Jens LimpertJens.Limpert@uni-jena.de

+49 (0) 36 41. 947 811 +49 (0) 36 41. 947 802

High power fiber lasers and amplifiers

Jens Limpert

Friedrich-Schiller University Jena, Institute of Applied Physics, Jena, Germanyand

Fraunhofer Institut of Applied Optics and Precission Engeniering, Jena, Germany

Outline of the talk

Properties of Fiber Lasers

Advanced Fiber Designs

Selected Experiments of High Power Fiber Lasers

Conclusion and Outlook

High power fiber lasers and amplifiers

rod

disk slab fiber

Solid-State Laser Concepts

power dependent thermal lensing and thermal stress-induced birefringence

reduced thermo-optical distortions

347 -- 352342 -- 347337 -- 342333 -- 337328 -- 333324 -- 328319 -- 324315 -- 319310 -- 315288

temperature [K]

Double-clad fiber laser

active corecladding

pumplight

laser radiation

mirror

mirror

inner cladding (pump core)

outer cladding

active core

laser radiation

refractive index profile

n

active corecladding

pumplight

laser radiation

mirror

mirror

Properties of Rare-Earth-Doped Fibers

• no or reduced free space propagation• immune against thermo-optical problems• excellent beam quality• high gain• efficient, diode-pumped operation

Power evolution of single-mode fiber lasers

1992 1994 1996 1998 2000 2002 2004 20060

500

1000

1500

2000

2500

3000

3500

2500 W

3000 W

2000 W

1530 W1360 W

1300 W800 W

600 W

135 W200 W

150 W170 W

485 W

110 W9,2 W5 W 30 W

Con

tinuo

us-w

ave

outp

ut p

ower

[W]

Year

V. Fomin et al, “3 kW Yb fibre lasers with a single-mode output,” InternationalSymposium on High Power Fiber Lasers and their applications (2006)

1050 1075 1100 1125 1150 1175 1200

Stokes-SignalLaser

Ausgangsleistung 100 W 110 W 120 W

Inte

nsitä

t [w

.E.]

Wellenlänge [nm]

Performance-limiting effectsEnd-facet damage

Thermal effects

Non-linear effects

Available pump power

Source: www.specialtyphotonics.com

1050 1075 1100 1125 1150 1175 1200

Stokes-SignalLaser

Ausgangsleistung 100 W 110 W 120 W

Inte

nsitä

t [w

.E.]

Wellenlänge [nm]

Performance-limiting effectsEnd-facet damage

Thermal effects

Non-linear effects

Available pump power

Source: www.specialtyphotonics.com

effR

effSRSthreshold Lg

AconstP

⋅

⋅=

.

Photonic Crystal FibersDouble clad fibers

Polymer coating

Active core

Pump cladding

•guiding properties of the core

•guiding of the pump light

Microstructured Fiber Step-index Fiber

Δn ~ 1·10-4

NA ~ 0.02Δn ~ 1·10-3

NA ~ 0.06

significantly larger single-mode core possible

Index control of doped fiber cores

n

r

ncore

nClad

0 a

neff-01

n

r

ncore

nClad

0 a

neff-01

n

r

ncore

0

neffclad

neff-01

405.22 22 ≤−⋅= cladcore nnaVλπ

Yb-doped corePump core area

Coating

0 200 400 600 800 1000 1200 1400 1600 1800 20000,0

0,2

0,4

0,6

0,8

1,0

350 nm 380 nm 400 nm

NA

Bridge width [nm]

25 µm

380 nm

The air-cladding region

high numerical aperture inner cladding no radiation has contact to coating material

1.5 mm outercladding

pump core(200 µm)

active core(80 µm)

air-clad 8 µm core

„rod-type“ fiber: 30 dB/m Pumplichtabsorption, 71 µm Modenfelddurchmesser, M2 ~ 1.2

“rod-type” photonic crystal fiber

Limpert et. al., "High-power rod-type photonic crystal fiber laser," Opt. Express 13, 1055-1058 (2005)

Rod-type photonic crystal fiber laser

0 50 100 150 200 250 300 350 400 4500

50

100

150

200

250

300

350slope efficiency ~ 78%

Out

put p

ower

[W]

Launched pump power [W]

320 W continuous-wave, >10 mJ ns-pulses extracted

Design freedom to tailor optical, mechanical and thermo-optical properties

Advantages of PCF

•higher index control•larger SM cores

•shorter fibers possible (0.5 m)•comparable heat dissipation

•intrinsically polarizing without drawbacks

Rare-earth doped photonic crystal fibers

fs source

Fiber

Fiber laser systems

fiber integrated mirror, e.g. fiber bragg grating

Fiber

Pump light coupling, e.g. tapered fiber bundles

all fiber laserCW, Q-switched

Fiber

low power light source

single pass amplifiers advanced fiber amplifiers

Pump(R=1) (R<1)

saturableabsorber

dispersioncompensation

activemedium

ultra-short fiber oscillators

Properties of Fiber Lasers

Advanced Fiber Designs

Selected Experiments of High Power Fiber Lasers

Conclusion and Outlook

High power continuous-wave fiber laser

0 500 1000 1500 2000 2500 3000 3500 40000

250500750

100012501500175020002250250027503000

slope efficiency of 75 % measurement

Out

put p

ower

[W]

launched pump power [W]

