Power Scaling of Tm:fiber Lasers to the kW Level

Peter F. MoultonQ-Peak, Inc.

CREOLIndustrial Affiliates Day 2009

High Power Optical Sources for the 21st CenturyApril 17, 2009

Outline

• Background• Fundamentals of Tm:silica fiber lasers• Fiber laser setup and results

Support:

HEL-JTO Contract Nos. FA9451-06-D-0009 andFA9451-08-D-0199

Technical work:

Q-Peak: Glen Rines, Evgueni Slobodtchikov, Kevin Wall, Nufern: Gavin Frith, Bryce Samson, Adrian Carter

Relative eye safety is obtainedfor > 1400-nm wavelengths

Retinal focusing can increase the power density by 105

Rare-earth laser transitionscan provide eyesafe wavelengths in fibers

Ener

gy (w

aven

umbe

r/100

00)

1080 nm

1950-2050 nm

1550 nm

1060 nm930 nm

Tm-ion cross relaxation allows excitation of two upper laser levels for one pump photon

Prior work with Tm:YAG lasers

E.C. Honea, R.J. Beach, S.B. Sutton, J. A. Speth, S.C. Mitchell, J.A. Skidmore, M.A. Emanuel, and S.A. Payne, “115-W Tm:YAG Diode-Pumped Solid-State Laser,” J. Quantum Electron. 33, 1592 (1997).

K. S. Lai, P. B. Phua, R. F. Wu, Y. L. Lim, E. Lau, S. W. Toh, B. T. Toh, and A. Chng, "120-W continuous-wave diode-pumped Tm:YAG laser ," Opt. Lett. 25, 1591-1593 (2000)

Recent advances in Tm-doped fiber-laser efficiencies show levels approaching Yb fibers

Fundamentals of Tm:silica fiber lasers

Absorption and emission cross sectionsfor Tm:silica

0.0E+00

5.0E-22

1.0E-21

1.5E-21

2.0E-21

2.5E-21

3.0E-21

3.5E-21

4.0E-21

4.5E-21

5.0E-21

1400 1500 1600 1700 1800 1900 2000 2100 2200 2300

Wavelength (nm)

Cro

ss s

ectio

n (c

m2)

AbsorptionEmission

Abs: NufernEm: Walsh (NASA)

Calculation of net gain in Tm:silica fiber laser

We define the inversion fraction as: F = N2 / (N2 + N1), where N1 and N2 are the inversion densities for the lower and upper Tm:silica laser levels. The net gain (or loss) cross section σ(λ) in the fiber as a function of wavelength, λ, is given by the relation: σ (λ) = F σe (λ) – (1-F) σa (λ), where σe (λ) and σa (λ) are the emission and absorption cross sections. The gain or loss coefficient is σ (λ) multiplied by the concentration of active ions.

Plot of net gain cross section in Tm:silicavs. inversion fraction

-5E-21

-4E-21

-3E-21

-2E-21

-1E-21

0

1E-21

2E-21

3E-21

4E-21

5E-21

1500 1600 1700 1800 1900 2000 2100 2200

Wavelength (nm)

Net

gai

n cr

oss

sect

ion

(cm

2)

1

0.9

0.8

0.7

0.6

0.5

0.4

0.3

0.2

0.1

0.05

Data on emission cross section from Walsh and absorption cross sections from Nufern

Tm:silica gain at low inversions

-2E-22

-1.5E-22

-1E-22

-5E-23

0

5E-23

1E-22

1.5E-22

1900 1920 1940 1960 1980 2000 2020 2040 2060 2080 2100 2120 2140 2160 2180 2200

Wavelength (nm)

Net

gai

n cr

oss

sect

ion

(cm

2)

0.1

0.09

0.08

0.07

0.06

0.05

0.04

0.03

0.02

0.01

Net gain cross sections needed for5-m fiber length, with gain of 25

Polished preforms and sample holder

Absorbance data from Lambda 9 measurements

0

0.05

0.1

0.15

0.2

0.25

0.3

600 650 700 750 800 850

Wavelength (nm)

Abs

orba

nce

HILO

Result:LO: 2.03-2.36 wt% (Tm2O3)HI: 2.47-2.87 wt%

The photodarkening issue has not appeared in pumping highly doped fibers at 790 nm

0

5000

10000

15000

20000

25000

30000

35000

40000

Ener

gy (c

m-1

)

3H6

3F4

3H5

3H4

3F2,3

1G4

1D2

1I63P03P1

3P2

790-nmpump

M.M. Broer, D.M. Krol and D.J. DiGiovanni, “Highly nonlinear near-resonant photodarkening in a thulium-doped aluminosilicate glass fiber,” Opt. Lett. 18, 799 (1993).

