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 Laboratory CryoYb:YAG-1 DJR 9/7/2010
Outline
• Cryogenic laser background • The case for power scalability and high efficiency in Yb lasers
• Laser demonstration results • Summary
CryoYb:YAG-2 DJR 9/7/2010
MIT Lincoln Laboratory
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 CryoYb:YAG-3 DJR 9/7/2010
MIT Lincoln Laboratory
Approaches to Generate HighBrightness 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 CryoYb:YAG-4 DJR 9/7/2010
MIT Lincoln Laboratory
Outline
• Cryogenic laser background • The case for power scalability and high efficiency in Yb lasers
• Laser demonstration results • Summary
CryoYb:YAG-5 DJR 9/7/2010
MIT Lincoln Laboratory
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
CryoYb:YAG-6 DJR 9/7/2010
MIT Lincoln Laboratory
Thermo-Optics Improve with Cooling 8
45
UNDOPED YAG
7
40
6
35
5
30
4
25
3
20
2
15
1
10 100
150
200
250
0 300
CTE (ppm/K), dn/dT (ppm/K)
Thermal Conductivity (W/m K)
Properties of Undoped YAG 50
Distortion (OPD) FOMd = κ / [ηhdn/dT]
Depolarization FOMb = κ / ηh α
ηh ≡ fractional thermal load κ ≡ thermal conductivity α ≡ thermal expansion dn/dT ≡ change in refractive index with temperature
Temperature (K)
Un-doped YAG Figures of Merit 100 K
300 K
κ (in W/mK)
47
11
dn/dT(ppm/K)
0.9
7.9
α (ppm/K)
2.0
6.2
Relative FOMd
87 (Yb:YAG)
1 (Nd:YAG)
31 (Yb:YAG)
1 (Nd:YAG)
(300-K Nd:YAG = 1)
Relative FOMb (300-K Nd:YAG = 1) CryoYb:YAG-7 DJR 9/7/2010
•
Larger material FOM’s give less OPD and less stress-induced birefringence
•
Key material properties (κ, α, dn/dT) scale favorably at lower temperature in bulk single crystals MIT Lincoln Laboratory
Thermo-Optic Properties of Host Crystals Thermal Conductivity
Yb:YAG Thermal Conductivity
Undoped Hosts
• 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 Aggarwal et al, JAP (2005) Fan et al, JSTQE (2007) CryoYb:YAG-8 DJR 9/7/2010
MIT Lincoln Laboratory
Energy Levels in Yb:YAG
Energy
Laser: 1030 nm Pump: 940 nm
3kBT @ 300K, 9kBT @ 100K
Absorption Coefficient (cm–1)
Efficiency Improves at Cryogenic Temperatures Yb:YAG Absorption Spectrum 10 77 K
8 6
Pump Array
Laser Wavelength
4 2 300 K 0 900
920
940
960
980 1000 1020 1040
Wavelength (nm)
• 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 CryoYb:YAG-9 DJR 9/7/2010
MIT Lincoln Laboratory
Thermal Sources for Yb:YAG Lasers Cooled Yb:YAG Unabsorbed Pump Quantum Defect Pump Photons
Laser Output
Absorbed Pump
Untrapped Fluorescence Trapped
•
•
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 –
CryoYb:YAG-10 DJR 9/7/2010
A 10-kW laser (3000 W of heat) will consume 1 LPM of L N2 MIT Lincoln Laboratory
Outline
• Cryogenic laser background • The case for power scalability and high efficiency in Yb lasers
• Laser demonstration results • Summary
CryoYb:YAG-11 DJR 9/7/2010
MIT Lincoln Laboratory
Typical Laser Breadboard Layout Yb:YAG Crystal
Beam Profile
LN2 Dewar
Laser Output Polarizers
Output Coupler Pump Lasers
• 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 CryoYb:YAG-12 DJR 9/7/2010
MIT Lincoln Laboratory
494-W CW Power Oscillator Near-Field Profile at 275 W (CW)
FiberCoupled Pump Laser
LN2 Dichroic Dewar Mirror
Yb:YAG Crystals
Laser Output Polarizers High Reflector
Output Coupler
Output Power (W)
500
• • • •
400 300 200 100
•
0 0
100
200
300
400
500
600
Incident Pump Power (W) CryoYb:YAG-13 DJR 9/7/2010
700
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 Fan et al, JSTQE (2007) MIT Lincoln Laboratory
255-W (CW) Single-Pass Amplifier Dewar and Crystal (Identical to Oscillator)
Polarization Isolator
255-W (CW) Average Power Near-Field Beam Profile M2 ~ 1.1
110-W (CW) Power Oscillator Thin-Film λ/4 Polarizers waveplate
150-W Diode Modules
Amplifier Performance 300
30-W Oscillator Data 70-W Oscillator Data 110-W Oscillator Data Theory Theory Theory
Output Power (W)
250 200 150 100
•
255-W (CW) generated by amplifying 110-W (CW) in a singlepass amplifier
• •
M2 ~ 1.1 measured from amplifier
•
Beam size ~ 0.9-mm radius
50 0 0
50
100
150
200
250
Incident Pump Power (W) CryoYb:YAG-14 DJR 9/7/2010
54% optical-optical efficiency of single-pass amplifier
300
Ripin et al, IEEE JQE (2005)
MIT Lincoln Laboratory
High-Average-Power Short-Pulse Laser
Hong et al, Optics Letters (2008)
Joint MIT Campus-Lincoln effort demonstrated 287-W ps-class laser CryoYb:YAG-15 DJR 9/7/2010
MIT Lincoln Laboratory
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) – 200-W Yb:YLF Laser Laser Schematic
• 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
LN2 Dewar
960-nm pump
Yb:YLF Output Coupler R = 44%
400-µm fiber Focusing Optics
Dichroic 20 cm
Output Power at 995 nm
Absorption Spectrum
Pump Feature
Zapata et al. (2010) CryoYb:YAG-19 DJR 9/7/2010
MIT Lincoln Laboratory
Summary
•
Cryogenically cooled Yb:YAG lasers enable highaverage-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
CryoYb:YAG-20 DJR 9/7/2010
MIT Lincoln Laboratory