Download - Ursula Keller
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ETH Zurich Ultrafast Laser Physics
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Compact lasers for frequency comb generation
Ursula Keller
ETH Zürich, Physics Department, Switzerland
The Sixth International Symposium on Ultrafast Photonics Technology University of Rochester, Rochester, New York
Oct. 21-22, 2013
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PD Dr. Lukas Gallmann
Prof. Dr. Ursula Keller
Dr. Matthias GollingDr. Alexandra Landsman
Dr. Hirofumi Yanagisawa Dr. Bauke Tilma
Dr. Jan van Beilen Volland Dr. Anna Garry Nadia Sigrist
Sandra Schmid
Dr. Claudio Cirelli Dr. Matteo Lucchini Dr. Jochen Maurer
Jens Hermann
Cinia SchriberAlexander Klenner
André Ludwig
Mario Mangold
Benedikt Mayer
Sebastian HeuserRobert Boge Florian Emaury
Cornelia Hofmann
Wolfgang Pallmann Reto Locher Christian ZauggMazyar SabbarOliver Sieber
Dr. Chris Phillips Dr. Clara Saraceno
Group Head Secretary
ULTRAFAST LASER PHYSICSDepartment of Physics
NCCR MUST
April 2013Institute of Quantum Electronics
Postdocs
Subgroup LeadersTechnical Staff
PhD Students
OP VECSEL-MIXSEL team Oliver Sieber (Ph.D. 2013) Christian Zaugg Mario Mangold Sandro Link Dr. Bauke Tilma previously: Valentin Wittwer (Ph.D. 2012) Martin Hoffmann (Ph.D. 2011) Dr. Thomas Südmeyer GHz Yb:KGW laser Alexander Klenner
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S/N in nearly all applications is limited by available power per comb line Increasing repetition rate leads to larger line spacing
Frequency combs: need for high repetition rates
Spectroscopy Information Metrology
Astronomical spectrograph calibration
Molecular spectroscopy
Tunable laser calibration
High speed communication Precision
ranging
Arbitrary waveform generation
Optical clocks Low noise
microwaves
Timing synchronization Frequency
transmission LIDAR
FTIR
frequency
intensity
Frequency comb with larger spacing: • higher power per mode • easier to access individual lines • more compact system
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Compact ultrafast lasers for “real world application”
Telecom & Datacom Interconnects Optical Clocking
Frequency comb
Multi-photon imaging
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S/N in nearly all applications is limited by available power per comb line Increasing repetition rate leads to larger line spacing
Frequency combs: need for high repetition rates
Spectroscopy Information Metrology
Astronomical spectrograph calibration
Molecular spectroscopy
Tunable laser calibration
High speed communication Precision
ranging
Arbitrary waveform generation
Optical clocks Low noise
microwaves
Timing synchronization Frequency
transmission LIDAR
FTIR
Passively modelocked laser • compact/robust laser system
• repetition rate >1 GHz
• pulse width <200 fs
• peak power >6 kW SC fCEO N = τ p2 ⋅Ppeak ⋅
γ|β 2|
⋅0.322
Coherence requirement for SCG:
J.M. Dudley et al., Rev. of Modern Physics 78, 1135 (2006)
< 10
soliton order N:
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GHz oscillators without compression or amplification
>1 GHz <200 fs >6 kW
N=10
coherent octave possible
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GHz oscillators without compression or amplification
>1 GHz <200 fs >6 kW
N=10
octave-spanning spectrum
N=10 PCF1
octave-spanning spectrum
20-fold better fractional frequency stability for Diode-Pumped Solid-State Laser (DPSSL)
1 10 100 1000
Er-fiber laserEr:glass DPSSL
10-6
10-8
10-10
10-12
10-13
10-15
10-17
10-19
!(s)
"#f
/ f C
EO(!)
"#f
/ f n(!)
