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
Photonics West’2013Paper 8599-10
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
Photonics West’2013Paper 8599-10
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
Photonics West’2013Paper 8599-10
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)
Photonics West’2013Paper 8599-10
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-pump #2~120 W at 1940 nm
Tm-pump #3~120 W at 1940 nm
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)
Photonics West’2013Paper 8599-10
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
Photonics West’2013Paper 8599-10
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
Photonics West’2013Paper 8599-10
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.
Photonics West’2013Paper 8599-10
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
Photonics West’2013Paper 8599-10
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
Photonics West’2013Paper 8599-10
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
Photonics West’2013Paper 8599-10
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
Photonics West’2013Paper 8599-10
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 (!!!)
Photonics West’2013Paper 8599-10
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
Photonics West’2013Paper 8599-10
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
Photonics West’2013Paper 8599-10
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 )
Photonics West’2013Paper 8599-10
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