High-energy, kHz-rate, picosecond, 2-µm laser pump source ...

Photonics West’2013Paper 8599-10

High-energy, kHz-rate, picosecond, 2-µm laser pump source for mid-IR nonlinear optical devices

Alex Dergachev

Q-Peak, Inc. 135 South Road, Bedford, Massachusetts 01730

E-mail: [email protected]

Acknowledgements:This work was supported by DoE Phase I SBIR program (Grant # DE-SC0007749)

In addition, special thanks to:Laser UT Center of Lockheed Martin (now LUT is part of PaR Systems)NASA Langley Research Center

mailto:[email protected]


Objectives 2-um system concept Prior art Ho:YLF spectroscopic properties and

modeling Experimental details

• Ho:YLF mode-locked osc• Ho:YLF regenerative amplifier• Ho:YLF single-pass amplifier

Conclusions

Outline


Objectives

Application:

Development of a CPA, high-energy 2-um-laser source as a pump for mid-IR parametric devices

Targeted specifications:

Pulse energies >50 mJ Repetition rate 1-2 kHz Pulse duration of 1-500 ps High-beam quality


CPA Ho-MOPA concept

Possible amplifier alternatives : Fiber amplifier? Bulk ? Ho-medium? This work: High-gain, bulk Ho:YLF laser amplifier

1-10 nJ~100 MHz0.1-10 ps

>1 mJ~ 1 kHz

Power AmpChain

Regenerative amplifier

Output>50 mJ~1 kHz>400 ps

2-um, fs/ps seed source

Pulse compressor

2-um Ho-MOPA system

Pulse stretcher

1-10 ps

Key objectives: Define optimum system configuration to achieve

energy/pulse rate targets• Seed osc and pulse stretching/compression

techniques relatively well developed • Emphasis on the amplifier development

Regenerative amplifier Power amplifier stage(s)


Prior Art: Q-Peak’s nanosecond Ho:YLF MOPAs

Ho-stage/ Regime CW 100 Hz 500 Hz 1000 Hz

Osc/Amp #1 39 W 55 mJ 50 mJ 33 mJ

Amp#2 76 W 110 mJ 95 mJ 68 mJ

Amp#3 115 W 170 mJ 140 mJ 105 mJ

Pump % 100% 70% 100% 100%

Tm-pump #1~120 W at 1940 nm

Osc/ Amp #1

Amp #2

Amp #3



Tm-fiber laser TLR-120-1940 IPG Photonicswww.ipgphotonics.com

Operation regime CWBeam Profile TEMooOutput power ≥ 120 WWavelength 1940 nmPolarization RandomLinewidth ≤ 2 nm

A. Dergachev, D. Armstrong, A. Smith, T. E. Drake, and M. Dubois, “High-power, high-energy ZGP OPA pumped by a 2.05-μm Ho:YLF MOPA system”, Proc. SPIE 6875, 687507 (2008).

Typical pulsewidth 20-25 ns (10-250 ns – range)


Prior art: High gain Ho:YLF amplifiers

Recent reported results re high-gain Ho-amplifiers: Dergachev, ASSP 2009• 23-dB Ho:YLF double-pass amplifier• 10 mW seed /2 W output at 2.05 um• single-frequency/broadband/pulsed

Coluccelli et al, Opt. Lett. 36, 2299 (2011)• 5-pass Ho:YLF amplifier for 2-um tail of Er:fiber comb

source (av. 30 dB gain in 2.05-2.07 um range)• 50-mm long, Brewster-cut Ho:YLF• 1.6-W comb in 2.05-2.07 um• 20-W Tm:fiber source

Dergachev, ASSP 2012• 45-dB, Compact, Single-Frequency, 2-µm Amplifier• 0.12 mW seed /4.5 W output at 2.05 um• single-frequency/broadband/pulsed

Ho:YLF

DM

DM

Ho:YLF

DM

DM

Pump #2Pump #1


Recent work: Ho:YAG CPA

Recent work :

P. Malevich, G. Andriukaitis, S. Alisauskas, A. Pugzlys, A. Baltuska, L. Tan, C. F. Chua, and P. B. Phua, "Femtosecond 3-mJ 5-kHz cw-pumped Ho:YAG CPA," in Frontiers in Optics Conference, OSA Technical Digest (online) (OSA, 2012), paper FW6B.10.