MHz-DPSSL vs. MHz-fiber laser
fCEO = 20 MHz fn ≈ 200 THz
GOAL: GHz-DPSSL frequency comb
S. Schilt et al., Opt. Express 19, 24171 (2011)
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coherent octave possible
GHz Yb:KGW laser for strong CEO beat N=10
22.7 kW 125 fs
First CEO-beat detection of a GHz DPSSL without pulse compression
A. Klenner et al., Opt. Express 8, 10351 (2013)
• Further investigations on the requirements for stabile frequency comb generation • Full stabilization of the 1 GHz Yb:KGW laser
Spec
tral p
ower
(dBm
)
Frequency (GHz)
100 kHz RBW unaveraged
b)
0 0.2 0.4 0.6 0.8 1.0 1.2
!70
!60
!50
!40
!30
!20
!10
fCEO frep-fCEO
-500 fs2
L1L2
M1M2
SESAM
M3M4
pump
Yb:KGW
-500 fs2
30 dB
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Motivation for semiconductor lasers: Wafer scale integration D. Lorenser et al., Appl. Phys. B 79, 927, 2004
MIXSEL modelocked integrated external-cavity surface emitting laser
SESAM
Passively modelocked VECSEL vertical external cavity surface emitting laser Review: Physics Reports 429, 67-120, 2006
D. J. H. C. Maas et al., Appl. Phys. B 88, 493, 2007
MIXSEL wafer scale integration
A. R. Bellancourt et al., “Modelocked integrated external-cavity surface emitting laser” IET Optoelectronics, vol. 3, Iss. 2, pp. 61-72, 2009 (invited paper)
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MIXSEL wafer scale integration
A. R. Bellancourt et al., “Modelocked integrated external-cavity surface emitting laser” IET Optoelectronics, vol. 3, Iss. 2, pp. 61-72, 2009 (invited paper)
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MIXSEL in a closed laser housing
• straight cavity with etalon for wavelength tuning • water-cooled Peltier-element for temperature stabilization • fiber-coupled industrial grade pump (35 W, 808 nm) • fix mounted optics (no translation stages)
pump-fiber
laser-output
output-coupler
intra-cavityetalon
MIXSEL chip
pump-optics
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Stabilized Noise Measurements
average output power
VECSEL [1] 53 mW
MIXSEL 700 mW Er:Yb:glass laser [2]
15 mW
laser repetition rate [GHz]
fmin [Hz]
fmax [MHz]
RMS timing
jitter [fs] Er:Yb:glass laser [2] 10 6 1.56 26
MIXSEL 2.004 6 1.56 44.2
[1] V. J. Wittwer et al., IEEE Photonics Journal 5, 1400107, 2012 [2] A. Schlatter et al., Opt. Lett. 30, 1536, 2005
rms amplitude noise: 0.1% [1 Hz, 40 MHz]
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CW optically pumped VECSEL
OP-VECSEL = Optically Pumped Vertical-External-Cavity Surface-Emitting Semiconductor Laser
M. Kuznetsov et al., IEEE Photon. Technol. Lett. 9, 1063 (1997)
• Semiconductor gain structure with reduced thickness IEEE JQE 38, 1268 (2002) • Pump: high power diode bar • External cavity
for diffraction-limited output
pump
laser
heat sink
gain structure
output coupler
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VECSEL gain structure
pump
laser
heat sink
gain structure
output coupler
gain structure heat sink
pump energy
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• 7 In0.13Ga0.87As QWs (8 nm) in anti-nodes of standing-wave pattern, designed for gain at ≈ 960 nm
• GaAs spacer layers
• Strain-compensating GaAs0.94P0.06 layers
• Pump at 808 nm
pump energy
VECSEL gain structure heat sink
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Power scaling in the thin disk geometry
