StingRay/BioRay Lasers · 2020-03-24 · StingRay/BioRay Lasers ... and …
High average power New frontiers in all-solid-state lasers:...
Transcript of High average power New frontiers in all-solid-state lasers:...
-
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)