High-power, high-energy diode-pumped

Tm:YLF-Ho:YLF laser

Alex Dergachev, and Peter F. Moulton

Q-Peak, Inc.

135 South Road, Bedford, Massachusetts 01730Tel.: (781) 275-9535, FAX: (781) 275-9726

E-mail: dergachev@qpeak.com

Acknowledgements:Lockheed MartinLaser Ultrasonic Technology Center

• Motivation• Previous Results• Tm:YLF Laser - Details• Ho:YLF Laser – Details• ZGP OPO• Conclusions

Outline

Motivation

Development of a 2-um laser source:• High-energy (up to 100 mJ)• High repetition rate (100-400 Hz) • High beam quality (TEMoo)

ZGPOPO

Ho:YLFlaser

Tm:YLFlaser

30 mJ3200 nm

100 mJ2050 nm

100 Hz - 400 Hz

CW1940 nm

Possible applications :• Pump source for other IR lasers • Industrial• Military

Approaches to diode-pumpingof Ho-doped lasers

780-790-nmdiode lasers Tm,Ho-laser

780-790-nm Tm-laser

1900-nm Ho-laser

Ho-laserdiode lasers

diode lasers

Advantages of Tm-pumped Ho-laser

• Compared to diode-pumped Tm, Ho-co-doped laser:

– Eliminates upconversion from Tm-Ho interaction that reduces efficiency and creates additional heating in crystal

– Eliminates energy sharing between Tm and Ho that limits energy extraction in Q-switched mode

• Compared to direct-diode-pumped Ho-laser

– Can operate at much higher power due to the availability of high-power diodes for Tm:YLF pumping

References on resonantly pumped Ho lasers

P.F. Moulton, “Industry R&D related to 2-μm lidars,” Second Review of 2-μm Solid State Laser Technology, NASA Headquarters, Washington, DC, May 18-19, 1992.

R.C. Stoneman and L. Esterowitz, Opt. Lett. 17, 736 (1992).

D.W Hart, M. Jani and N.P. Barnes, Opt. Lett. 21, 728 (1996).

M. Petros, J. Yu, U. N. Singh and N.P. Barnes, “High energy directly pumped Ho:YLF laser,” in Advanced Solid State Lasers, OSA Technical Digest (Optical Society of America, Washington, DC, 2000), pp. 79-81.

P.A. Budni, M.L. Lemons, J.R. Mosto, and E.P. Chicklis, IEEE J. Sel. Topics in Quantum Electron. 6, 629 (2000).

P.A. Budni, M.L. Lemons, C.A. Miller, P.A. Ketteridge, L.A. Pomeranz, T.M. Pollak, P.G.Schuneman, K.L. Lanier, J.R. Mosto, and E.P. Chicklis, “High power 1.9 micron pumped solid state holmium lasers,” in Conference on Lasers and Electro-Optics, OSA Technical Digest (Optical Society of America, Washington, DC, 2000), p 564.

L.D. DeLoach, S.A. Payne, L.L. Chase, L.K. Smith, W.L. Kway and W.F. Krupke, IEEE J. Quantum Electron. 29, 1179 (1993).

W.F. Krupke and L.L. Chase, Optical and Quantum Electron. 22, S1 (1989).

Previous results – Ho-lasers

Tm:YLF pumped Ho:YAGP. A. Budni et al., “High-power/high-brightness diode-pumped 1.9-µm Thulium and resonantly pumped 2.1-µm Holmium lasers,” IEEE J. on Selected Topics in Quantum Electron., 6, 629-635 (2000).

• Tm:YLF pump– 36 W CW output at 1.907 mm (σ-line)– Multimode, M2 ~ 2

• Ho:YAG– CW: 19 W– QCW: 16 W at 15 kHz

Ho:YLF vs Ho:YAG

Why Ho:YLF?• Long upper laser level lifetime ~ 15 ms• Higher emission cross-section• Naturally birefringent material• Low dn/dT –> weak thermal lensing

Ho:YAG•Isotropic

•Lifetime (5I7) 7 ms

•Strong thermal lensing

•Excellent thermo-mechanical properties

Gain Calculation – 3.5% Tm:YLF

-0.25

-0.20

-0.15

-0.10

-0.05

0.00

0.05

0.10

0.15

0.20

0.25

1800 1825 1850 1875 1900 1925 1950 1975 2000

Wavelength, nm

Gai

n co

effic

ient

, cm

-1

0.25, π

0.25, σ

0.15, π

0.15, σ

Inversion fraction

Polarized gain in Tm:YLF at two values of inversion fractiong(λ) = N [ p⋅σem(λ) - (1-p)⋅σabs(λ) ],where p – inversion fraction, N - Tm-concentration

Pumping Ho:YLF with Tm:YLF laser

0

0.5

1

1.5

2

2.5

3

3.5

1800 1850 1900 1950 2000 2050 2100 2150

Wavelength, nm

Abs

orpt

ion

coef

ficie

nt, c

m-1

Ho abs - PiTm-tuning

2.0% Ho:YLF

Tm:YLF Active Element:Rectangular slab: 22-mm longClear aperture 2x6 mm.

