a hybrid diode-gas laser approach to high power and ... · a dpal utilizes a neutral alkali vapor...
TRANSCRIPT
Bill Krupke
WFK Lasers, LLC.
A hybrid diode-gas laser approach to high power and brightness (DPAL)
CREOL Industrial Affiliates Day
Orlando, Florida
April 17, 2009
Outline
What is a DPAL? Why DPALs? - basics
Summary of surrogate pumping experiments
Advances in narrowband laser diode pump sources
Summary DPAL experiments
Scaling to high power
Some concluding observations
W. Krupke, CREOL April 17, 2009
What is a DPAL? --- a hybrid electric gas laser – Why DPALs?
Laser diode or diode array pump
Gas (Vapor) laser gain medium
Output Beam
high efficiency (~60-70 %)high power, but poor beam quality
no stress birefringenceno stress fracturelow index of refraction (density)convective removal of waste heatreduced thermal focusing
high average output power with high beam quality single aperture power scaling
W. Krupke, CREOL April 17, 2009
D2 lineνpump, λpump
D1 lineνlaser, λlaser
2S1/2
2P1/2
2P3/2δE
collisional relaxation (buffer gas: He, CH4, etc)
Alkali atom (Cs, Rb, K, Na, Li)
levels ideally in Boltzmann equilibriumQuantum defect = ∆ = δΕ/νpump
A DPAL utilizes a neutral alkali vapor atom as the active specie
DPAL: a quasi-two-level laser with a small quantum defect W. Krupke, CREOL April 17, 2009
W. Krupke, CREOL April 17, 2009
atom λpump (D2), nm λlaser (D1), nm ∆E, cm-
1Q-defect
K 770.11 766.70 57.7 0.0044Rb 780.25 794.98 237.5 0.019Cs 852.35 894.95 554.1 0.0472S1/2
2P3/22P1/2
∆E
λpump λlaser
atom λpump nm ∆ν (GHz) ∆λ (nm)K 766 0.852 0.00164
Rb 780 0.569 0.00116Cs 852 0.419 0.00102
Alkali atoms have small quantum defects & narrow Doppler widths
Very good!
Problematic!W. Krupke, CREOL April 17, 2009
History of alkali atom kinetics
K - Glushko, et. al., Opt. Spectrosc (USSR), 52, 458 (1982)
Na - Konefal and Ignaciuk, Opt. and Quantum Electron, 28, 169 (1993)
Rb - Movsesyan, et. al., Opt. Spectrosc (USSR), 61, 285 (1986)
K - Davtyan, et. al., ”, Opt. Spectrosc (USSR), 66, 686 (1989)
Rb - Konefal, Opt. Communications. 164, 95 (1999)
Population inversion and ASE had been observed on the D1 transition of the D2-transition-pumped alkali atoms with buffer gases
The main issue for practical DPALs is efficient diode pumping:
alkali atoms have quite narrow linewidth transitions
pump diodes have relatively broad emission linewidths (0.2-2nm)
main issue is how to achieve efficient alkali atom pumping?
W. Krupke, CREOL April 17, 2009
HP Diode Array Alkali Mode Converter
Ppump > kWs ∆λ ~2-4 nm(M2)slow ~1000
form pop. inversion;TEM00 mode extraction
Bright Source
B ~ ηconv *Ppump /(Aout∆Ω)
∆λ << nm M2 ~ 1
ηconv
DPAL: a spatial and spectral mode converter of LDs
Additionally,
DPALs are attractive for intra-cavity harmonic generation
- atomic precision and wavelength stability
- high gain coefficients
- short operating wavelengths W. Krupke, CREOL April 17, 2009
776 778 780 782 7840.0
0.2
0.4
0.6
0.8
1.0Comparison of Relevant Linshapes
Line
Sha
pe
nm
Radiative Doppler (127 C) Pressure (10 atm He) Diode (2nm FWHM)
collisionally-broaden alkali transitions with a buffer gas, making them spectrally homogeneous, with Lorentzian lineshapes
this enables greatly enhances wing absorption, compared to Gaussian lineshapes
The inherently large transition dipole moments result in high pump opacity even in the wings of the collisionally broadened transitions
Efficient DPAL pumping can be realized even when the diode pump emission linewidth is many times the alkali absorption linewidth
How to efficiently pump intrinsically narrow-band alkali transitions with a relatively broadband pump sources? Solution:
W. Krupke, CREOL April 17, 2009
Alkali – helium collisional broadening rates
∆νL (FWHM) = π-1 ZL = γ(vT) NT (He)
Alkali Atom γ (GHz/amg)K, potassium 26.7Rb, rubidium 18.6Cs, cesium 21.7
Alkali Atom Phomogeneous (atm) ∆λL, min (nm) K, potassium 0.436 0.0164Rb, rubidium 0.420 0.0116Cs, cesium 0.264 0.0102
D1,2 transitions become predominately Lorentzian when ∆νL > 10 x ∆νD
W. Krupke, CREOL April 17, 2009
Typical DPAL gain medium parameter ranges
alkali cold temperatures 110 –170 oC
