FILAS’2012paper FTh4A.2
45-dB, Compact, Single-Frequency, 2-µm Amplifier
Alex Dergachev
Q-Peak, Inc. 135 South Road, Bedford, Massachusetts 01730
E-mail: [email protected]
Acknowledgements:This work was supported by NASA Phase II SBIR program (Grant # NNX11CB77C)
FILAS’2012paper FTh4A.2
Objectives Proposed system layout Ho:YLF spectroscopic properties and
modeling Experimental details
• Ho:YLF gain module• Ho:YLF amplifier
Conclusions
Outline
FILAS’2012paper FTh4A.2
Objectives
Application:
Development of a pulsed, single-frequency 2-um-laser source suitable for coherent LIDAR/ DIAL applications
Targeted specifications:
Eye-safe wavelength (~2 um) Pulse energies of 1-10 mJ Repetition rate 1-5 kHz Pulse duration of 100-400 ns (due to required linewidth) High-beam quality
FILAS’2012paper FTh4A.2
Proposed system layout
Possible Power Amplifier alternatives : Fiber amplifier? Bulk ? This work: High-gain bulk laser Ho-amplifier providing:
• Damage-free operation at mJ-level pulse energy• Immunity to Brillouin scattering
Approach : MOPA Low-average power pulsed seed
source • Single-frequency• 1-100 mW• 100-400 ns
FILAS’2012paper FTh4A.2
Prior Art: Tm-fiber amplifiers
Just a few examples – recent work:
Goodno et al, Opt. Lett. 34, 1204-1206 (2009)• 608 W output• CW, single-frequency, TEMoo• 3-stage front end + a power amp
W. Shi et al Opt. Lett. 36, 3575-3577 (2011)• Pulsed, single-frequency • 220 uJ output /80 ns/ 20 kHz• QS fiber osc + 3 amplifier stages
FILAS’2012paper FTh4A.2
Prior Art - Ho-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• ~20-W Tm:fiber source
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
Over the last 10 years Q-Peak has successfully applied Ho:YLF amplifiers in a variety of laser systems – typically, high-power (up to 115 W), high-energy (up to 170 mJ), high rate (up to 1 kHz)
FILAS’2012paper FTh4A.2
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 20-50 W Possible alternatives:
• Diode-pumped Tm-bulk solid state lasers • Direct diode-pumping (at ~1.9 um) – not too bright!
This work: Tm:fiber laser (IPG Photonics):• TEM00• < 50 W• 1940 nm• Randomly polarized• < 2 nm linewidth
FILAS’2012paper FTh4A.2
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
FILAS’2012paper FTh4A.2
Ho:YLF – Calculated gain (||c) vs wavelength(various inverted fractions)
The net gain coefficient:g(ν) = N [ p σem(ν) - (1-p) σabs(ν) ]
-0.60
-0.40
-0.20
0.00
0.20
0.40
0.60
1850 1900 1950 2000 2050 2100
Wavelength, nm
Gai
n, c
m-1 0.56
0.30.250.2
L, cm Gain (dB)
5 10.6
10 21.7
15 32.6
1-pass small signal gain (G)
FILAS’2012paper FTh4A.2
Ho-amplifier – Numerical modeling
Single or double pass One or two crystals Model accounts for:
• Pump depletion• Almost all parameters are from experimental data
Assumptions:• Collimated beam through the crystal – no diffraction• Gaussian beam profile (seed/pump) for single-pass amp• Plane waves for double-pass amp – faster calcs
Practical limits/compromises• Limit on max crystal length –> “no” divergence of the pump beam• Beam dia large enough to accommodate desired pulse energy• Beam dia small enough to provide desired gain• Accommodate pumping with unpolarized Tm-fiber laser
FILAS’2012paper FTh4A.2
Ho:YLF gain module
Ho:YLF
DM
DM
TE-cooled crystal mount
Ho:YLF:• 0.5 % Ho• Rectangular bar• 4x4 mm aperture• Length – 30-70 mm• AR/AR coated at 1940/2050 nm
Ho:YLF
DM
DM
Pump #2Pump #1
Dimensions:12.5 x 10 x 2.5 cm
Adapted for 2-beam pumping with pol.-split
unpol. fiber laser
FILAS’2012paper FTh4A.2
Ho:YLF amplifier – Experimental set-up
HR Ho:YLFTm-fiber laser
DMDM
PBS λ/2 Ho:YLFDM DM
Seed Osc2050 nm
0.07-25 mWCW or pulsed 2-pass
output
Power Monitor
λ/2 Isolator
Input (seed osc beam) Output (2nd pass)
1940 nm<50 W CW
(IPG Photonics)
Pump (waste)
1-pass output HR mirror (for 2nd pass)
Dimensions:40 x 15 x 5 cm
FILAS’2012paper FTh4A.2
Ho:YLF amplifier – Output vs pump power(single- and double-pass)
FILAS’2012paper FTh4A.2
Ho:YLF amplifier – Gain vs pump power (single- and double-pass)
FILAS’2012paper FTh4A.2
Ho:YLF amplifier – Output power, and gain vs seed power (single- and double-pass)
Seed(mW)
Out (W)
Gain (dB)
0.12 4.26 45.5
1 8.6 39.3
25 14.4 27.6
100 17.0 22.3
2-pass data
FILAS’2012paper FTh4A.2
Ho:YLF amplifier – Pulsed regime
Output power – same for ≥5 kHz as for CW
Seed input: 25 mW (5 uJ at 5 kHz)
Pump power: 46 W
Rep.ratekHz
Av. Power
W
Pulse energy
mJ
tpns
PeakPower
kW5 14.1 2.8 350 8.12 12.9 6.4 200 32.2
1.5 11.9 7.9 150 52.8
FILAS’2012paper FTh4A.2
Conclusions
Ho:YLF amplifier:• Compact, single-stage, dual-crystal, double-pass amplifier with >45 dB gain• Peak power of up to ~50 kW for 150-400-ns-long laser pulses at kHz rates – single-
frequency or broadband• Possible to extract up to ~40% in 2-pass regime with seed power < 100 mW• The use of bulk amplifier medium permits single-frequency output with peak power
levels 1-2 orders of magnitude higher than with fiber-based amplifiers.• Maximum pulse energy up to ~ 8 mJ in current set-up (could be increased)• Further scaling to >100 mJ is straightforward (additional amp stage(s))
Pros:• Reliability• High damage threshold -> Ability to generate high pulse energy• Immunity to such deleterious effects as stimulated Brillouin and Raman scattering which
limit the performance of fiber amplifiers• Damage-free operation even if the seed is absent
Cons:• Not fiber?