Dynamics measurements of Tm:silica

800-nm emission

Decay data for 3F4 (upper laser) level showstwo-lifetime dynamics

0

10000

20000

30000

40000

50000

60000

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0

Time (msec)

Sign

al (a

rb. u

nits

)

LO data broadband

Double exponential fit

633 usec lifetime

281 usec lifetime

Initial portion of 3F4 signal showsfeeding from pumped level

0

10000

20000

30000

40000

50000

60000

0.000 0.005 0.010 0.015 0.020 0.025 0.030 0.035 0.040

Time (msec)

Sign

al (a

rb. u

nits

)

800-nm fluorescence provides dataon cross-relaxation efficiency

0

10000

20000

30000

40000

50000

60000

0 10 20 30 40 50 60 70 80 90 100

Time (microseconds)

Sign

al le

vel (

arb.

uni

ts)

LO sample

HI sample

5.6 usec to 1/e

7.9 usec to 1/e

Decay in tail: 24.3 usec (LO), 21.0 usec (HI)

Assuming 45 usec lifetime for low Tm doping, efficiency of cross relaxation:

74% for LO80% for HI

Fiber laser setup and results

Approach to scaling follows on work done by SOTON on Yb:fiber lasers

Diode stack@975 nm, 1.2 kW

Double-clad Yb-doped fibre

HT @975 nmHR @~1.1 µm

Signal output@~1.1 µmHT @975 nm

HR @~1.1 µm

Diode stack@975 nm, 0.6 kW

HT @975 nmHR @~1.1 µm

0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.80.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

Slope efficiency: 83%

Sign

al p

ower

[kW

]

Launched pump powwer [kW]

Measured Linear fit

Details of the 790-nm pump band (2 wt. % Tm) showing broad absorption

0

1

2

3

4

5

6

7

750 760 770 780 790 800 810 820 830 840 850

Wavelength (nm)

Atte

nuat

ion

(dB

/m)

350-W Laserline pump laser (1 of 2)

Rack unit with diodes, power supply and cooler

5-m delivery fiber

High-powerconnector

1:1 lens focusing optics

Pump laser wavelengths were 795 nm at full power

Spectral emission data for pump lasers #1 and #2, respectively at a drive current of 55A, approximately 350 W of power output.

Q-Peak fiber-laser testbed

powermeter

Pump Laser A

Pump Laser B

focusinghead

Meniscus2.5-cm R concave surface

HR at 2050 nmHT at 790 nm

Dichroicmirror

HR at 2050 nmHT at 790 nm

clamp

Heat sink

clamp

Active fiber coil

focusinghead

2050 nm output

793-nm pump400-um, 0.2 NAfiber delivery

Single-ended pump

Characteristics of Nufern-supplied fibers

Fiber ID MM-TDF-20/400-LO

MM-TDF-20/400-Hi

LMA-20/35/400-Hi

Core diameter 17-23µm 17-23µm 17-23µm Clad diameter 385-415µm 385-415µm 385-415µm Core NA (effective) 0.2 0.2 0.1 Cladding NA 0.46 0.46 0.46 V value at 2µm >6 >6 <4 # of modes (2µm) 7 7 2 Cladding absorption (795nm)

~2dB/m ~2.6dB/m ~2.6dB/m

Tm-concentration 2.7wt% 3.5wt% 3.5wt% Cladding Shape Octagon Octagon Octagon

At 790.1 nm (2.5-nm linewidth) we measured 1.09 db/m for LO fiber (10-m length)and 1.54 db/m for HI fiber (7-m length)

Summary of highest-powerLO and HI Tm:fiber lasers

0

25

50

75

100

125

150

175

200

225

250

0 50 100 150 200 250 300 350 400 450 500 550

Launched pump power (W)

Out

put p

ower

(W)

LO fiber data

LO linear fit

HI fiber data

46.3% slope

`

LMA HI2 fiber design used undoped, spliced ends

Fiber assembly: 5-m length of Tm-doped fiber (3), with two undoped,3-m-long fibers (1) fusion-spliced (2) to the ends of the doped fiber