Experimental Set-Up – Tm:YLF Laser

Tm:YLF DL

HR

OC

DL

BRF

Tm:YLF DL

DL

Tm:YLF – Dual GM Oscillator – 1 pass

0

5

10

15

20

25

30

35

0 20 40 60 80 100 120 140 160

Total diode power, W

Out

put p

ower

, W

Slope efficiency 38%

Tm:YLF – Dual GM Oscillator – 3 passes

0

5

10

15

20

25

30

0 20 40 60 80 100 120 140 160 180

Diode pump power, W

Out

put p

ower

, W

25% slope efficiency

Calculations for Tm:YLF-pumped Ho:YLF laser at low pulse rates

Calculations are based on work by W.F. Krupke and L.L. Chase, Optical and Quantum Electron. 22, S1 (1989).

Crystal doping (%) 0.5N0 (cm-3) 7 x 1019

Crystal length (cm) 3.6Scaled pump fluence 1.6Pump pulsewidth (msec) 15Pump power (W) 20

average inversion fraction 0.44ηs pumping efficiency 0.52Fp (J/cm2) 21.1Pump energy (J) 0.3Pump-beam radius (cm) 0.048Stored energy in crystal (J) 0.16g0, Gain coefficient (cm -1) 0.36G, Single-pass gain 3.7

Schematic layout of the end-pumped Ho:YLF laser

HR

OC

AOM

Ho:YLF

Tm:YLF laser #1

Tm:YLF laser #2

DM

DM

DM – Dichroic Mirror, AOM – Acousto-Optic Modulator, OC – Output Coupler, HR – High Reflector

CW Ho:YLF Laser Operation (TEMoo)

0

5

10

15

20

25

0 10 20 30 40 50 60

Total Tm pump power, W

Ho:

YLF

CW

out

put,

W

10%15%40%70%

54% slope efficiency

45% slope efficiency

Toc

Ho:YLF – Q-Switched Operation (TEMoo)

0

2

4

6

8

10

12

14

16

18

0 500 1000 1500 2000 2500

Repetition rate, Hz

Out

put p

ower

, W

0

5

10

15

20

25

30

35

40

Puls

e en

ergy

, mJ

PE

Ho:YLF – Pulsewidth vs repetition rate

0

5

10

15

20

25

0 200 400 600 800 1000 1200

Repetition rate, Hz

Puls

ewid

th, n

s

ZGP OPO - Layout

ZGP OPO:• ZGP 1 cm-long• Type I, 53o-cut • Flat/flat resonator• Singly resonant cavity• Pump – double pass• ~6 cm-long resonator• OC 38% at 3.2 um

R=38% 3.2 um

HR 2.05 um

HT 5.7 um

HR 3.2 um

HT 2.05 and 5.7 um

ZGP

ZGP Operation – 400 Hz

0

0.5

1

1.5

2

2.5

3

3.5

4

4.5

0 1 2 3 4 5 6 7 8 9 10

Pump power, W

OPO

out

put,

W

0.00

0.05

0.10

0.15

0.20

0.25

0.30

0.35

0.40

0.45

0.50

Con

vers

ion

effic

ienc

y

PoutEff.

Slope Efficiency 63%

400 Hz

ZGP OPO Operation – Pulse energy

0

2

4

6

8

10

12

14

0 5 10 15 20 25 30

Pump pulse energy, mJ

OPO

pul

se e

nerg

y, m

J

50 Hz200 Hz400 Hz

Slope Efficiency:50 Hz 60%200 Hz 56%400 Hz 63%

Development of an efficient Tm:YLF - Ho:YLF – ZGP laser system:

Conclusions

Ho:YLF laser:• Highest (to the best of our knowledge) CW output of

21 W for 2-μm Ho:YLF laser• Efficient Q-switched operation (up to 37 mJ per pulse)• Repetition rates in wide range from Hz to kHz,

particularly, in 100-400 Hz • High beam quality TEMoo beam

ZGP OPO • Demonstrated > 10 mJ (total) output at 50-400 Hz

rep. rates