alkali pressures 5 - 20 mtorr
alkali number densities 3 - 5 x 1013/cc
He buffer pressures 0.5 - 3 atm
Small-hydrocarbon buffer pressures 70 -150 torr
alkali peak Lorentzian cross-sections 5 – 30 x 10-14/cc
alkali Lorentzian transition linewidths 0.01 – 0.05 nm
Double-pass pump absorbed fraction >90%
W. Krupke, CREOL April 17, 2009
Several DPAL laser design parameters differ by orders of magnitude from those of solid state lasers:
Parameter Unit Rb-DPAL# Yb:YAG Rb/Yb
Laser transition X-section 10-20 cm2 30,000,000 2 1.5 x 107
Laser transition linewidth nm 0.03 ~ 9 ~ 3x 10-3
Upper laser level lifetime* µsec 0.028 1080 ~3 x10-5
Laser saturation fluence** mJ / cm2 0.001 ~10,000 ~10-7
~ ss gain coefficient cm-1 1 0.05 ~2 x101
~ operating intensity kW / cm2 ~10 ~20 ~ 0.5
# 1 atm helium buffer gas *sat fluence ≡ hc/λσ
W. Krupke, CREOL April 17, 2009
Outline
What is a DPAL? Why DPALs? - basics
Summary of surrogate pumping experiments
Advances in narrowband laser diode pump sources
Summary DPAL experiments
Scaling to high power
Some concluding observations
gain cell
pump diode
laser gain cell
pump diode
laser
Early DPAL experiments have utilized a classic “end-pumped” geometry
• pump-laser mode matching is facilitated for TEMoo operation
• pump and laser wave have orthogonal polarizations
• full length of gain medium is pumped (overcome resonance loss)
• power scaling is achieved by increasing mode diameter
• power scaling is constrained by:
induced radial thermal gradient in gain medium
pump spatial brightness
W. Krupke, CREOL April 17, 2009
Rb laser setup* – Titanium Sapphire surrogate pump
780 nm pump
795 nmlaser
20 cm radius concave output coupler
flat HR Mirrorgain cell
Oventhin film polarizer
TiS laser∆λ ~0.1 nm
Rubidium density = 10 microns (1.7 x 1013 / cc)Ethane pressure 75 torrHelium pressure 525 torr
*Krupke, et. al, Optics Letters, 28, 2336 (2003)
W. Krupke, CREOL April 17, 2009
795 nm Rb laser oscillation*
0
5
10
15
20
25
30
35
120 130 140 150 160 170 180
Absorbed pump power [mW]
Out
put p
ower
[mW
]
slope power efficiency = 49%
Rb density = 1.7 E13/cc
Output Coupler Reflectivity = 50%
*Krupke, et. al, Optics Letters, 28, 2336 (2003)
The pump width was 4 times the Lorentzian pump transition width
• Effective (homogeneous) wing-pumping was quantitatively confirmed
W. Krupke, CREOL April 17, 2009
DPAL energetics are accounted for using a simple quasi-two-level model**Beach, et. al, JOSA B21, 2151 (2004)
TiS pumped Cs DPAL*The Beach quasi-three-level, end-pumped Yb laser model** was adapted to a quasi-two-level model, appropriate to a DPAL:
end-pumped geometry
rate equation kinetics
plane-wave only
literature spectroscopic data
literature collisional dataexperiment : Xmodel :
The next stage in modeling will treat transverse modal properties
The literature has all necessary spectroscopic-kinetic data to estimate DPAL laser energetics
W. Krupke, CREOL April 17, 2009
Some groups now active in DPAL R&D
US Air Force Academy Zhdanov, Knize
Phillips Lab Hostutler
AF Inst. Technology (AFIT) Perrman, Hagen
U. Illinois, CU Areospace Carroll, Verdyn, Eden
Emory University Heaven
LLNL Beach, Wu
General Atomics Zweiback, Krupke
Newport-Spectra-Physics Petersen, Lane
Hamamatsu Zheng, Kan, Haruma
W. Krupke, CREOL April 17, 2009
Summary of surrogate-pumped alkali resonance lasers*
Author Atom Buffer gas ∆νp(GHz) Ppump (mW) Pout (mW) ηslope(%)
Krupke Rb He, ethane 50 180 30 54
Zweiback Rb He, methane 98 7 mJ 4.1 mJ 72
Wu Rb He4 9 2000 130 70-0
Wu Rb He3 9 1700 350 23
Beach Cs He, ethane 30 780 230 59
Zhdanov Cs He, ethane 0.0002 570 350 81
Zhdanov K He, ethane 0.0002? 860 14 20
Zhdanov K He ? 1000 40 18
Zweiback K He 98 23 mJ 14 67
*for references, see Krupke, Proc. SPIE, 7005-75 (2008)
W. Krupke, CREOL April 17, 2009
Outline
What is a DPAL? Why DPALs? - basics
Summary of surrogate pumping experiments
Advances in narrowband laser diode pump sources
Summary DPAL experiments
Scaling to high power
Some concluding observations
Power scalable to several tens of watts, ∆ν ~ 10 GHz
Pout ~10 Watts at 852 nm with ∆ν ~ 1.8 MHz