•fiber length 15 m, forced air-cooling

•two side pump coupling of 2 kW

•fiber temperature max. 120°C

•beam quality M² < 1.4 at 2 kW

•max. 2.53 kW laser output

75% slope efficiency (below 1.5 kW, above wavelength drift of pump diode)

Scaling approach: Incoherent Combining

transmission grating

Λ= 800 nm)(cos LittΘ⋅Λ

Δ=ΔΘ

λ

(in near and far field)

Polarizing PCF, 1.5 m, 40 µm core

153 W

Combining-efficiency 95 %Degree of Polarization 98%

Scalable while maintaining beam quality

S. Klingebiel,F. Röser,B. Ortac, J. Limpert, A. Tünnermann, ”Spectral beam combining of Yb-doped fiber lasers with high efficiency,”JOURNAL OF THE OPTICAL SOCIETY OF AMERICA B-OPTICAL PHYSICS 24 (8): 1716-1720 (2007)

Combining of pulsed fiber lasers

two beams (ΔΤ ~ 10ns):

Inte

nsity

[a.u

]

Time [50 ns/Div]

M²x =1.17M²y =1.10

M²x =1.22M²y =1.13

M²x =1.17M²y =1.18

SPIRICONTM M2 measurementscaling of MW peak power pulsed fiber sources beyond self-focusing limit of 4 MW

Q-switching of fiber lasers

AOM air-cooledHR @ > 1010 nmHT @ < 990 nm

Q-switching elementHR @ > 1010 nm

rod-type fiber

output

Yb-dopedHT @ < 990 nm

diode @ 976 nm

pump PCF

R = 0.08

end caps on

Microscope image of the rod-type photonic crystal fiberand close-up to the inner cladding and core region.

Fiber parameter:

outer diameter: up to 2 mm60 µm Yb-doped core, Aeff ~ 2000 µm2,

180 µm (NA ~ 0.6) inner cladding30 dB/m pump absorption @ 976 nm,

(0.5 m absorption length)

0 20 40 60 80 100

4

6

8

10

Aver

age

outp

ut p

ower

[W]

Repetition rate [kHz]

0.0

0.5

1.0

1.5

2.0

2.5

3.0

Pul

se e

nerg

y [m

J]

output characteristics @ 100 kHz

0 50 100 1500

25

50

75

100

Ave

rage

out

put p

ower

[W]

Launched pump power [W]

0

20

40

60

80

100

120

Pul

se d

urat

ion

[ns]up to 100 W

down to 12 ns

output characteristics vs. rep. rate

pulse @ 2.5 mJ energy

-20 -10 0 10 20 30 40 500,0

0,2

0,4

0,6

0,8

1,0

Δτ = 8 ns

Inte

nsity

[a.u

.]

Time [ns]

Q-switching of fiber lasers

O. Schmidt, J. Rothhardt, F. Röser, S. Linke, T. Schreiber, K. Rademaker, J. Limpert, S. Ermeneux, P. Yvernault, F. Salin, A. Tünnermann, “Millijoule pulse energy Q-switched short-length fiber laser,” Optics letters Vol.32, No.11, 1551-1553, 2007

Quasi-monolithic, passively Q-switched microchip laser

808nm, 0.5W100µm, 0,22NA

f=30mm f=20mm a) b) c) ‏

AR@808nm, HR@1064nm

a) 0.2mm, 3% doped Nd:YVO4, AR@808nm, T=10% @ 1064nm

b) Spin on glass glue,c) SESAM, ΔR=20%,

TR= 320ps,

-Unmatched simplicity

-No moving parts in theresonator

-Simple gluing technique

-Spin on glass glue*high dielectric strength*high transparency

•1µJ, 50ps, 40kHz•0.5µJ, 110ps, 170kHz

Current production costs:~300€

3mm

D. Nodop, J. Limpert, R. Hohmuth, W. Richter, M. Guina, and A. Tünnermann,"High-pulse-energy passively Q-switched quasi-monolithic microchip lasers

operating in the sub-100-ps pulse regime," Opt. Lett. 32, 2115-2117 (2007)