• CPA Ho:YAG• 3 mJ, 5 kHz, ~250 ps• 0.5 ps - after compression


Ho-Laser Media: YLF vs YAG

Notes: • Basic host parameters are from D.N. Nikogosyan “ Handbook of Properties of Optical

and Laser-Related Materials”, Wiley and Sons (1997).• *Damage data for YLF from “Laser induced damage in optical materials”, NBS special

publication 509, p. 402 (1977).• **Critical power for self-focusing is calculated as Pcr=α(λ2/4πnn2), where α=1.8962 for a

Gaussian beam.• Ho:YAG data from S.A.Payne et al. IEEE J. of QE, 28, 2619-2630 (1992)).

Ho:YLF Ho:YAG Host parameters

Material type Birefringent Isotropic

Transparency range (undoped), µm 0.18-6.7 0.21-5.3

Thermal conductivity, Wm/K 6.3 13

Refractive index (at 2 µm) 1.44 (no)1.46 (ne)

1.80

dn/dT (at 1.0 µm) -4.3x10-6(IIc),-2.0x10-6(⊥c) 7.3x10-6/K

Nonlinear refractive index (n2), m2/W 1.7x10-20 8.1x10-20

Bulk damage threshold, GW/cm2 18.9 (at 4.6 ns)* 10.1 (at 4.6 ns)*

Critical power (self-focusing), MW 24** 5.3**

Ho-doped mediumλabs, nm 1940 1907

λem, nm 2051 2098

σem, cm2 1.84⋅10-20 0.98⋅10-20

Quantum defect (1-λabs/λem), % 5.4 9.1

tem, ms 15 7

Ho:YLF:

• Natural birefringence,• Low dn/dT resulting in negligible

thermal lensing,• Low nonlinearity (4.8 times less than

for Ho:YAG),• High emission (gain) cross-section

(twice that of Ho:YAG),• Long emission lifetime (twice that of

Ho:YAG),• Readily available material with large

size and high optical quality.


Ho:YLF – Absorption/ Emission (E||c)

Cross-section determination - reciprocity method:σem(ν) = σabs(ν) ( Zl /Zu ) exp [ (EZL - hν) / kT ](Following S.A.Payne et al. IEEE J. of QE, 28, 2619-2630 (1992)).

0.0E+00

2.0E-21

4.0E-21

6.0E-21

8.0E-21

1.0E-20

1.2E-20

1.4E-20

1.6E-20

1.8E-20

2.0E-20

1800 1850 1900 1950 2000 2050 2100 2150

Wavelength, nm

Abs

orpt

ion/

Emis

sion

cro

ss-s

ectio

n, c

m-2

Abs (E||c)Em (E||c)

Pump

Lasing


Ho:YLF – Calculated gain (||c) vs wavelength(various inverted fractions)

The net gain coefficient:g(ν) = N [ p σem(ν) - (1-p) σabs(ν) ]

L, cm Gain

5 ~7

10 ~46

1-pass small signal gain (G)

0.00

0.10

0.20

0.30

0.40

0.50

0.60

2040 2045 2050 2055 2060 2065 2070 2075 2080

Wavelength, nm

Gai

n, c

m-1

0.560.3

Av. gain 0.38 cm-1

2045-2070 nm


Tm:fiber Laser Pumping of Ho-Laser Media

Relatively high-brightness sources are required• Efficient GSD laser pumping requires high optical density αL>>1• The use of a Tm-fiber laser with diffraction-limited beam quality is

essential to provide long, collimated gain regions enabling high gain operation of the bulk Ho-amplifier

1850-1950 nm wavelength range High average power up to 150 W – commercial products Possible alternatives:

• Diode-pumped Tm-bulk solid state lasers • Direct diode-pumping (at ~1.9 um) – not too bright!

