A hybrid diode-gas laser approach to high power and ... · A DPAL utilizes a neutral alkali vapor atom as the active specie DPAL: a quasi-two-level laser with a small quantum defect

Bill Krupke

WFK Lasers, LLC.

[email protected]

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


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


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?


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:


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


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%


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


Outline







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


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)


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


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


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


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)


Outline







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


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


*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







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)








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


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).


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


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


Documents

A hybrid diode-gas laser approach to high power and ... · A DPAL utilizes a neutral alkali vapor atom as the active specie DPAL: a quasi-two-level laser with a small quantum defect