Swiss Federal Institute of Technology ZürichUltrafast Laser Physics
New frontiers in all-solid-state lasers:High average power
High pulse repetition rate
Ursula Keller
Ultrafast Laser PhysicsSwiss Federal Institute of Technology Ë
Zürich, Switzerland
Ultrafast laser oscillators:perspectives from past to futures
Ultrafast laser oscillators:perspectives from past to futures
Swiss Federal Institute of Technology ZürichUltrafast Laser Physics
Research Group of Prof. KellerUltrafast diode-pumped solid-state lasers (R. Paschotta)
Sub-10-femtosecond pulse generation (G. Steinmeyer)
Novel materials: III-V/fluoride MBE (S. Schön)
Attosecond Science (J. Tisch, J. Biegert)
Swiss Federal Institute of Technology ZürichUltrafast Laser Physics
Current status in ultrafast lasersKerr-lens modelocked Ti:sapphire lasers
Pulse duration of about two optical cycles (≈≈≈≈ 5.5 fs)
Ultrafast diode-pumped solid-state lasersSESAM modelocking is becoming the “standardapproach”Compact reliable lasers commercially availableNew Frontier: High average powerfs lasers: 22 W, 240 fs, 25 MHz, 3.3.MW peak (Yb:KYW)ps lasers: 60 W, 6 - 24 ps, 34 MHz, 1.7 µJ (Yb:YAG)New Frontier: High pulse repetition rateUp to 157 GHz (Nd:Vanadate miniature laser)
Swiss Federal Institute of Technology ZürichUltrafast Laser Physics
Mode locking
I (ω)
φ (ω)
0
I (t)
+π
-π
~
~φ (t)
• axial modes in laser not phase- locked
• noise
I (ω) I (t)
φ (ω)
0
+π
-π
τ ≈ 1∆ν
φ (t)~
~
• axial modes in laser phase- locked
• ultrashort pulse
• inverse proportional to phase- locked spectrum
Swiss Federal Institute of Technology ZürichUltrafast Laser Physics
Ultrashort pulse generation (Science 286, 1507, 1999)
1960 1970 1980 1990 2000
First ML Laser Ti:Sapphire
KLM
Chirped Mirror
CEO control
FWH
M p
ulse
wid
th (s
ec)
20001990198019701960 Year
10 fs
100 fs
1 ps
1 fs
10 ps
Ti:sapphire laser≈5.5 fs with ≈200 mW
dye laser27 fs with ≈10 mW
compressed
Swiss Federal Institute of Technology ZürichUltrafast Laser Physics
D. E. Spence, P. N. Kean, W. Sibbett, Opt. Lett. 16, 42, 1991
Effective Saturable Absorber Fast Self-Amp. Modulation
Pulse
Gain
Loss
Time
Kerr Lens Modelocking (KLM)
Incident beam
Nonlinear mediumKerr lens
Low intensity light
Aperture
Intense pulse
Loss
Pulse fluence on absorber
Saturation fluence
Swiss Federal Institute of Technology ZürichUltrafast Laser Physics
Passively modelocked solid-state lasersA. J. De Maria, D. A. Stetser, H. HeynauAppl. Phys. Lett. 8, 174, 1966
200 ns/div
50 ns/div
1960 1970 1980 1990 2000
Nd:glassFirst passively modelocked laser
Q-switched modelockedTi:Sapphire
KLM
SESAM
First passively modelocked(diode-pumped) solid-state laserwithout Q-switching
U. Keller et al. Opt. Lett. 17, 505, 1992
Flashlamp-pumped solid-state lasers
Diode-pumped solid-state lasers(first demonstration 1963)
Q-switching instabilitiescontinued to be a problem until 1992
Swiss Federal Institute of Technology ZürichUltrafast Laser Physics
U. Keller et al., IEEE JSTQE 2, 435, 1996Chapter 4 in Semiconductors and Semimetals, vol. 59, Academic Press, 1999
R ≈ 0 %Saturableabsorber(Sat. abs.) Sat. abs.