Pump spot (typical diameter >100 µm)
Thin semiconductor structure (≈ 8 µm) 1D heat flow
Copper or diamond heat sink 3D heat flow
Increase output power by increasing pump power and mode size
First room temperature VECSEL: 20 µW average power: J.V. Sandusky et al., IEEE Photon. Technol. Lett. 8, 313 (1996) High-power cw operation: 0.5 W in TEM00 beam: M. Kuznetsov et al., IEEE Photon. Technol. Lett. 9, 1063 (1997) 1.5 W: W. J. Alford et al., J. Opt. Soc. Am. B 19, 663 (2002) 30 W: J. Chilla et al., Proc. SPIE 5332, 143 (2004)
High power TEM00 cw-operation
SESAM diamond heat
spreader
808-nm pump
B. Rudin et al., Opt. Lett. 33, 2719-2721 (2008)
material k [W/Km]
GaAs 55
copper 400
diamond >1800
• MBE or MOVPE growth of structure in reverse order
• cleaving of small pieces
• metalization for soldering
• flipping over
• soldering on heat spreader with high thermal conductivity
• substrate removal by selective wet etching
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§ Maximum power P = 20.2 W § opt.-to-opt. efficiency up to 43% § M² < 1.1
SESAM diamond heat
spreader 960 nm
808-nm pump
VECSEL pump rad. 240 µm
output coupler (0.7%)
High power TEM00 cw-operation B. Rudin et al., Opt. Lett. 33, 2719-2721 (2008)
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OP-VECSEL milestones
• First room temperature VECSEL: 20 µW average power: J.V. Sandusky et al., IEEE Photon. Technol. Lett. 8, 313 (1996)
• High-power cw operation: 0.5 W in TEM00 beam: M. Kuznetsov et al., IEEE Photon. Technol. Lett. 9, 1063 (1997) 1.5 W: W. J. Alford et al., J. Opt. Soc. Am. B 19, 663 (2002) 30 W: J. Chilla et al., Proc. SPIE 5332, 143 (2004) - Coherent 20 W in TEM00 beam: B. Rudin et al., Optics Lett. 33, 2719, 2008
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SESAM Semiconductor Saturable Absorber Mirror
Ultrafast VECSELs: Modelocking with SESAMs
pump
modelocked laser
heat sink
gain structure
output coupler
SESAM
cw laser
Review article for modelocked VECSELs: U. Keller and A. C. Tropper, Physics Reports, vol. 429, Nr. 2, pp. 67-120, 2006
Saturday, October 19, 13 Ultrafast Laser Physics
Dynamic gain saturation in semiconductor lasers
time
loss
gain
pulsetime
loss
gain
pulse
Solid-state lasers No dynamic gain saturation Soliton modelocking F. X. Kärtner, U. Keller, Opt. Lett. 20, 16, 1995
Semiconductor and dye lasers Dynamic gain saturation G.H.C. New, Opt. Com. 6, 188, 1974
gain structure
SESAM
Important difference between semiconductor lasers and diode-pumped solid-state lasers
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OP-VECSEL milestones
• First room temperature VECSEL: 20 µW average power: J.V. Sandusky et al., IEEE Photon. Technol. Lett. 8, 313 (1996)
• High-power cw operation: 0.5 W in TEM00 beam: M. Kuznetsov et al., IEEE Photon. Technol. Lett. 9, 1063 (1997) 1.5 W: W. J. Alford et al., J. Opt. Soc. Am. B 19, 663 (2002) 30 W: J. Chilla et al., Proc. SPIE 5332, 143 (2004) - Coherent 20 W in TEM00 beam: B. Rudin et al., Optics Lett. 33, 2719, 2008
• Passive mode locking with SESAM: 20 mW: S. Hoogland et al., IEEE Photon. Technol. Lett. 12, 1135 (2000) 200 mW: R. Häring et al., Electron. Lett. 37, 766 (2001) 950 mW: R. Häring et al., IEEE JQE 38, 1268 (2002) 2.1 W, 4.7 ps, 4 GHz, 957 nm 2.2 W, 6 ps, 1.5 GHz, 957 nm A. Aschwanden et al., Opt. Lett. 30, 272 (2005) Modelocked VECSEL review: Physics Reports 429, 67, 2006