Fiber based amplification of ps-µchip lasers

Pump diode

amplified signal

low costmicro-chip

laser

Isolator

60 ps, 0.75 µJ, 50 kHz

60 ps, 80 µJ, 50 kHz peak power: 1.33 MW

micromachining

35 µm, 1.5 m

Pump(R=1) (R<1)

ultra-shortpulsed laser

SA

DC

AM

•active medium (AM)-e.g. Yb-doped fiber

•saturable absorber (SA)-favours pulse against noise background-initiates mode-locking

•dispersion compensation (DC)- keeps the pulse short during roundtrip

Dispers

ion

Nonlinea

rity

gain

loss

Theory: dissipative, nonlinear system:

Ultra-short pulse generation

High-energy femtosecond fiber laser

Pump diode975 nm

OutputOptical IsolatorM1

HR

4λ

M2

Yb-doped rod-type fiberL1 L2 L3

SAM L4

HR

HR

HR

Total cavity dispersion : + 0.012 ps2

Repetition rate : 10.18 MHz

Modulation depth 30%Fast relaxation time 200 fsSlow relaxation time 500 fs

dispersion compensation free

B. Ortaç, O. Schmidt, T. Schreiber, J. Limpert, A. Tünnermann, A. Hideur,“High-energy femtosecond Yb-doped dispersion compensation free fiber laser,” Optics Express, vol. 15, pp. 10725– 10732, 2007.

-20 -10 0 10 200.0

0.2

0.4

0.6

0.8

1.0

In

tens

ity (a

. u.)

Delay time (ps)

Autocorrelation trace

• Output pulse duration = 4 ps

-6 -4 -2 0 2 4 60.0

0.2

0.4

0.6

0.8

1.0

In

tens

ity (a

. u.)

Delay time (ps)

Extra-cavity compression

• Compressed pulse duration = 400 fs

Compression efficiency: 75 %

Energy per pulse: 200 nJ

Peak power: 500 kW

Average output power: 2.7 W

Energy per pulse: 265 nJ

Single pulse characterization:

High-energy femtosecond fiber laser - Results

Optical Spectrum

• Spectral bandwidth = 8.4 nm

-6 -4 -2 0 2 4 60.0

0.2

0.4

0.6

0.8

1.0

In

tens

ity (a

. u.)

Delay time (ps)

Extra-cavity compression

• Compressed pulse duration = 400 fs

Compression efficiency: 75 %

Energy per pulse: 200 nJ

Peak power: 500 kW

Average output power: 2.7 W

Energy per pulse: 265 nJ

Single pulse characterization:

1010 1020 1030 1040 1050

10-3

10-2

10-1

100

Inte

nsity

(a. u

.)

Wavelength (nm)

High-energy femtosecond fiber laser - Results

Ultra-short pulse fiber amplification systemsHigher average power

Higher pulse energy Higher repetition rate

1 10 100 1000

10

100

1000

10000

100 W

10 W1 W

100 mW

Puls

e en

ergy

[µJ]

repetition rate [kHz]

Higher average power

Higher pulse energy Higher repetition rate

1 10 100 1000

10

100

1000

10000

100 W

10 W1 W

100 mW

Puls

e en

ergy

[µJ]

repetition rate [kHz]

fiber-basedsystems*

* Röser et. al., „Millijoule pulse energy high repetition rate femtosecond fiber chirped-pulse amplification system,“ Opt. Lett. 32, 3495 (2007)* Röser et. al., „90 W average power 100 μJ energy femtosecond fiber chirped-pulse amplification system,” Opt. Lett. 32, 2230 (2007)

Ultra-short pulse fiber amplification systems

-4 -2 0 2 40,0

0,2

0,4

0,6

0,8

1,0 B = 0 B = 0.6 B = 8 B = 16

SHG

inte

nsity

[a.u

.]

Time delay [ps]

( )∫ ⋅⋅

=L

dzzInB0

22λπ

Accumulated nonlinear phase (B-integral):

Influence of self-phase modulation (SPM)

Reduction of pulse quality

Simulated autocorrelation traces

( ) ( ) zTzATzSPMNL

2,, γφ =

Nonlinear phase:

Chirped Pulse Amplification (CPA)

reduced pulse peak power during amplification

mode-locked laser amplifier grating compressorgrating stretcher

D. Strickland and G. Mourou, “Compression of amplified optical pulses,”Opt. Comm. 56, 3, 219 (1985).

reduced nonlinearityenhanced damage threshold

Yb:KGW oscillator Stretcher -Compressor -

Unit

AOMPre-amplifier

Main amplifier

ISO

Output

State of the art FCPA System

Schematic Setup

Yb:KGW oscillator9.7 MHz, 1.6 W, 400 fs , 1030 nm

Stretcher -Compressor -

Unit

AOM

ISO

Output

State of the art FCPA System

Schematic Setup

Pre-amplifier

Main amplifier

Yb:KGW oscillator9.7 MHz, 1.6 W, 400 fs , 1030 nm

Stretcher -Compressor -

Unit1740 l/mm

dielectric gratings

AOM

ISO

Output

State of the art FCPA System

Schematic Setup

Pre-amplifier

Main amplifier

Yb:KGW oscillator9.7 MHz, 1.6 W, 400 fs , 1030 nm

Stretcher -Compressor -

Unit1740 l/mm

dielectric gratings

AOM

Pre -amplifier40 µm core PCF, 1.2 m

Main amplifier80 µm core PCF, 1.2 m

ISO

Output

State of the art FCPA System

Schematic Setup

1.5 mm outercladding

pump core(200 µm)