R ≈ 95 %
R ≈ 30 %
High-finesseA-FPSA
Thin absorberAR-coated
Low-finesseA-FPSA,SBR
D-SAMSaturableabsorber and negativedispersion
Sat. abs. Sat. abs.R ≈ 30 %
April 92 Feb. 95 June/July 95 April 96
R ≈ 100 % R ≈ 100 % R ≈ 100 % R ≈ 100 %
Enabling Technology: SESAMSemiconductor saturable absorber mirror (SESAM)
Swiss Federal Institute of Technology ZürichUltrafast Laser Physics
∝Aeff,L
em,Lσ
= A F Reff,A sat,A∆=
P
fintra
rep
2E E E RP sat,L sat,A
2 > ∆
C. Hönninger, R. Paschotta, F. Morier-Genoud, M. Moser, and U. Keller,JOSA B 16, 46 (1999)
cw mode locking
Lase
r pow
er
403020100Time (multiples of round trip time)
Q-switched mode locking
Lase
r pow
er
403020100
Time (multiples of round trip time)
Q-switched mode locking is avoided if...
Swiss Federal Institute of Technology ZürichUltrafast Laser Physics
E E E RP sat,L sat,A2 > ∆
C. Hönninger, R. Paschotta, F. Morier-Genoud, M. Moser, and U. Keller,JOSA B 16, 46 (1999)
Saturation fluence and modulation depth
100
95
90
Ref
lect
ivity
(%
)
300250200150100500
Incident pulse fluence Fp ( µJ/cm2)
∆R Modulation depth
Fsat, A Saturation fluence ∆R ns
Non-saturable losses
SESAM
Semiconductor saturable absorber mirror A F Reff,A sat,A∆F
Asat,A ∝
1σ
Absorber σ A cm2[ ]ion-doped solid-state
0 1019 22− −−
dye 0 16−
semiconductor 0 14−
Swiss Federal Institute of Technology ZürichUltrafast Laser Physics
Recovery times in semiconductors
Density of states D
D
E
IntrabandThermalization
≈ 100 fs
Density of states D
D
E
InterbandRecombination
≈ nsLT grown materials:
Electron trapping≈ ps - nsA
bsor
ptio
nTime Delay
τ τ τA p p≤ to 10 30
R. Paschotta, U. Keller, Applied Physics B 73, 653, 2001
Swiss Federal Institute of Technology ZürichUltrafast Laser Physics
KLM vs. SESAM modelocking
Kerr lens modelocking (KLM)
- fast/broadband saturable abs.
- critical cavity adjustment: KLM
better at cavity stability limit
- typically not self-starting
SESAM modelocking
- “not so fast” saturable absorber
- absorber independent of cavity
design
- self-starting
pulse
gain
loss
time time
loss
gain
pulse
Swiss Federal Institute of Technology ZürichUltrafast Laser Physics
time
loss
Slow saturable absorber modelockingR. Paschotta, U. Keller, Appl. Phys. B submitted
leading edge of pulse
has significant loss from
the saturable absorber
Fully saturated absorber:
negligible loss for
trailing edge of pulse
absorber delays pulse
Dominant stabilization process:
Picosecond domain: absorber delays pulse
The pulse is constantly moving backward and
can swallow any noise growing behind itself
Femtosecond domain: dispersion in soliton modelocking
{A(T , t ) = Asech tτ exp i Φ0 TTR +Soliton Perturbation Theory:Frequency domain Time domain
soliton
{
“continuum”only GVD & SAM
small perturbations
spreading
F. X. Kärtner, U. Keller, Optics Lett. 20, 16, 1995Invited Paper: F. X. Kärtner, I. D. Jung, U. Keller, IEEE JSTQE, 2, 540, 1996
fs domain: soliton modelocking
Dispersion spreads continuum out where it sees more loss
Continuum
Time
Pulse
Gain
Loss
GDD GDD
Frequency
Gain
Pulse
Continuum
Swiss Federal Institute of Technology ZürichUltrafast Laser Physics
Motivation for Mode-LockedHigh-Power Lasers
Multi-kW to MW peak powers, ≈ µJ pulse energiesApplications:
Material processingMedical applicationsNonlinear frequency conversione.g. with high-power optical parametric oscillators:
➔ RGB laser displays
➔ mid-infrared sources
➔ tunable femtosecond sources
Swiss Federal Institute of Technology ZürichUltrafast Laser Physics
16-pass arrangement
Thin-Disk Laser HeadS. Erhard, A. Giesen, M. Karszewski, T. Rupp, C. Stewen, I. Johannsen, and K. Contag,
in OSA Topical Meeting, Advanced Solid-State Lasers, 1999
efficient pump absorption
• efficient cooling• high pump intensities possible• very weak thermal lensing
