Power Scaling of High-Efficiency, Tm-doped Fiber Lasers

Peter F. MoultonQ-Peak, Inc.

SPRC 2008 Annual SymposiumSeptember 17th

Stanford UniversityPalo Alto, CA

Outline

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

Support:

HEL-JTO Contract No. FA9451-06-D-0009

Technical work:

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

Relative eye safety is obtained for > 1400-nm wavelengths

Retinal focusing can increase the power density by 105

Rare-earth laser transitions can 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

0

10

20

30

40

50

60

70

80

90

100

1995 2000 2005 2010Date

Slop

e Ef

ficie

ncy

(%) 2:1 limit

Recent work: efficient, high-power Tm-doped fiber lasers

G. Frith, D.G. Lancaster and S.D. Jackson, “85 W Tm3+-

doped silica fibre laser,”

Electron. Lett. 41, 1207 (2005).

D.Y. Shen, J.L. Mackenzie, J.K. Sahu, W.A. Clarkson and S.D. Jackson, “High-power and ultra-efficient operation of a Tm3+-

doped silica fiber laser,”

OSA Topical Meeting on Advanced Solid State Photonics, 2005 (Vienna), Paper MC-6.

J. Wu, Z. Yao, I. Zong

and S. Jiang, “Highly Efficient High Power Thulium Doped Germanate Glass Fiber Laser, “

Opt. Lett. 32, 638 (2007

Also: Nufern, 100 W Tm:silica laser

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

Tm-fiber laser120 W

Ho-osc

Ho-Amp #1

Tm-fiber laser110 W

Ho-Amp #2

Ho-Amp #3

Tm-fiber laser100 W

Ho-Amp#4

Ho-Amp #5

CW 500 Hz 1000 Hz

Master osc 19 W 25 mJ 17 mJ

Amp 1 42 W 55 mJ 38 mJ

Amp 2 60W 90 mJ 58 mJ

Amp 3 78 W 115 mJ 73 mJ

Amp 4 97 W *** ***

Amp 5 113 W *** ***

*** work in progress

Used IPG fiber lasers that in-band pumped by Yb, Er fiber lasers and are <10% efficient, overall

Note: IPG has scaled this design to 405 W.

Fundamentals of Tm:silica fiber lasers

Absorption and emission cross sections for 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)

Abs: NufernEm: Walsh (NASA)

Plot of net gain cross section in Tm:silica vs. 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 for 5-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% (Tm2 O3 )HI: 2.47-2.87 wt%

Result:LO: 2.03-2.36 wt% (Tm2 O3 )HI: 2.47-2.87 wt%

Retiring the photodarkening issue?

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 shows two-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 shows feeding 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 data on 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

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

NAa

o 2V

a is core radius, is wavelength

V < 2.405 for single-mode fiber

Optical damage fluence (dielectric breakdown): scales as l

Raman gain: scales as 1/ l

Brillouin gain: scales as 1/ (l)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

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 m

Signal output@~1.1 mHT @975 nm

HR @~1.1 m

Diode stack@975 nm, 0.6 kW

HT @975 nmHR @~1.1 m

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%

Sig

nal 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-power connector

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

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 m in diameter, NA: 0.08.Pump claddings: 400-m in diameter, octagonal cross sectionPump attenuation: 2.9 dB/m

x21

3

x12

Results with 350-W pump lasers, LMA HI2 fiber

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

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

( m

)

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

Vert. axis, My2=1.16

Horiz. axis, Mx2=1.21

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

0

2

4

6

8

10

12

14

16

18

20

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 m

HT @790 nmHR @~2 m

Signal output@~2 m

HT @790 nmHR @~2 m

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

350 Wpumps

1 kWpump

Hole forsecond

1 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

Application to laser accelerators

Motivation

• The maximum energy generated by conventional, microwave- driven electron accelerators is starting to reach a practical limit imposed by size, either in circumference for storage rings, or in length for linear accelerators. The logical evolution for increasing the energy of linear accelerators (or decreasing their size for the same energy) is to increase the acceleration field. By substituting lasers for microwave sources, one can obtain both higher peak powers and also, because of the ability to focus the much more tightly, much smaller beam areas. The net result in increased intensity is that an accelerating field of, say, a 1 TW, 1000-nm laser can exceed that of a 100 MW, 10-cm microwave source by 7 orders-of-magnitude

Design for laser-based electron accelerator

www.slac.stanford.edu/grp/arb/Byer.pdf

Dielectric structure for accelerator

With an index of ~3.5, a good thermal conductivity and availability silicon would appear to be an ideal choice for the dielectric, however its

transparency range requires the use of λ~2μm lasers that at present are not a mature technology. (PFM – never mind!)

SLAC-PUB-12143 October 2006 Proposed few-optical cycle laser–driven particle

accelerator structure T. Plettner, P. Lu and R.L. Byer

E.L. Ginzton Laboratories, Stanford University, Stanford, CA 94305

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 1.8 laser photons

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

• Potential use for electron accelerators would enable Si dielectric structures, among other possibilities

Backup

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 () in the fiber as a function of wavelength, , is given by the relation: () = F e () – (1-F) a (), where e () and a () are the emission and absorption cross sections. The gain or loss coefficient is () multiplied by the concentration of active ions.

Atmospheric transmission over 5-km path shows 90% transmission in 2.035-2.04 m region

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

1.94 1.96 1.98 2 2.02 2.04 2.06 2.08 2.1 2.12

Wavelength (um)

Tran

smis

sion

CO2

HITRAN-PC calculation, standard atmosphere

CO2 and H2 O

Characteristics of Nufern-supplied fibers

Fiber ID MM-TDF-20/400-LO

MM-TDF-20/400-Hi

LMA-20/35/400-Hi

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