Frequency narrowing of a 25 W broad area diode laser**J. F. Sell, et. al., Applied Physics Letters, 94, 051115 2009
W. Krupke, CREOL April 17, 2009
Laser diode bar output set and narrowed by volume Bragg grating (VGB)*
*Gourevitch, et. al., Optics Letters, 33, 702 (2008)
Pout = 30 Watts at 780 nm with
∆ν~ 10 GHz (0.020 nm)
Power scalable to ~100 watt
W. Krupke, CREOL April 17, 2009
*Petersen and Lane, Proc. SPIE 6571-58; Petersen and Gloyd, ASSP, paper MD-2 (2008)
Spectra-Physics fiber-coupled, tunable VGB-line-narrowed pump source*
778.8
779
779.2
779.4
779.6
779.8
780
cent
er w
avel
engt
h (n
m)
(unc
orre
cted
)
780.2
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
-20 0 20 40 60grating mount temperature (deg C)
Comet s/n 027 with temp-tuned grating
80
38.5 W output
5.6 W output
100 120
• 54.5 W from a bare diode bar• 38.5 W from a fiber coupled module• 0.1 nm linewidth• 0.8 nm tuning range
Power scalable to >100 WattsW. Krupke, CREOL April 17, 2009
What is a DPAL? Why DPALs? - basics
Summary of surrogate pumping experiments
Advances in narrowband laser diode pump sources
Summary DPAL experiments
Scaling to high power
Some concluding observations
Outline
1st Author Atom Buffer gas ∆λpump(nm)
Ppump(W) Pout (W) ηslope(%)
Ehrenreich Cs ethane 0.027 0.4 0.13 41
Zhdanov Cs ethane 0.027 16 10 68
Zheng Cs ethane 0.2 50peak 6.9peak 14
Page Rb He, ethane 0.3 13peak 1peak 10
Zhdanov Rb ethane 0.027 37 17 53
Petersen Rb He, methane 0.25 8 7.8 15
Zhdanov Cs He, ethane 0.027 100peak 48peak 52
Summary of DPAL experimental results to date*
Power scaling of static, end-pumped, cell-based DPALs is constrained by gain medium heating and availability of higher brightness pump diode arrays
*for references, see Krupke, Proc. SPIE, 7005-75 (2008)
W. Krupke, CREOL April 17, 2009
What is a DPAL? Why DPALs? - basics
Summary of surrogate pumping experiments
Advances in narrowband laser diode pump sources
Summary DPAL experiments
Scaling to high power
Some concluding observations
Outline
Laser resonator axis
Flow directionPump arrays
Power scalable transverse-pumped, flowing DPALs – for very high powers
• Aperture transverse to flow is not limited by gain medium temp gradient
• Flow velocity set by allowed gain medium temperature rise
• Two-sided pumping or double-pass pumping options available
• Pump flux freely propagates (no wall reflections needed)
• Demand pump brightness is greatly reduced (<1/10 end-pumped DPALs)
• Pump and laser beams do not share optics (no polarizing dichroics, etc.)
>100 kW-class, transversely-pumped, flowing DPALs seem feasible
W. Krupke, CREOL April 17, 2009
Total population inversion (displaced upward for clarity)
Left Pump Right Pump
δ, δ = bleach wave thickness = 1/αpump
Population (gain) distribution under bleachwave pumping*
pop. inversion pop. inversion
Transverse gain distribution is not governed by Beer’s Exponential Absorption Law under bleachwave pumping; it is much more uniform due to pump saturation effects. Quantitative modeling is required
*W. F. Krupke, Opt. & Quantum Electronics, 22, S1 (1990).
W. Krupke, CREOL April 17, 2009
Some concluding comments
Several laboratory demonstrations have validated basic DPAL physics
A simple rate equation models predicts end-pumped DPAL energetics
Great advances have been made in power-scaling narrowband pumps
More complex models (transverse pumping, pooling, etc) will be needed
- transverse modal properties
- pooling, associative ionization, etc.
- transverse pumping geometries, spatial gain variations, etc.
DPAL power scaling > few 100 watts will likely use:
- a flowing gain medium
- transverse pumping
W. Krupke, CREOL April 17, 2009
Acknowledgements
I am pleased to acknowledge many insightful DPAL
discussions and collaborations with:
Dr. Ray Beach (LLNL)
Dr. Jason Zweiback (General Atomics)
Dr. Alan Petersen (Spectra-Physics)
For comprehensive descriptions of current DPAL research, see papers from the DPALs symposium at the SPIE High Power and Laser Ablation (HPLA) conference, Santa Fe, 2008 (Proc SPIE, 7005)
DPAL R&D support by the DOD Joint Technology Office is gratefully acknowledged
W. Krupke, CREOL April 17, 2009