Progress in Hybrid Fiber/Bulk Solid State Lasers

Alex Dergachev, Peter F. Moulton

Q-Peak, Inc. 135 South Road, Bedford, Massachusetts 01730 E-mail: [email protected]

Thomas E. Drake

Lockheed Martin Aeronautics Company One Lockheed Blvd. MZ 6852 Bldg.4 22C, Fort Worth, TX 76108

Photonics West’06 Outline

 Motivation

 General Info on GSD lasers • Pump sources for GSD lasers

 Ho-properties – brief overview

 Tm:fiber Laser – Details

 Previous Results • Q-Peak’ Previous Results

 Ho:YLF Laser – Details

 Conclusions

Photonics West’06 Motivation

 Development of an IR laser source: • High-energy (> 200 mJ) • High repetition rate (200-1000 Hz)

• High beam quality (TEM00)

 Immediate applications: • Pump source for OPOs • Lidar • Medical, industrial, military

Photonics West’06 Ground state depleted lasers

 Yb, Er, Ho, Tm lasers are examples of ground state depleted (GSD) lasers

 GSD laser operates as a quasi-4-level system: • Pumping is accomplished into the 1st excited manifold • Lower laser level belongs to the ground manifold and is partially populated at room temperature

• GSD laser can not be [1-quantum-defect]-efficient at RT !!!

 Fundamental spectroscopic and laser properties pertinent to GSD lasers are systematically and quantitatively analyzed in prior work at LLNL

• W.F. Krupke and L.L. Chase, Optical and Quantum Electron. 22, S1 (1990). • S.A. Payne, L.L. Chase, L.K. Smith, W.L. Kway and W.F. Krupke, IEEE J. Quantum Electron. 28, 2619 (1992).

• 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).

Photonics West’06 Why Ho-lasers?

 Most of the laser transitions in 2-um region have such a low gain cross-section that efficient, high-energy laser oscillation or amplification is impossible:

• The energy fluence required for efficient extraction of stored energy in the laser material is so high that they will lead to optical damage of the laser crystal or associated optics.  Only Ho-doped crystals, including Ho:YAG and Ho:YLF, have a large enough gain cross-section for effective high- energy operation. -20 2 • Ho:YAG em~1x10 cm -20 2 • Ho:YLF em~2x10 cm

Photonics West’06 Approaches to diode-pumping of Ho-doped lasers

780-790-nm Tm,Ho-laser diode lasers

780-790-nm Tm-laser Ho-laser diode lasers

940-980-nm Er-laser Tm-laser Ho-laser diode lasers

~1900-nm diode lasers Ho-laser

Photonics West’06 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 availability of high-power Tm-lasers

Photonics West’06 Pump sources for Ho-laser

 Relatively high-brightness sources are required (preferably M2<10) • GSD laser pumping requires high optical density L>>1  1850-1950 nm wavelength range

 High power to achieve Ho-output >>10 W

 Possible alternatives:

• Diode-pumped Tm-bulk solid state lasers

• Diode-pumped Tm:fiber lasers Er or Tm

Photonics West’06 Ho-laser power scaling

 Cw Tm:fiber lasers with output >100 W emerge as an alternative to bulk Tm-laser:

• Turn key operation • Cost-effective • Maintenance-free • Fiber delivery (no surprise!) • Excellent beam quality • Scalable power • Wide tuning range • Commercially available

Photonics West’06 Previous results – Ho-lasers

 Tm:YLF pumped Ho:YAG P. 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 1907 nm (-line) – Multimode, M2 ~ 2

• Ho:YAG

– CW: 19 W

– QS: 16 W at 15 kHz

 Tm:YLF pumped Ho:YLF A. Dergachev, P.F.Moulton, "High-power, high-energy diode-pumped Tm:YLF-Ho:YLF-ZGP laser system", OSA TOPS Vol. 83, ASSP, (OSA, 2003), pp. 137-141 • Tm:YLF pump – 2 x ~25 W CW output at 1940 nm (-line) – M2 ~ 1.05 x 7