• excellent thermal properties• broad emission bandwidth
nearly one-dimensional longitudinal heat flow
Yb:YAG as gain material
fiber coupleddiode laser
collimating lens
heat sink withcrystal in focal plane
laser output
parabolic mirror
roof prism
Swiss Federal Institute of Technology ZürichUltrafast Laser Physics
➤ saturation parameter S := Ep/(Fsat,A·Aeff,A) in our thin disk laser: S < 10 ⇒ far below damage threshold (S > 100-200) negative group delay dispersion generated with a GTI linear polarization enforced by Brewster plate
Passively Mode-Locked Thin Disk Laser
GTI
wedged Yb:YAG diskon cooling finger
R=1.5 m
output coupler
Brewster plate
R=0.5 m
SESAM: Fsat,A ≈ 100 µJ/cm2 ∆R ≈ 0.5% ∆Rns ≈ 0.3%
SEmiconductor Saturable
Absorber Mirror
R=1 mheat sink
Swiss Federal Institute of Technology ZürichUltrafast Laser Physics
1.0
0.8
0.6
0.4
0.2
0.0Aut
ocor
rela
tion
trac
e
-3 -2 -1 0 1 2 3
Time delay (ps)
τp = 730 fs
1.0
0.8
0.6
0.4
0.2
0.0Spe
ctra
l int
ensi
ty (
a.u.
)
10341032103010281026
Wavelength (nm)
1.55 nm
Passively ML Yb:YAG thin-disk laser
frep = 34.6 MHz
Ep ≈ 0.47 µJ
S ≈ 7M 2 < 1.5
Pavg = 16.2 W
τp = 730 fs
Ppeak ≈ 560 kW
∆ν τp = 0.32
optical-to-optical efficiency: 28%
far away fromSESAM damage(S > 100-200)
J. Aus der Au et al., Opt. Lett. 25, 859, 2000
Swiss Federal Institute of Technology ZürichUltrafast Laser Physics
Thin disk laser head:
double pump power and modearea in gain medium
SESAM:
double mode area on SESAM,keep SESAM parametersunchanged
Power Scaling:How to Double the Output Power
• unchanged temperature rise (1-dim. heat flow)• unchanged intensities no SESAM damage• thermal lensing not increased• Q-switching tendency not increased
Swiss Federal Institute of Technology ZürichUltrafast Laser Physics
Passively ML Yb:KYW thin-disk laser
Ppeak ≈ 3.3 MW
Ep ≈ 0.9 µJ
Ipeak = 2 x 1014 W/cm2 , 2 µm radius
Pavg = 22 W
τp = 240 fs
frep = 24.6 MHzM 2 ≈ 1.1
F. Brunner et al., CLEO 2002, accepted
1.0
0.8
0.6
0.4
0.2
0.0Spe
ctra
l int
ensi
ty (
norm
aliz
ed)
1040103010201010Wavelength (nm)
6.9 nm
1.0
0.8
0.6
0.4
0.2
0.0
Aut
ocor
rela
tion
sign
al
-0.4 0.0 0.4Time delay (ps)
240 fs
Swiss Federal Institute of Technology ZürichUltrafast Laser Physics
New frontiers: high pulse repetition rates
100
101
102
103
104
Aver
age
Out
put P
ower
[mW
]
1 10 100 1000Repetition Rate [GHz]
Nd:BEL
Nd:YLF
Cr:YAG
Ti:sapphire
Miniature Nd:YVO4
Fiber lasersSemicon. lasers
Semicon. lasers
Er:Yb:glass
High Power Nd:YVO4
VECSEL
Passive ML Active ML
Swiss Federal Institute of Technology ZürichUltrafast Laser Physics
Quasi-Monolithic Cavity Setup
Crystal lengths: 0.9 - 2.3 mm (FSR ~ 77 - 29 GHz)
Nd:YVO4 doping: 3 % (90 µm absoption length)
L. Krainer et al., Electron. Lett. 35, 1160, 1999 (29 GHz)APL 77, 2104, 2000 (up to 59 GHz), Electron. Lett. 36, 1846, 2000 (77 GHz)
4
Swiss Federal Institute of Technology ZürichUltrafast Laser Physics
Passively modelocked Nd:VanadateAppl. Phys. Lett., 77, 14, (2000)
39 GHzCrystal length = 1.76 mm
ττττP = 5 psEp = 1.5 pJ
Pout = 60 mW
ττττP = 2.7 psEp = 0.8 pJ
Pout = 65 mW
Electron. Lett., submitted
77 GHzCrystal length = 0.9 mm
Electron. Lett., 34, 14, (1999)
29 GHzCrystal length = 2.31 mm
ττττP = 6.8 psEp = 2.8 pJ
Pout = 81 mW
Aut
ocor
rela
tion
-40 -20 0 20 40
Time, ps
34 ps
Aut
ocor
rela
tion
-20 -10 0 10 20Time, ps
26 ps
Opt
ical
spe
ctru
m
1064.41064.01063.61063.2Wavelength, nm
Aut
ocor
rela
tion
-20 -10 0 10 20Time, ps
13 ps
Opt
ical
Spe
ctru
m
1064.41064.01063.61063.2Wavelength, nm
Opt
ical
spe
ctru
m
1064.51064.01063.5Wavelength, nm
Swiss Federal Institute of Technology ZürichUltrafast Laser Physics
150 GHz Nd:Vanadate Laser
Autocorrelation trace of the ≈157 GHz pulse train.The pulses are about 6.4 ps apart.
