jens limpert - swissphotonics :: home
TRANSCRIPT
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Jens [email protected]
+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
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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
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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]
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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
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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
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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)
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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
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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
⋅
⋅=
.
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Photonic Crystal FibersDouble clad fibers
Polymer coating
Active core
Pump cladding
•guiding properties of the core
•guiding of the pump light
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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
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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
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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)
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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
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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
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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
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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)
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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)
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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
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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)
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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
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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)
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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
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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
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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.
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-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
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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
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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]
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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
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-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:
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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
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Yb:KGW oscillator Stretcher -Compressor -
Unit
AOMPre-amplifier
Main amplifier
ISO
Output
State of the art FCPA System
Schematic Setup
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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
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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
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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
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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
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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
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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
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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%
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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
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-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
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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 !
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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
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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 …