Mixsel

ETH ZurichUltrafast Laser Physics

nano-tera.ch Annual Meeting 12. 5. 11

Vertical integration of ultrafast semiconductor lasers for wafer-scale mass production

Vertical integration of ultrafast semiconductor lasers for wafer-scale mass production

Prof. Bernd WitzigmannComputational Electronics and Photonics, University of Kassel (previously ETH Zurich)

Prof. Ursula Keller & Dr. Thomas Südmeyer Physics Department, ETH Zurich

Prof. Eli Kapon & Dr. Alexei SirbuInstitut de Photonique et d‘Electronique Quantiques, EPFL, Lausanne

Prof. Pierre ThomannInstitut de Physique, Université de Neuchâtel

Jan. 17, 2011



OutlineOutline

Motivation and research targets

VECSELs and SESAMs

MIXSEL concept

Highlights • 6.4 W modelocked OP-MIXSEL chip at 960 nm.

• 1 W femtosecond SESAM-modelocked OP-VECSEL at 960 nm.

• 2.62 W cw average power from a 1550 nm OP-VECSEL realized with wafer fusion. First Modelocking result at 1550 nm.

• 120 mW cw average power from an EP-VECSEL.

• Full stabilization of a frequency comb (CEO beat and laser repetition rate) using a SESAM modelocked diode-pumped Er:Yb:glass

Summary and outlook



Compact ultrafast lasers for “real world application”Compact ultrafast lasers for “real world application”Telecom & Datacom Interconnects Optical Clocking

Frequency comb

Multi-photon imaging



The first VECSELs conference at Photonics West

Jan. 24 - 25, 2011



OutlineOutline


VECSELs and SESAMs

MIXSEL concept

Highlights• 6.4 W modelocked OP-MIXSEL chip at 960 nm.





Summary and outlook



CW Optically-Pumped VECSELsCW Optically-Pumped VECSELs

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

outputcoupler



VECSEL gain structureVECSEL gain structure

pump

laser

heat sink

gain structure

outputcoupler

gain structureheatsink

pump energy



Optically pumped semiconductor laser?Optically pumped semiconductor laser?

• Maybe a bad idea coming from semiconductor diode lasers?

• But for sure a good idea coming from diode-pumped solid-state lasers:- more flexibility in operation wavelengths- broad tunability - efficient mode conversion from low-beam-quality high-power diode lasers

- modelocking possible with SESAMs- waferscale integration - cheaper ultrafast lasers in the GHz pulse repetition rate regime



Semiconductor materials: bandgap engineeringSemiconductor materials: bandgap engineeringWavelength of interest 960 nm, 1.3 µm, and 1.5 µm

1.5 µm



VECSELs: cw spectral coverage (Jennifer Hastie)VECSELs: cw spectral coverage (Jennifer Hastie)

• 2‐2.8 μm – GaInAsSb / AlGaAsSb

• 1.5 μm – InGaAs / InGaAsP

• 1.2‐1.5 μm – AlGaInAs / InP (fused)

• 1.2‐1.3 μm – GaInNAs / GaAs

• 1‐1.3 μm – InAs QDs

• 0.9‐1.18 μm – InGaAs / GaAs

• 850‐870 nm – GaAs / AlGaAs

• 700‐750 nm – InP QDs

• 640‐690 nm – InGaP / AlGaInP

• Frequency‐doubled VECSELs have been reported throughout the visible and into the UV

Infrared review: N. Schulz et al., Laser & Photonics Reviews 2, 160 (2008)Visible and UV review: S. Calvez et al., Laser & Photonics Reviews 3, 407 (2009)

updated by Jennifer Hastie, University of Strathclyde, group of Prof. Martin Dawson



SESAMSemiconductorSaturableAbsorber Mirror

Ultrafast VECSELs: Modelocking with SESAMs Ultrafast VECSELs: Modelocking with SESAMs

pump

modelockedlaser

heat sink

gain structure

outputcoupler

SESAM

cwlaser

Review article for VECSELs: U. Keller and A. C. Tropper, Physics Reports, vol. 429, Nr. 2, pp. 67-120, 2006



OutlineOutline


VECSELs and SESAMs

MIXSEL concept






Summary and outlook



Motivation for semiconductor lasers: Wafer scale integrationMotivation for semiconductor lasers: Wafer scale integrationD. Lorenser et al., Appl. Phys. B 79, 927, 2004

MIXSELmodelocked integrated external-cavity surface emitting laser

SESAM

Passively modelocked VECSELvertical 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 integrationMIXSEL 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)



Review article for VECSELs: U. Keller and A. C. Tropper, Physics Reports, vol. 429, Nr. 2, pp. 67-120, 2006