L. Krainer et al., CLEO 2002
1.0
0.5
0.0
s.h
. in
ten
sity
, a
.u.
-20 0 20
time, ps
Swiss Federal Institute of Technology ZürichUltrafast Laser Physics
10 GHz Er:Yb:glass laserL. Krainer et al., Electron. Lett., to be published March 1, 2002
10
8
6
4
2
0
P out a
t QM
L th
resh
old
(mW
)
15701560155015401530
Wavelength (nm)
10
8
6
4
2
0
Pulse duration (ps)
-80
-60
-40
-20
0
Phot
o de
tect
or s
igna
l (dB
c)
10.52610.52410.522Frequency (GHz)
span: 5 MHzres. bw.: 30 kHz
0.01
0.1
1
Aut
ocor
rela
tion
sign
al
-10 0 10Time delay (ps)
measured
sech2 fit
τp
= 3.8 ps
Swiss Federal Institute of Technology ZürichUltrafast Laser Physics
What about diode-pumped semiconductor lasers?
Edge emitting lasersStripe width limited by beam quality requirementsFacet damage limits peak power
Surface emitting deviceExternal cavity needed (repetition rate: 1–100 GHz)Electrical pumping: ring electrode limits sizeOptical pumping: large area with homogeneousinversion
Optical pumped Vertical-External-CavitySurface-Emitting Laser (VECSEL)*
* M. Kuznetsov, F. Hakimi, R. Sprague, and A. Mooradian, JSTQE 2, 435-453 (1996)
Swiss Federal Institute of Technology ZürichUltrafast Laser Physics
Optically pumped VECSEL
First demonstration of passively modelocked optically pumped VECSEL:
S. Hoogland et al., IEEE Photon. Technol. Lett. 12, 1135 (2000).
Simple cavityfiber coupled diode arraylarge pump diametercurved output couplerspot size smaller on SESAMthan on gain structure
time
loss
gain
pulse
Swiss Federal Institute of Technology ZürichUltrafast Laser Physics
Autocorrelation at 530 mW
Pulses with low chirpSESAM absorber: 8 nm In0.15Ga 0.85As (∆∆∆∆R ≈≈≈≈ 1.5%)
Gaussian pulse shape3.9 ps FWHM durationonly 1.5 times over Fourier limit
1.0
0.8
0.6
0.4
0.2
0.0Aut
ocor
rela
tion
sign
al (a
.u.)
-10 -5 0 5 10Delay time (ps)
measured3.9 ps gaussian
1.0
0.5
0.0O
ptical density (a.u.)954953952951Wavelength (nm)
0.5 nm
Swiss Federal Institute of Technology ZürichUltrafast Laser Physics
Microwave Frequency at 530 mW
Stable mode-lockingResolution 300 kHzNoise free to -55 dBcRepetition rate = 5.9533 GHz
Polarized: >100:1
nearly diffraction limitedM2 < 1.05
18 W pump power300 µm pump diameter3°C heat sink temperature
-60
-50
-40
-30
-20
-10
0
RF
pow
er d
ensi
ty (d
Bc)
5.975.965.955.94Frequency (GHz)
-60
-40
-20
151050Frequency (GHz)
Swiss Federal Institute of Technology ZürichUltrafast Laser Physics
1.0
0.8
0.6
0.4
0.2
0.0Aut
ocor
rela
tion
sign
al (a
.u.)