This work: Tm:fiber lasers (IPG Photonics):• TEM00• < 20 W• 1940 nm• Linearly polarized• < 2 nm linewidth


Ho:YLF mode-locked oscillator

Mode-locked oscillator:• 0.5 % Ho:YLF, 30-mm long, wedged• AR/AR-coated at 1940/2050 nm• TE-cooled at 20C• Active mode-locking (TeO2 AOM, 41 MHz)• Resonator length: 184 cm• Tm-pump power up to 15W• Output: up to 4W (~38% slope)• 81 MHz• 250-300 ps

Output

AOM

OC

Ho:YLF

Tm-fiber laser

DM


Ho:YLF regenerative amplifier

Seed osc~1.8 nJ81 MHz

Regen output

Tm-fiber laser

EOcell

TFP2

HRλ/4λ/2Rot

HR

TFP1

Main pulse

SatelliteSatellite

1 ns/div

5 ns/div

tp~300 ps

Regen amp:• 0.5 % Ho:YLF, 40-mm long, wedged• AR/AR-coated at 1940/2050 nm• TE-cooled at 20C• EO: RTP 20-mm long, ¼-wave voltage• Resonator length: ~184 cm• Triggering off RF signal to AOM

• Rate: 1-10 kHz• Gate width ~165-200 ns (~14- 17 round trips)• Tm-pump power up to 15W• Output: Typical av. power 1-2 W• ~1.7 mJ at 1 kHz

Ho:YLF

Iso

DM

Regen target: >1 mJ at 1 kHz (!!!)


Ho:YLF single-pass, 2-xtal amplifier

HRHo:YLF

Tm-fiber laser≤75 W DMDM

PBSλ/2

Input

Ho:YLFDM DM

Regen amp2050 nm

~1mJ at 1 kHz

Output

~7 mJ at 1 kHz

2-xtal power amp:• Single pass• 0.5 % Ho:YLF, 30-70-mm long, wedged• AR/AR-coated at 1940/2050 nm• TE-cooled at 20C• Adapted for 2-beam pumping with pol.-split

unpol. fiber laser• Tm-pump power up to 75W


Ho:YLF single-pass, 2-xtal amplifier - Model

2-xtal power amp:• Single pass• 0.5 % Ho:YLF, 70-mm long – each• Pump power 60W – Total

Experiment:• 1 mJ (1 kHz) -In• ~11 mJ - Out• ~300 ps• 1.3-mm dia• ~2.5 GW/cm2

Model:• 1 mJ/ 1 kHz/ 300 ps

seed• Fixed pump power (60W)• Vary beam size for the

seed and pump beams


Ho:YLF single-pass, 2-xtal amplifier

1 kHz (11 mJ)

5 kHz (2.6 mJ)

10 kHz (1.4 mJ)

At pulse rates ≥5 kHz average power is the same as in CW

Av. power:~11 W (1 kHz)~13 W (≥5 kHz )


Conclusions

Ho:YLF regenerative amplifier:• Up to 1.7 mJ at 1 kHz (nominal regime)• 1-10 kHz – operating pulse rate• ~300 ps

Ho:YLF power amplifier:• ~11 mJ at 1 kHz (nominal regime)• Further scaling to ~20 mJ is straightforward

Concerns:• Damage limitations -> laser damage tests of Ho:YLF at 2

um• Fine-tuning of the power amp design

Further work:• Operation with chirped-pulse seed• Spectral shaping• Scaling to reach ~50 mJ target -> additional amp stage(s)• Compression of amplified pulses• Packaging

High-energy, kHz-rate, picosecond, 2-µm laser pump source ...

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