Comparison of Ultrafast GHz LasersComparison of Ultrafast GHz Lasers



OutlineOutline


VECSELs and SESAMs

MIXSEL concept






Summary and outlook



Optically pumped ultrafast VECSELs / MIXSELsOptically pumped ultrafast VECSELs / MIXSELs

B. Rudin, V. J. Wittwer, D. J. H. C. Maas, M. Hoffmann, O. D. Sieber, Y. Barbarin, M. Golling, T. Südmeyer, U. Keller,

Opt. Express 18, 27582, 2010



Resonant vs. antiresonant MIXSEL designResonant vs. antiresonant MIXSEL design

Initial MIXSEL demonstration had a resonant design:D. J. H. C. Maas et al., Appl. Phys. B 88, 493, 2007 • sensitive to growth errors

• high GDD - long pulses

• tolerant to growth errors

• low GDD - short pulses

Here: MIXSEL demonstration with antiresonant design

growth error simulation: layer thickness variations < 1%



MIXSEL: improved thermal managementMIXSEL: improved thermal management

Finite Element (FE) temperature simulations

• exchange the copper with CVD diamond

reasonable temperatures

• leads to highest output power from a ultrafast semiconductor laser

heat sink material

thermal conductivity (W m-1K-1)

estimated heating power (pump power)

pump/ laser mode radius

temp. rise(FE sim.)

heat sink temperature

output power

GaAs 45 1.5 W (1.7 W) 80 µm 149 K -15 °C 41.5 mW

copper 400 3.2 W (4.3 W) 80 µm 98 K +10 °C 660 mW

diamond 1800 26.6 W (36.7 W) 215 µm 100 K -15 °C 6400 mW



High power MIXSELHigh power MIXSEL

• Optical pumping 36.7 W at 808 nm

• Pump / laser spot radius: ~215 m

• Efficiency (opt-opt): 17.4 %

• Cavity length: 60.8 mm 2.47 GHz

• Output coupling: 0.7%

• TBP: 1.35 (4.2 times sech2)

B. Rudin, V. J. Wittwer, D. J. H. C. Maas, M. Hoffmann, O. D. Sieber, Y. Barbarin, M. Golling, T. Südmeyer, U. Keller, Opt. Express 18, 27582, 2010



OutlineOutline


VECSELs and SESAMs

MIXSEL concept

Highlights • 6.4 W modelocked OP-MIXSEL chip at 960 nm.





Summary and outlook




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 VECSELFemtosecond all Quantum Dot VECSELSeparate pump mirrorDBR separation tuning for maximum absorption

higher efficiencyActive regionchirped QD-layer positions

• each layer stack resonant for different laser wavelength

• according to absorption intensity broader gain

AR sectionhybrid semiconductor / fused silica

reduction of the GDD

pump

modelockedlaser

CVD-diamond QD-gainstructure

outputcoupler QD-SESAM



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-VECSELFemtosecond QD-VECSEL

pump

modelockedlaser

CVD-diamond QD-gainstructure

outputcoupler QD-SESAM

repetition rate: 5.4 GHzTBP: 1.3 sech2

peak power: 219 W

pulse duration: 784 fsoutput power: 1.05 Wcenter 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



OutlineOutline


VECSELs and SESAMs

MIXSEL concept






Summary and outlook



2.62 W wafer fused VECSEL at 1550 nm2.62 W wafer fused VECSEL at 1550 nm

Opt. Express 16, 21881-21886 (2008)

• Combine advantages of InP-based active medium with GaAs/AlGaAs reflector

• Intra-cavity diamond for good heat dissipation

2.62 W cw



First wafer-fused modelocked VECSEL at 1550 nmFirst wafer-fused modelocked VECSEL at 1550 nm• First wafer-fused passively modelocked VECSEL at 1550 nm!

• Combine advantages of InP-based active medium with GaAs/AlGaAs reflector

• Intracavity diamond for good heat dissipation

• Beam-spot diameters: 210 µm on gain chip; 50 µm on GaInNAs-based SESAM

• 600 mW in 16 ps pulses at 1.29 GHz with 10 W pump power

E. J. Saarinen, J. Puustinen, A. Sirbu, A. Mereuta, A. Caliman, E. Kapon, O. Okhotnikov, Optics Letters, 34, 3139 (2009)



OutlineOutline


VECSELs and SESAMs

MIXSEL concept






Summary and outlook



Bottom disk contact

DBR

Top ring contact

Electrical vs. optical pumpingElectrical vs. optical pumpingOutput coupler

Active region

Heat spreader

DBR

OP-VECSEL EP-VECSEL

~ 50 μm

Pump laser



ETH Zurich EP-VECSEL designETH Zurich EP-VECSEL design

Design guidelines: P. Kreuter, B. Witzigmann, D.J.H.C. Maas, Y. Barbarin, T. Südmeyer and U. Keller, Appl. Phys. B, 91, 257, 2008