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MIXSEL concept
integration of absorber
MIXSEL Modelocked Integrated External- Cavity Surface Emitting Laser
VECSEL VECSEL
QD-SESAM QW-SESAM
focusing on SESAM
no focusing required stronger saturation
VECSEL Vertical External Cavity Surface Emitting Laser
SESAM Semiconductor Saturable Absorber Mirror
dynamic gain saturation
loss has to saturate faster than gain
absorber integration: same mode size on absorber and gain Aa= Ag ð 1:1 modelocking ð Fsat,a < Fsat,g
an absorber with low saturation fluence 8 µm
mm
to c
m
D. J. H. C. Maas et al., APB 88, 493, 2007
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Resonant design analogue to resonant QD SESAMs
D. J. H. C. Maas et al., Applied Physics B 88, 493-497 (2007)
First MIXSEL demonstration: 35 ps, 40 mW, 2.8 GHz
Sections: • 30 pair bottom mirror for the laser • 1 layer of self-assembled InAs QD • DBR to increase field in absorber • 9 pair mirror for the pump • active region with 7 InGaAs QWs • AR coating
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average output power: 6.4 W repetition rate: 2.47 GHz pulse duration: 28.1 ps center wavelength: 959 nm
Antiresonant MIXSEL (2010) tolerant to growth errors
B. Rudin et al., Opt. Exp. 18, 27582, 2010
average output power: 2.4 W
repetition rate: 10 GHz pulse duration: 16.9 ps center wavelength: 963 nm
Wittwer et al., Electron. Lett. 48, 1144, 2012
relaxed growth ð thermal management by flip-chip bonding on CVD-diamond
growth error simulation: layer thickness variations < 1%
10 GHz ð cavity round trip time: 100 ps
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1 mW
10 mW
100 mW
1 W
10 W
aver
age
outp
ut p
ower
10 fs 100 fs 1 ps 10 ps 100 pspulse duration
QW-VECSEL QD-VECSEL MIXSEL
Optically pumped ultrafast VECSELs / MIXSELs
M. Hoffmann, O. D. Sieber, V. J. Wittwer, I. L. Kestnikov, D. A. Livshits, T. Südmeyer, U. Keller, Opt. Express 19, 8108, 2011
Femtosecond all quantum dot VECSEL Separate pump mirror DBR separation tuning for maximum absorption
è higher efficiency Active region chirped QD-layer positions
• each layer stack resonant for different laser wavelength
• according to absorption intensity è broader gain
AR section hybrid semiconductor / fused silica
è reduction of the GDD
pump
modelocked laser
CVD-diamond QD-gain structure
output coupler QD-SESAM
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heat sink: thinned QD gain structure on CVD substrate
output coupler: 100 mm
output coupler transmission: 2.5%
laser mode radius on QD-VECSEL: 115 µm
laser mode radius on QD-SESAM: 115 µm
heat sink temperature: -20°C
Femtosecond QD-VECSEL pump
modelocked laser
CVD-diamond QD-gain structure
output coupler QD-SESAM
repetition rate: 5.4 GHz TBP: 1.3 sech2
peak power: 219 W
pulse duration: 784 fs output power: 1.05 W center wavelength: 970 nm
M. Hoffmann, O. D. Sieber, V. J. Wittwer, I. L. Kestnikov, D. A. Livshits, T. Südmeyer, U. Keller, Opt. Express 19, 8108, 2011
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novel ultrafast absorber: QW absorber embedded
in AlAs
1.0
0.8
0.6
0.4
0.2
0.0
norm
aliz
ed s
atur
atio
n
12080400time delay (ps)
MIXSEL absorber fast QD-SESAM absorber
problem: QD are annealed during the long
growth of the MIXSEL!