Gain fiber: LMA HI2Cores: 25 µm in diameter, NA: 0.08.Pump claddings: 400-µm in diameter, octagonal cross sectionPump attenuation: 2.9 dB/m

x21

3

x12

300 W from 25/400 Tm:fiber laser

0

25

50

75

100

125

150

175

200

225

250

275

300

325

0 50 100 150 200 250 300 350 400 450 500 550 600

Launched pump power (W)

Out

put p

ower

(W)

LMA HI2 fiber data conduction cooled, new clampsLinear fitLMA HI2 fiber data conduction cooledLinear fitLMA HI2 fiber data water cooledLinear fit

59.1% slope

61.8% slope

301 W

64.5% slope

Slope efficiency data, corrected for absorbed power, is 71.7% in good agreement with the value of 69.8% calculated by spectroscopy. The pump quantum efficiency is 1.84.

LMA HI2 fiber laser beam quality close to D.L.

0

500

1000

1500

200 250 300 350 400EP 7290 camera distance (mm)

Bea

m w

idth

( µm

)

Horiz. raw dataVert. raw dataHoriz. processed dataVert. processed data

Vert. axis, My2=1.16

Horiz. axis, Mx2=1.21

Tuning of LMA HI2 laser limitedon short-wavelength end by high gain

0

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8

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1960 1980 2000 2020 2040 2060 2080 2100

Wavelength (nm)

Pow

er o

utpu

t (W

)

The laser was pumped from one end with 47 W, and had a 600g/mm Littrow grating as an end mirror

Next: Scale the Tm-doped fiber laser to 1 kW

Fiber coupled diode stacks1000 W at 790 nm, 1000 um 0.22 NA

Double-clad Tm-doped fiber

Cladding 625 um, 0.46 NACore 35 um

HT @790 nmHR @~2 µm

HT @790 nmHR @~2 µm

Signal output@~2 µm

HT @790 nmHR @~2 µm

Rack of pump lasers, 1- kW Q-Peak pump data

350 Wpumps

1 kWpump

Hole forsecond1 kWpump

I/O data from Laserlines 1-kW LDM(S/N 760420)

0

200

400

600

800

1000

1200

1 1.5 2 2.5 3 3.5 4

Current Monitor (V)

Pow

er O

utpu

t (W

)

Power through undoped fiberMM-GDF-625/35

Power through lens assembly

86% transmission through fiber(93% maximum with uncoated ends)

New world’s record for Tm:fiber power

0

100

200

300

400

500

600

700

800

900

1000

0 250 500 750 1000 1250 1500 1750 2000

Launched pump power (W)

Out

put p

ower

(W)

Fiber 2

Fiber 1

51% slope

885 W @ 50.7% efficiency

Cutback measurements on 35/625 fiber show absorption to be double exponential

0

0.1

0.2

0.3

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0.9

1

0 1 2 3 4 5 6 7

Length (m)

Rel

ativ

e tr

ansm

issi

on 1.21.31.4Short fitLong fitTotal fit

Source setpoint (V)

and fit

T = As exp (- αs L) + Al exp (- αl L)

As 0.493αs (/m) 1.38Al 0.395αl (/m) 0.50

Cutback absorption measurements on 400-um cladding fibers - single exponential absorption

0

1

2

3

4

5

6

7

8

0 2 4 6 8 10

Length (m)

Pow

er (W

)LO data

LO fit

LMA HI1 data

LMA HI1 fit

LMA HI2 data

LMA HI2 fit

Fiber Nominal doping

(%)

Cladding absorption coefficient

(/m)

Core absorption coefficient

(/m)

Area ratio Predicted cladding

absorption coefficient

(/m)

Measured/predicted

LO 2.7 0.292 158 0.00250 0.395 0.74 LMA-HI1 3.5 0.438 192 0.00250 0.480 0.91 LMA-HI2 4.5 0.664 247 0.00391 0.965 0.69

Thermal modeling for 200 W/m indicates cladding/buffer temperature well below 100 C

The cladding/ buffer interface reaches 100 C sooner for the smaller diameter fiber even with a thinner buffer

35/625/819 25/400/550Pcritical = 382 W/m Pcritical = 323 W/m

Publication from BAA1

Scaling issues for Tm-doped fiberscompared to Yb-doped fibers

NAa

oλπ= 2V

a is core radius, λ is wavelength

V < 2.405 for single-mode fiber

Optical damage fluence (dielectric breakdown): scales as λ

Raman gain: scales as 1/ λ

Brillouin gain: scales as 1/ (λ)2 x 1/linewidth

Thus, for the same V parameter, compared to Yb-doped fibers,Tm-doped fibers have:

8 X higher fiber end-facet damage threshold

8X higher stimulated Raman scattering threshold

TBD higher stimulated Brillouin scattering threshold

High-power single-frequency results from NGAS

Gregory D. Goodno, Lewis D. Book, and Joshua E. Rothenberg SPIE Photonics West January 27, 2009

Applications of high-power Tm-doped fiber lasers

• Directed energy• IRCM• Remote Sensing for CBW detection• Remote Sensing of Global Winds• Coherent laser radar• Driver for laser ultrasonics NDT for aircraft parts• Pump source for mid-IR ultrafast systems

Ho:YLF MOPA chain produces recordfor hybrid system with Tm:fiber pumps

Ho-stage/ Regime CW 100 Hz 500 Hz

Osc/Amp #1 39 W 55 mJ 50 mJ

Amp#2 76 W 110 mJ 95 mJ

Amp#3 115 W 170 mJ 140 mJ

Tm-pump #1~120 W at 1940 nm

Osc/ Amp #1

Amp #2

Amp #3

Tm-pump #2~120 W at 1940 nm

Tm-pump #3~120 W at 1940 nm

Tm-fiber laser TLR-100-1940 IPG Photonicswww.ipgphotonics.com

Operation regime CWBeam Profile TEMooOutput power ≥ 120 WWavelength 1940 nmPolarization RandomLinewidth ≤ 2 nm

ZGP OPO/OPA layout for 3200-nm generation

Ho-MOPA

Ristra OPO

DM1

DM2

DM4

DM3

BS

OPA output

Ch. 2<100 mJ

Ch. 1<60 mJ

OPA

HR

HR

ZGP 10 mm

ZGP 10 mm

ZGP obtained from Inrad

0

5

10

15

20

25

30

35

0 20 40 60 80 100 120

Ho-pump energy (mJ)

OPA

out

put (

mJ)

100 Hz300 Hz500 Hz

Signal seed ~10 mJ

Signal seed ~9 mJ

Signal seed ~9 mJ

ZGP OPA - various repetition rates

500 Hz

OPA pump beam size remains optimized for max pulse energy of ~100 mJ (100 Hz)

Cr:ZnSe CPA system based on Tm:fiber pumps

1.9-2.0-µm5-W P M T m:fiberC W pump laser

2.5-µmC E P -s tabilized C r:Z nS e

femtosecond laser(100 fs , 1 nJ , 100 MHz)

C E P -s tabilization

setup

P ulse s tretcher100 fs 100 ps

2.5-µmC r:Z nS e regenerative

amplifier(4.5 mJ , 1 kHz)

2.05-µmHo:Y L F Q-s witched

pump laser(17 mJ , 1 kHz)

1.94-µm50-W T m:fiber

C W pump laser

P ulse compressor

Output:2.5 µm, 100 fs

3 mJ , 1 kHz

1.9-2.0-µm5-W P M T m:fiberC W pump laser

2.5-µmC E P -s tabilized C r:Z nS e

femtosecond laser(100 fs , 1 nJ , 100 MHz)

C E P -s tabilization

setup

P ulse s tretcher100 fs 100 ps

2.5-µmC r:Z nS e regenerative

amplifier(4.5 mJ , 1 kHz)

2.05-µmHo:Y L F Q-s witched

pump laser(17 mJ , 1 kHz)

1.94-µm50-W T m:fiber

C W pump laser

P ulse compressor

Output:2.5 µm, 100 fs

3 mJ , 1 kHz

CW Cr:ZnSe laser generates130 fs pulses at 2530 nm

-200 -100 0 100 200

-0.2

0

0.2

0.4

0.6

0.8

1

Time [fs]

Am

plitu

de [a

u]

T m:fiberlaser

S E S AM

C r:Z nS e

R O C = 150 mm

R O C = 100 mm

2% O .C .

C aF 2 prisms

T m:fiberlaser

S E S AM

C r:Z nS e

R O C = 150 mm

R O C = 100 mm

2% O .C .

C aF 2 prisms

Conclusions

• Tm:silica fiber lasers may provide power levels and efficiencies approaching that of Yb:silica fibers

• We have measured some fundamental properties of Tm:silica to better understand laser operation

• With a 25/35/400 Tm:silica fiber laser, we generated 301 W, with 60% conversion of launched pump power to laser output

• The laser slope efficiency indicates that each pump photon generates 1.84 laser photons

• With a 35/625 Tm:silica laser we have generated 885 W of power, a new record for this technology.