A hybrid diode-gas laser approach to high power and brightness (DPAL)

Transcription

1 A hybrid diode-gas laser approach to high power and brightness (DPAL) Bill Krupke WFK Lasers, LLC. CREOL Industrial Affiliates Day Orlando, Florida April 17, 2009

2 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

3 What is a DPAL? --- a hybrid electric gas laser Why DPALs? Laser diode or diode array pump high efficiency (~60-70 %) high power, but poor beam quality Gas (Vapor) laser gain medium no stress birefringence no stress fracture low index of refraction (density) convective removal of waste heat reduced thermal focusing Output Beam high average output power with high beam quality single aperture power scaling

4 A DPAL utilizes a neutral alkali vapor atom as the active specie Quantum defect = = δε/ν pump levels ideally in Boltzmann equilibrium 2 P 3/2 δe collisional relaxation (buffer gas: He, CH 4, etc) 2 P 1/2 D 2 line ν pump, λ pump D 1 line ν laser, λ laser 2 S 1/2 Alkali atom (Cs, Rb, K, Na, Li) DPAL: a quasi-two-level laser with a small quantum defect

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6 Alkali atoms have small quantum defects & narrow Doppler widths 2 P 3/2 2 P 1/2 E atom λ pump (D 2 ), nm λ laser (D 1 ), nm E, cm - 1 Q-defect λ pump 2 S 1/2 λ laser K Rb Cs atom λ pump nm ν (GHz) λ (nm) K Rb Cs Very good! Problematic!

7 History of alkali atom kinetics Population inversion and ASE had been observed on the D 1 transition of the D 2 -transition-pumped alkali atoms with buffer gases 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) 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?

8 DPAL: a spatial and spectral mode converter of LDs HP Diode Array Alkali Mode Converter Bright Source P pump > kws λ ~2-4 nm (M 2 ) slow ~1000 η conv form pop. inversion; TEM 00 mode extraction B ~ η conv *P pump /(A out Ω) λ << nm M 2 ~ 1 Additionally, DPALs are attractive for intra-cavity harmonic generation - atomic precision and wavelength stability - high gain coefficients - short operating wavelengths

9 How to efficiently pump intrinsically narrow-band alkali transitions with a relatively broadband pump sources? Solution: 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 Line Shape Comparison of Relevant Linshapes Radiative Doppler (127 C) Pressure (10 atm He) Diode (2nm FWHM) nm Efficient DPAL pumping can be realized even when the diode pump emission linewidth is many times the alkali absorption linewidth

10 Alkali helium collisional broadening rates ν L (FWHM) = π -1 Z L = γ(v T ) N T (He) Alkali Atom γ (GHz/amg) K, potassium 26.7 Rb, rubidium 18.6 Cs, cesium 21.7 Alkali Atom P homogeneous (atm) λ L, min (nm) K, potassium Rb, rubidium Cs, cesium D 1,2 transitions become predominately Lorentzian when ν L > 10 x ν D

11 Typical DPAL gain medium parameter ranges alkali cold temperatures o C alkali pressures 5-20 mtorr alkali number densities 3-5 x /cc He buffer pressures atm Small-hydrocarbon buffer pressures torr alkali peak Lorentzian cross-sections 5 30 x /cc alkali Lorentzian transition linewidths nm Double-pass pump absorbed fraction >90%

12 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 cm 2 30,000, x 10 7 Laser transition linewidth nm 0.03 ~ 9 ~ 3x 10-3 Upper laser level lifetime * µsec ~3 x10-5 Laser saturation fluence ** mj / cm ~10,000 ~10-7 ~ ss gain coefficient cm ~2 x10 1 ~ operating intensity kw / cm 2 ~10 ~20 ~ 0.5 # 1 atm helium buffer gas * sat fluence hc/λσ

13 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

14 Early DPAL experiments have utilized a classic end-pumped geometry pump diode gain cell laser pump-laser mode matching is facilitated for TEM oo 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

