Cryogenically cooled solid-state lasers: Recent developments and future prospects *

1. 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.

2. Outline • Cryogenic laser background • The case for power scalability and high efficiency in Yb lasers • Laser demonstration results • Summary

3. 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

4. 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

5. Outline • Cryogenic laser background • The case for power scalability and high efficiency in Yb lasers • Laser demonstration results • Summary

6. 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

7. Thermo-Optics Improve with Cooling Properties of Undoped YAG CTE (ppm/K), dn/dT (ppm/K) Thermal Conductivity (W/m K) Temperature (K) Distortion (OPD) Depolarization • Larger material FOM’s give less OPD and less stress-induced birefringence • Key material properties (k, a, dn/dT) scale favorably at lower temperature in bulk single crystals FOMd = k / [hhdn/dT] FOMb =  / h h fractional thermal load k thermal conductivity a thermal expansion dn/dT  change in refractive index with temperature Un-doped YAG Figures of Merit

8. Thermo-Optic Properties of Host Crystals Thermal Conductivity Yb:YAG Thermal Conductivity • 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 Undoped Hosts Aggarwal et al, JAP (2005) Fan et al, JSTQE (2007)

9. Efficiency Improves at Cryogenic Temperatures 10 77 K 8 Laser:1030 nm Absorption Coefficient (cm–1) 6 Pump Array LaserWavelength Pump:940 nm 4 Energy 2 3kBT @ 300K, 9kBT @ 100K 300 K 0 900 920 940 960 980 1000 1020 1040 Wavelength (nm) Yb:YAG Absorption Spectrum Energy Levels in Yb:YAG • 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

10. 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 • A 10-kW laser (3000 W of heat) will consume 1 LPM of L N2

11. Outline • Cryogenic laser background • The case for power scalability and high efficiency in Yb lasers • Laser demonstration results • Summary

12. Typical Laser Breadboard Layout Yb:YAG Crystal Beam Profile LN2 Dewar Laser Output Polarizers OutputCoupler 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

13. 494-W CW Power Oscillator Fiber-Coupled Pump Laser Near-Field Profile at 275 W (CW) LN2 Dewar Yb:YAG Crystals Dichroic Mirror • 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 Laser Output Polarizers High Reflector OutputCoupler Fan et al, JSTQE (2007)

14. 255-W (CW) Single-Pass Amplifier Dewar and Crystal (Identical to Oscillator) Polarization Isolator 110-W (CW) Power Oscillator Thin-Film Polarizers l/4 waveplate 150-W Diode Modules 255-W (CW) Average Power Near-Field Beam Profile M2 ~ 1.1 • 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 radius Amplifier Performance Ripin et al, IEEE JQE (2005)

15. High-Average-Power Short-Pulse Laser Hong et al, Optics Letters (2008) Joint MIT Campus-Lincoln effort demonstrated 287-W ps-class laser

16. 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

17. Candidates for Ultrashort Pulse Lasers

18. Cryogenic Yb:YLF Provides Path to High-Power Short-Pulse Lasers YLF Properties • 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 40 -8 30 -6 dne/dT (ppm/K) Thermal Conductivity (W/m K) 20 -4 10 -2 Yb:YLF Gain Spectrum 0 0 100 150 200 250 300 Temperature (K) Fan et al. (2007) Data from Aggarwal et al. (2005) ~17 nm FWHM

19. >200-W Yb:YLF Laser Laser Schematic LN2 Dewar Yb:YLF • 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 960-nm pump Output Coupler R = 44% 400-µm fiber Dichroic Focusing Optics 20 cm Absorption Spectrum Output Power at 995 nm Pump Feature 68% slope Zapata et al. (2010)

20. 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