-40 -20 0 20 40Delay time (ps)
measured15.3 ps sech2
1.0
0.5
0.0O
ptical density (a.u.)958957956955Wavelength (nm)
1 nm
Autocorrelation at 950 mW
Higher power / longer pulsesech2 shape, 15.3 ps FWHM duration
1 nm optical bandwidth ⇒⇒⇒⇒ chirp
continuous wave: 2.2 W
Swiss Federal Institute of Technology ZürichUltrafast Laser Physics
4
3
2
1
0Ref
ract
ive
inde
x
6000 4000 2000 0Position (nm)
Mirror ARQWs
Gain structure
Swiss Federal Institute of Technology ZürichUltrafast Laser Physics
4
3
2
1
0Ref
ract
ive
inde
x
6000 4000 2000 0Position (nm)
Mirror ARQWs
Gain structure
R > 99.95% for 950 nmR ≈ 97% for 805 nm, 45°double pass pump light
100
50
0
Ref
lect
ivity
(%)
1000950900850800Wavelength (nm)
Swiss Federal Institute of Technology ZürichUltrafast Laser Physics
4
3
2
1
0Ref
ract
ive
inde
x
6000 4000 2000 0Position (nm)
Mirror ARQWs
Gain structure
R < 1% for 950 nmR ≈ 10% for 805 nm, 45°R > 99.95% for 950 nmR ≈ 97% for 805 nm, 45°
double pass pump light
100
50
0
Ref
lect
ivity
(%)
1000950900850800Wavelength (nm)
100
50
0
Ref
lect
ivity
(%)
1000950900850800Wavelength (nm)
Swiss Federal Institute of Technology ZürichUltrafast Laser Physics
4
3
2
1
0Ref
ract
ive
inde
x
6000 4000 2000 0Position (nm)
Mirror ARQWs
Gain structure
R < 1% for 950 nmR ≈ 10% for 805 nm, 45°
5 InGaAs Quantum wellsSpacer absorbs pump,carrier trapped in QWs
R > 99.95% for 950 nmR ≈ 97% for 805 nm, 45°double pass pump light
100
50
0
Ref
lect
ivity
(%)
1000950900850800Wavelength (nm)
100
50
0
Ref
lect
ivity
(%)
1000950900850800Wavelength (nm)
Swiss Federal Institute of Technology ZürichUltrafast Laser Physics
Thermal impedance: Idea
Consider epitaxial lift-off structure(substrate replaced with a heat sink)
heat source is a thin sheetd ≈ 1 µm, Ø ≈ 500 µm
1-dimensional heat flow in vicinity of source
power scalable approache.g. double pump spot, keep pump intensity constant
⇒ temperature is unchanged, output power doubled
Swiss Federal Institute of Technology ZürichUltrafast Laser Physics
Thermal impedance
Check of validity
Simulationconstant intensityvaried pump spotcopper heat sink
Critical radiusheat sink andsemiconductorcontribute equally
100
80
60
40
20
0
∆T (K
)
4 6 810
2 4 6 8100
2 4 6 8
Radius (µm)
wcrit ∆T
1d model
∆T3d
model
Swiss Federal Institute of Technology ZürichUltrafast Laser Physics
Success story is base on ... Transition from dye to solid-state lasers– Kerr lens modelocking– Ti:sapphire laser produces shorter pulses and more
average power
Diode-pumped solid-state lasers– development of high-power and high-brightness diode
lasers for direct pumping of solid-state lasers– efficient, compact and reliable sources
Semiconductor saturable absorbers– stable passive modelocking of diode-pumped solid-state
lasers (self-starting and no Q-switching instabilities)– many different parameter regimes such as laser
wavelength, pulse duration and power levels– engineering of linear and nonlinear optical response
Swiss Federal Institute of Technology ZürichUltrafast Laser Physics
Hot topics in the near future
Ultrafast diode-pumped solid-state lasers
High average power in the 100 W regime for picosecondto sub-100-fs pulse durations
Very simple (“single-pass”) and efficient nonlinearfrequency conversion (SHG, OPG, fiber OPO, ….)
Many 10 GHz pulse repetition rates at longer wavelength(1.3 µm and 1.5 µm, telecom application)