active core(80 µm)

air-clad

Rod-type photonic crystal fiber

High pump light absorption (30dB/m) -> short fiber length+ large mode area ( >100x of standard fiber) -> ultralow Nonlinearity

Limpert et. al., "High-power rod-type photonic crystal fiber laser," Opt. Express 13, 1055-1058 (2005)

State of the art FCPA System

8 µm core

200/80 Rod-type PCF, 1.2m length

MFD = 71 µm-> MFA ~ 4000 µm2

Near field image

State of the art FCPA System

M2x = 1.17 , M2

y = 1.26(Spiricon™, 4σ method)

Beam quality-measurement

Yb:KGW oscillator9.7 MHz, 1.6 W, 400 fs , 1030 nm

Stretcher -Compressor -

Unit1740 l/mm

dielectric gratings

AOM

Pre -amplifier40 µm core PCF, 1.2 m

Main amplifier80 µm core PCF, 1.2 m

ISO

Output

State of the art FCPA System

Schematic Setup

• multilayer dielectric reflection gratings – average power scalable• 70% compressor throughput efficiency

0 50 100 150 200 2500

20

40

60

80

100

Com

pres

sed

outp

ut p

ower

[W]

Launched pump power [W]

slope efficiency = 46%

100 W compressed @ 200 kHz-> 0.5 mJ pulse energy

Output characteristics

State of the art FCPA System

50 W compressed @ 50 kHz-> 1 mJ pulse energy

0 50 100 150 2000

10

20

30

40

50

Com

pres

sed

outp

ut p

ower

[W]

Launched pump power [W]

total slope efficiency = 25%

1020 1030 1040 1050

-80

-70

-60

-50

-40

-30

-20

1000 1020 1040 1060 1080

-80

-60

-40

Inte

nsity

, log

sca

le [a

.u.]

Wavelength [nm]

Wavelength [nm]

1020 1030 1040 1050-80

-70

-60

-50

-40

-30

-20

1000 1020 1040 1060 1080

-80

-70

-60

-50

-40

-30

Inte

nsity

, log

sca

le [a

.u.]

Wavelength [nm]

Wavelength [nm]

State of the art FCPA System

Spectrum @ highest power

ASE supression better than 30 dBExtended measurement shows no sign of Stimulated Raman Scattering

(1st stokes expected at 1080nm)

Spectrum @ highest pulse energy

-10 -5 0 5 100.0

0.2

0.4

0.6

0.8

1.0

ΔτAC = 1.23 ps ΔτAC = 1.2 ps ΔτAC = 0.93 ps

SH In

tens

ity [a

.u.]

Time delay [ps]

At low pulse energy

200 kHz / 500 µJ(B-Integral 4.7)

50 kHz / 1 mJ(B-Integral 7)

Autocorrelation

1mJ, 800 fs => pulse peak power ~ 1 GW!

State of the art FCPA System

0 200 400 600 800 1000 12000

100

200

300

400

500

600

700

800

slope efficiency 66%

Out

put p

ower

[W]

Launched pump power [W]

Average power scalability of main amplifier

710 W max. output (pump power limited)

570 W/m with no thermal degradation(passively cooled)

diode laser80 µm core Rod-type PCF1.2 m Lengthdiode laser DM DM

HR R=10%Schematic Setup cw cavity

1 kW, 1 MHz, 1 mJ, sub-1 ps !

75 msbreakthrough time

106 rounds/srotating speed

75 µmtrepanning radius

Copper (Cu 99.9%)

Thickness: 0.5 mm

Rep.rate.: 975 kHz

Pulse Energy : 70 μJ

Focal length: 80mm

Fluence: ~ 2.32 J/cm2

Number of rounds: 50

Laser trepanning with high average power on copper

Conclusion and outlook

Success story is based on …RE-doped fibers are a power scalable solid-state laser concept !

significant progress in fiber manufacturing technologyrecent developments of reliable high power all solid-state pump sources

Outstanding performance in a variety of operation regimescontinuous-wave: >10 kW diffraction-limited

ultra-short pulse: >1 kW, 1 MHz, 1 mJ, sub-1 ps

RE-doped fibers are good for …