cryogenically cooled solid-state lasers: recent developments and future prospects *

CryoYb:YAG-1DJR 04/22/23

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 04/22/23

Outline

• Cryogenic laser background

• The case for power scalability and high efficiency in Yb lasers

• Laser demonstration results

• Summary


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


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


Outline




• Summary


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


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


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)


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 04/22/23

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


DJR 04/22/23

Outline




• Summary


DJR 04/22/23

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


DJR 04/22/23

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)


DJR 04/22/23

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)


DJR 04/22/23

High-Average-Power Short-Pulse Laser

Joint MIT Campus-Lincoln effort demonstrated 287-W ps-class laser

Hong et al, Optics Letters (2008)


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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


DJR 04/22/23

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


DJR 04/22/23

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)


DJR 04/22/23

>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)


DJR 04/22/23

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

cryogenically cooled solid-state lasers: recent developments and future prospects *

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