15 Rb laser setup* Titanium Sapphire surrogate pump *Krupke, et. al, Optics Letters, 28, 2336 (2003) thin film polarizer Oven TiS laser λ ~0.1 nm 780 nm pump gain cell flat HR Mirror 795 nm laser 20 cm radius concave output coupler Rubidium density = 10 microns (1.7 x / cc) Ethane pressure 75 torr Helium pressure 525 torr

16 795 nm Rb laser oscillation* *Krupke, et. al, Optics Letters, 28, 2336 (2003) Output power [mw] Rb density = 1.7 E13/cc Output Coupler Reflectivity = 50% slope power efficiency = 49% Absorbed pump power [mw] The pump width was 4 times the Lorentzian pump transition width Effective (homogeneous) wing-pumping was quantitatively confirmed

17 DPAL energetics are accounted for using a simple quasi-two-level model* *Beach, et. al, JOSA B21, 2151 (2004) The Beach quasi-three-level, end-pumped Yb laser model** was adapted to a quasi-two-level model, appropriate to a DPAL: TiS pumped Cs DPAL* end-pumped geometry rate equation kinetics plane-wave only literature spectroscopic data literature collisional data experiment : model : The literature has all necessary spectroscopic-kinetic data to estimate DPAL laser energetics The next stage in modeling will treat transverse modal properties X

18 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

19 Summary of surrogate-pumped alkali resonance lasers* *for references, see Krupke, Proc. SPIE, (2008) Author Atom Buffer gas ν p (GHz) P pump (mw) P out (mw) η slope (%) Krupke Rb He, ethane Zweiback Rb He, methane 98 7 mj 4.1 mj 72 Wu Rb He Wu Rb He Beach Cs He, ethane Zhdanov Cs He, ethane Zhdanov K He, ethane ? Zhdanov K He? Zweiback K He mj 14 67

20 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

21 Frequency narrowing of a 25 W broad area diode laser* *J. F. Sell, et. al., Applied Physics Letters, 94, P out ~10 Watts at 852 nm with ν ~ 1.8 MHz Power scalable to several tens of watts, ν ~ 10 GHz

22 Laser diode bar output set and narrowed by volume Bragg grating (VGB)* *Gourevitch, et. al., Optics Letters, 33, 702 (2008) P out = 30 Watts at 780 nm with ν ~ 10 GHz (0.020 nm) Power scalable to ~100 watt

23 Spectra-Physics fiber-coupled, tunable VGB-line-narrowed pump source* *Petersen and Lane, Proc. SPIE ; Petersen and Gloyd, ASSP, paper MD-2 (2008) Comet s/n 027 with temp-tuned grating center wavelength (nm) (uncorrected) W output 5.6 W output grating mount temperature (deg C) 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 Watts

24 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

25 Summary of DPAL experimental results to date* *for references, see Krupke, Proc. SPIE, (2008) 1 st Author Atom Buffer gas λ pump (nm) P pump (W) P out (W) η slope (%) Ehrenreich Cs ethane Zhdanov Cs ethane Zheng Cs ethane peak 6.9 peak 14 Page Rb He, ethane peak 1 peak 10 Zhdanov Rb ethane Petersen Rb He, methane Zhdanov Cs He, ethane peak 48 peak 52 Power scaling of static, end-pumped, cell-based DPALs is constrained by gain medium heating and availability of higher brightness pump diode arrays

26 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

27 Power scalable transverse-pumped, flowing DPALs for very high powers Pump arrays Flow direction Laser resonator axis 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

28 Population (gain) distribution under bleachwave pumping* *W. F. Krupke, Opt. & Quantum Electronics, 22, S1 (1990). Total population inversion (displaced upward for clarity) pop. inversion pop. inversion Left Pump Right Pump δ, δ = bleach wave thickness = 1/α pump 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

29 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

30 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) DPAL R&D support by the DOD Joint Technology Office is gratefully acknowledged 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)