• Ho:YLF

– CW: 21 W

– QS: 16 W at 1 kHz

 Ho-lasers pumped with Tm:fiber lasers: • ORC, Univ. of Southampton, UK (Ho:YAG) ASSP’03 (A.Agdolvand et al., TOPS Vol.83, 7 (2003)) • NASA (Ho:YLF) • FFI (Forsvarets forskningsinstitutt), Norway (Ho:YAG) ASSP’03 (E.Lippert et al., TOPS Vol.83, 292 (2003)) • BAE Systems (Ho:YAG) • Q-Peak (Ho:YLF) ASSP’05

Photonics West’06 Ho:YLF vs Ho:YAG

 Ho:YLF • Long upper laser level lifetime ~ 15 ms • Higher emission cross-section • Naturally birefringent material • Low dn/dT –> weak thermal lensing • ~5% quantum defect

Ho:YAG •Isotropic

5 •Lifetime ( I7) 7 ms •Strong thermal lensing

•Excellent thermo-mechanical properties •~10% quantum defect

Photonics West’06 Ho:YLF – Absorption/ Emission

2.5E-20 Lasing

2 2.0E-20 Pump

1.5E-20

Pi-abs Pi-em

1.0E-20

Abs./Emiss. cross-section, cm cross-section, Abs./Emiss. 5.0E-21

0.0E+00 1800 1850 1900 1950 2000 2050 2100 2150 Wavelength, nm

Cross-section determination - reciprocity method:

em( ) = abs () ( Zl /Zu ) exp [ (EZL -h) / kT ]

(Following S.A.Payne et al. IEEE J. of QE, 28, 2619-2630 (1992)). Photonics West’06 Ho:YLF – Calculated gain () vs inverted fraction

0.60

0.40

0.20 -1 0.5 0.00 0.3 1800 1850 1900 1950 2000 2050 2100 2150 0.2 Gain, cm

-0.20

-0.40

-0.60 Wavelength, nm

Photonics West’06 Q-Peak’ Ho-laser development

2001 • Tunable Tm:YLF laser (>15 W )

2003 • Single Ho-crystal oscillator pumped with two Tm:YLF lasers (~50 W total power)

2005 • Double Ho-crystal oscillator pumped with one 100-W Tm:fiber laser

2006 • Single Ho-crystal oscillator followed by three amplifiers pumped with two Tm:fiber lasers ( ~230 W total power)

Photonics West’06 Q-Peak’ prior results: Pumping Ho:YLF with Tm:YLF laser

3.5 2.0% Ho:YLF

3 -1 2.5

2 Ho abs - Pi Tm-tuning 1.5

1 Absorption coefficient, cm

0.5

0 1850 1900 1950 2000 2050 2100 Wavelength, nm

Photonics West’06 Q-Peak’ prior results: Experimental Set-Up – Tm:YLF Laser

Tm:YLF DL

Tm:YLF DL OC HR DL BRF DL

 Dual-Gain Module oscillator  Beam quality: M2 ~ 1.05 x 7

 Wavelength tuning with BRF element • Alternative: Volume Bragg Grating reflector  Average power (CW) > 30 W at 1940 nm

Photonics West’06 Q-Peak’ Prior Results: End-pumped single-crystal Ho:YLF laser

Tm:YLF laser #2 AOM

DM CW output: 21 W OC (max)

Ho:YLF Pulse energy (max): 100 Hz 35 mJ 400 Hz 27 mJ DM HR Pulsewidth:

Tm:YLF laser #1 100 Hz 12 ns 400 Hz 15 ns

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

Photonics West’06 Scaling of Ho-laser: Tm-pump source

 Specific requirements for Tm-laser as a pump source for Ho:YLF: • Linear polarization (preferably) • Lasing wavelength at ~ 1940 nm • Linewidth < 6 nm

Tm-fiber laser TLR-100-1940 (IPG Photonics, www.ipgphotonics.com )

Operation regime CW Operational temperature RT Output power ≥ 100 W Lasing wavelength range: 1750-2200 nm

Polarization: Random Linewidth ≤ 2 nm

Photonics West’06 Schematic layout of the double-crystal end-pumped Ho:YLF laser

PBS Tm:fiber laser /2

DM DM HR

OC AOM Ho:YLF Ho:YLF

DM DM

DM – Dichroic Mirror, AOM – Acousto-Optic Modulator, OC – Output Coupler, HR – High Reflector Photonics West’06

