Ursula Keller

10/19/13

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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:

6

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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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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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

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Ursula Keller