Suitable for modelocking • Relatively low GDD: AR section• Confined current injection for good beam profile

• 6 µm current spreading layer• bottom p-doped, top n-doping• small bottom disk p-contact

Power scalability• Wafer removal• Large apertures possible

Trade off between electrical and optical losses• Optimized doping profile

• High doping → high free carrier absorption• Low doping → high resistivity

• Intermediate n-DBR for increased gain

top contact

bottom contact

CuW wafer

p-DBR

current spreading

layer

AR section

n-DBR

SiNx

SiNx

active region

SEM

14 µm

11



Growth, processing, and evaluation implemented 60 different EP-VECSEL lasing in cw Output power up to 120 mW (cw) achieved Good homogenous electroluminescence profiles measured for

devices up to 100 µm (excellent agreement with our simulations)

First EP-VECSEL resultsFirst EP-VECSEL results



EP-VECSEL cw resultsEP-VECSEL cw results40 EP-VECSELs with different bottom contact diameters

Power scaling considerations• Output power should scale with area (P α Ø2)

and current density (P α J )• Ideal power scaling, ∆T independent of device size

17



OutlineOutline


VECSELs and SESAMs

MIXSEL concept






Summary and outlook



A key application: optical frequency combsA key application: optical frequency combs

www.faszination-uhrwerk.de

- Phase stable link between optical (100s THz) and microwave frequencies (GHz)

- Counting of arbitrary optical frequencies practicable for the first time

offer

- Fundamental physics- Optical clocks- Satellite navigation- Large bandwidth telecommunication- Spectroscopy- Medical applications, noninvasive

diagnostics

impact



Femtosecond Er:Yb:glass laserFemtosecond Er:Yb:glass laser

Stumpf, Zeller, Schlatter, Südmeyer, Okuno, Keller, Opt. Express 16, 10572 (2008)

Telecom center wavelength (1.55 µm) Reliable telecom grade pump diode Low power consumption (< 1.5 W electrical) Clean soliton pulses Polarized output

Total resonator losses below 3 %



Moving of the laser from ETH to NeuchatelMoving of the laser from ETH to Neuchatel



Noise performance of DPSSLs and VECSELsNoise performance of DPSSLs and VECSELs

+ high-Q cavity, low nonlinearities⇒ extremely low intrinsic noise

+ convenient and robust

DPSSLs

Compare 75 MHz 1.5-µm Er:Yb glass DPSSL with commercial 1.5-µm Er-fiber laser

Example excellent noise performance of DPSSLs: Optical ultra-stable microwave oscillator

S.Schilt, M. C. Stumpf, L. Tombez, N. Bucalovic, V. Dolgovskiy, G. Di Domenico, D. Hofstetter, S. Pekarek, A. E. H. Oehler, T. Südmeyer, U. Keller, P. Thomann,“Phase noise characterization of a near-infrared solid-state laser optical frequency comb for ultra-stable microwave generation”,Optical Clock Workshop, Torino, Italy, December 1-3, 2010

Relative frequency stability of the CEO frequency measured with the same feedback loop

(right scale: relative frequency stability with respect to the optical carrier)



fCEO detected with a DPSSL without pulse compression or amplification

targeted VECSEL

VECSELs for Frequency Comb GenerationVECSELs for Frequency Comb Generationcrucial for frequency comb stabilization:

detection of the carrier envelope offset frequency (fCEO)

278 fs

74 mW

75 MHz

3.1 kW1550 nm

p

Pav

frep

Ppeak

λcenter

200 fs

1 W

1 GHz

4.4 kW960 nm

Femtosecond VECSEL: promising candidate for compact, low cost frequency comb generation

Stumpf, Pekarek, Oehler, Südmeyer, Dudley, Keller, Appl. Phys. B 99, 401 (2010)



OutlineOutline


VECSELs and SESAMs

MIXSEL concept






Summary and outlook






Gantt chartGantt chart

• Excellent result of 960 nm MIXSEL => 1550 nm SESAM and MIXSEL delayed (Tasks 1.1, 1.2 and 2.1)

• Femtosecond VECSEL demonstrated (Task 4.2) => high expectation for fs-MIXSEL (Task 4.3)

• First EP-VECSEL in a university,120 mW realized => 200 mW achievable in a next realization (Task 5.3)

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