Ho:YLF Laser – Power output

45 Mode size (dia): "Tight" 40 focusing (w0) w0 = 0.8 mm 35 w1 = 1.0 mm w2 = 1.3 mm 30 w0-cw 42% slope efficency w1-cw 25 w1-400 "Moderate" w1-200 20 focusing (w1) w2-cw w2-200 w2-400 15 Ho:YLF output power, W 10

"Loose" 5 focusing (w2)

0 0 20406080100120 Total Tm-pump power, W

Photonics West’06 Ho:YLF Laser – Pulse energy

50 20 ns 45 Mode size (dia): w1 = 1.0 mm 17 ns 22 ns 40 "Loose" w2 = 1.3 mm focusing 35 28 ns

30 w1-1k 32 ns w1-400 25 w1-200 w2-200 20 w2-400

"Moderate" 15 focusing Ho:YLF pulse energy, mJ pulse Ho:YLF 10

5

0 0 20406080100120 Total Tm-pump power, W

Photonics West’06 Current work: Specific objectives

CW >200 mJ 1940 nm 2050 nm Tm:fiber Ho:YLF Ho:YLF laser OSC AMPs

500 Hz - 1000 Hz

 Our approach for Tm-pumped Ho-laser: • Tm-pump CW Tm-fiber lasers • Ho-laser: Ho:YLF oscillator with AO Q-switching • Ho-Amplifiers Single-pass

Photonics West’06 Tm:fiber laser pumped single-crystal Ho:YLF laser

/2 PBS Pump to Tm-fiber laser AMP #1

OC DM

Master Ho:YLF Oscillator AO HR

DM

DM – Dichroic Mirror, AOM – Acousto-Optic Modulator, OC – Output Coupler, HR – High Reflector Photonics West’06

Ho:YLF Master Oscillator

30 60 1 kHz 0.5 kHz

25 50

20 40 1 kHz 0.5 kHz

15 30

10 20 Pulsewidth, ns Energy per pulse, mJ

5 10

0 0 0 10203040506070 Input power, W

Photonics West’06 Ho:YLF MOPA

Tm-fiber laser Ho-osc 120 W CW 500 Hz 1000 Hz Ho-Amp #1 Master 19 W 25 mJ 17 mJ osc

Amp 1 42 W 55 mJ 38 mJ Tm-fiber laser Ho-Amp #2 110 W Amp 2 60W 90 mJ 58 mJ Ho-Amp #3

Amp 3 78 W 115 mJ 73 mJ

Tm-fiber laser Ho-Amp#4 100 W Amp 4 97 W *** ***

Ho-Amp #5 Amp 5 113 W *** ***

*** work in progress

Photonics West’06 Ho-laser efficiency

 In order to achieve efficient Ho-operation • High optical density

• Tight focusing to deplete (bleach) the ground state  This works well in CW regime

 However, in Q-switched regime it is necessary to avoid the damage of all optical components  A few examples: • We assumed generation of 10 mJ pulses with 15 ns pulsewidth

• We took the values for the mode size and OC reflectivity from different Ho-papers and calculated intracavity energy/power density Roc Beam Radius, Av. energy Av. power density,

um density, J/cm2 MW/cm2 Q-Peak 0.3 0.45 2.9 195

ORC 0.8 0.23 54 3600

BAE 0.85 0.28 50 3340

Photonics West’06 Conclusions

Development of an efficient 2-um Ho:YLF MOPA pumped with cw-Tm-fiber-lasers:

 Scalable (modular) Ho:YLF MOPA configuration

• Highest (to the best of our knowledge) CW output of 113 W • Efficient Q-switched operation (> 115 mJ per pulse)

 Repetition rates in wide range (Hz to kHz),

particularly, in 500-1000 Hz range

 High beam quality (TEM00 beam)

Future work:  Optimize Ho-amplifiers

 Test complete Ho-MOPA in QS regime  Pump the OPO Photonics West’06

Acknowledgements

We would like to thank:

 Lockheed-Martin Center for Laser Ultrasonics for continuous support of our work

 Dr. Valentin Gapontsev for sharing his vision on the progress of fiber lasers in general, and Tm:fiber lasers in particular.

 Dr. Denis Gapontsev and Dr. Nikolai Platonov for fruitful technical discussions

 All our vendors who provided crystals, optics and coatings

Photonics West’06