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

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...
Author: Gary Reynolds
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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 / [ηhdn/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

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