonator
• Limited by available pump power
Near-Field Profile at 275 W (CW)
0 100 200 300 400 500 600 7000
100
200
300
400
500
Ou
tpu
t P
ow
er (
W)
Incident Pump Power (W)
Laser Output
OutputCoupler
LN2 Dewar
Yb:YAG Crystals
Fiber-Coupled
Pump Laser
High Reflector
Dichroic Mirror
Polarizers
Fan et al, JSTQE (2007)
MIT Lincoln LaboratoryCryoYb:YAG-14
DJR 04/21/23
255-W (CW) Single-Pass Amplifier
• 255-W (CW) generated by amplifying 110-W (CW) in a single-pass amplifier
• M2 ~ 1.1 measured from amplifier
• 54% optical-optical efficiency of single-pass amplifier
• Beam size ~ 0.9-mm radius0 50 100 150 200 250 300
0
50
100
150
200
250
300
Ou
tpu
t P
ow
er (
W)
Incident Pump Power (W)
30-W Oscillator Data 70-W Oscillator Data 110-W Oscillator Data Theory Theory Theory
255-W (CW) Average PowerNear-Field Beam Profile
M2 ~ 1.1
Amplifier Performance
Thin-Film Polarizers
/4 waveplate
150-W Diode
Modules
110-W (CW) Power Oscillator
Dewar and Crystal (Identical to Oscillator)Polarization
Isolator
Ripin et al, IEEE JQE (2005)
MIT Lincoln LaboratoryCryoYb:YAG-15
DJR 04/21/23
High-Average-Power Short-Pulse Laser
Joint MIT Campus-Lincoln effort demonstrated 287-W ps-class laser
Hong et al, Optics Letters (2008)
MIT Lincoln LaboratoryCryoYb:YAG-16
DJR 04/21/23
Ultrafast Cryo-Yb Lasers
• Relatively simple and inexpensive to generate high average power
• Hosts available for picosecond and femtosecond operation
• Key attributes are– Large bandwidth at cryogenic temperature– Favorable thermo-optics
• Examples of possible gain media:– Yb:YAG – ps-class
– Yb:YLF (LiYF4) – <100-fs class
– Yb:YSO (Y2SiO5) - <50-fs class
MIT Lincoln LaboratoryCryoYb:YAG-17
DJR 04/21/23
Candidates for Ultrashort Pulse Lasers
Laser ParameterNd:Glass
300 KTi:Al2O3
300 KNd:YAG
300 KYb:YAG
100 KYb:YLF100 K
Yb:YSO100 K
Thermal Conductivity (W/mK) ~ 1 30 11 39 20
Thermal Expansion (ppm/K) ~ 10 5 6.2 2 3
dn/dT (ppm/K) ~ 3 11 7.9 0.9 -1.8 (ne)
Quantum-Limited Thermal Load per Unit Output Power
0.18(p= 870 nm)
0.52(p= 532 nm)
0.32(p= 808nm)
0.11(p= 940 nm)
0.09(p= 940 nm)
Nominal Gain Bandwidth (nm) 20 300 0.5 1.5 17 >50
Isat (kW/cm2) at laser 16 240 2.6 1.2 5.7 14
Expected Efficiency
MIT Lincoln LaboratoryCryoYb:YAG-18
DJR 04/21/23
Cryogenic Yb:YLF Provides Path to High-Power Short-Pulse Lasers
• Direct diode-pumping for simplicity and ease of use
• Thermo-optic effects scale favorably at cryogenic temperatures
• 4-level laser with small quantum defect for high efficiency
Yb:YLF Gain Spectrum
YLF Properties
~17 nm FWHM
Th
erm
al C
on
du
ctiv
ity (
W/m
K)
dn e
/dT
(p
pm
/K)
0
10
20
30
40
0
-2
-4
-6
-8
Temperature (K)100 150 200 250 300
Data from Aggarwal et al. (2005)
Fan et al. (2007)
MIT Lincoln LaboratoryCryoYb:YAG-19
DJR 04/21/23
>200-W Yb:YLF Laser
• High-power cw Yb:YLF laser shows the potential for power scaling fs sources
• Pump at 960-nm, output at 995 nm with 44% R output coupler
• M2 of 1.1 at 60 W, M2 of 2.6 at 180 W– Multi-transverse mode operation at
higher power
Absorption Spectrum
960-nmpump
400-µmfiber
Yb:YLFLN2 Dewar Output
CouplerR = 44%
DichroicFocusing Optics 20 cm
Laser Schematic
Output Power at 995 nm
68% slope
PumpFeature
Zapata et al. (2010)
MIT Lincoln LaboratoryCryoYb:YAG-20
DJR 04/21/23
Summary
• Cryogenically cooled Yb:YAG lasers enable high-average-power with excellent beam quality
– High efficiency and low thermo-optic distortion
• Laser designs relatively simple and inexpensive
• Further power scaling– Increase pump power
– Combine cryogenic cooling with orthogonal power-scaling approaches