1.0
0.8
0.6
0.4
0.2
0.0
norm
aliz
ed s
atur
atio
n
12080400time delay (ps)
MIXSEL absorber fast QD-SESAM absorber novel QW-absorber
Limitations of previous QD-MIXSEL
absorber recombination • slow recombination of the QD absorber in the QD-MIXSEL
• compared to the absorber in the QD SESAM for femtosecond pulses
M. Hoffmann, et al., Optics Express 19, 8108, 2011
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Femtosecond MIXSEL structure
• 24-pair AlAs/GaAs laser mirror (for 960 nm) • single InGaAs quantum-well saturable absorber • 9-pair AlAs/Al0.2Ga0.8As (for 808 nm) • 10 active quantum-wells embedded in GaAs • hybrid semiconductor / fused-silica anti-reflection coating for low
group-delay dispersion and small pump reflection
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Quantum-well saturable absorber • single low-temperature grown
InGaAs quantum-well saturable absorber
• embedded in AlAs • 25 nm detuning at room
temperature to 935 nm • operation at 60-80°C inside the
pumped MIXSEL structure
low saturation fluences fast recovery dynamics
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Femtosecond MIXSEL
pulse duration: 620 fs average output power: 101 mW center wavelength: 967.7 nm FWHM spectral width: 2.90 nm repetition rate: 4.83 GHz
• optical pumping 24.9 W at 808 nm
• pump / laser spot radius: ~145 µm
• cavity length: 31 mm ð 4.83 GHz
• fluence on the MIXSEL : 8.5 µJ/cm2
• heat sink temperature: + 11 °C
• output coupling: 0.35 % (ROC 200 mm)
First femtosecond MIXSEL M. Mangold et al. Opt. Express 21, 24904, 2013
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High-power ultrafast MIXSEL
pulse duration: 2.4 ps average output power: 1.05 W center wavelength: 952.3 nm FWHM spectral width: 0.80 nm
• optical pumping 16.9 W at 808 nm
• pump / laser spot radius: ~148 µm
• cavity length: 29.5 mm ð 5.1 GHz
• fluence on the MIXSEL : 30.1 µJ/cm2
• heat sink temperature: 0°C
• output coupling: 1% (ROC 200 mm)
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Optically pumped ultrafast VECSELs / MIXSELs
1 mW
10 mW
100 mW
1 W
10 W
aver
age
outp
ut p
ower
10 fs 100 fs 1 ps 10 ps 100 pspulse duration
QW-VECSEL QD-VECSEL MIXSEL
More updates also on webpage of Prof. Keller: www.ulp.ethz.ch/research/VecselMixsel
most recent MIXSEL results
100 mW, 620 fs, 4.8 GHz
1.3 W, 3.9 ps, 10 GHz 1 W, 2.4 ps, 5 GHz 0.61 W, 2.4 ps, 21 GHz
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coherent octave possible
Outlook N=10
22.7 kW 125 fs
Promising alternatives: for more compact and low noise frequency combs
Vertical External
Cavity Surface
Emitting Laser
VECSEL
Modelocked Integrated
External-Cavity Surface
Emitting Laser
MIXSEL
V. J. Wittwer, et al., IEEE Photonics Journal 5, 1400107, 2013
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More info on the web ...
• Web page of Prof. Ursula Keller at ETH Zurich: http://www.ulp.ethz.ch • All papers are available to download as PDFs:
http://www.ulp.ethz.ch/publications/paper • SESAM milestones: http://www.ulp.ethz.ch/research/Sesam • Ultrafast solid-state laser: get started with the book chapter ...
http://www.ulp.ethz.ch/research/UltrafastSolidStateLasers • VECSEL and MIXSEL: http://www.ulp.ethz.ch/research/VecselMixsel • Frequency combs: http://www.ulp.ethz.ch/research/FrequencyComb • Attoscience, Attoclock, Attoline: • http://www.ulp.ethz.ch/research/Attosecondscience • http://www.ulp.ethz.ch/research/Attoline • http://www.ulp.ethz.ch/research/Attoclock • Viewgraphs to graduate lecture course: Ultrafast Laser Physics
http://www.ulp.ethz.ch/education/ultrafastlaserphysics/viewgraphs