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

Vertical integration of
ultrafast semiconductor lasers
for wafer-scale mass production
Prof. Eli Kapon & Dr. Alexei Sirbu
Institut de Photonique et d‘Electronique Quantiques, EPFL, Lausanne

Prof. Bernd Witzigmann
Computational Electronics and Photonics, University of Kassel
(previously ETH Zurich)
Prof. Pierre Thomann
Institut de Physique, Université de Neuchâtel
Prof. Ursula Keller & Dr. Thomas Südmeyer
Jan. 17, 2011 Laser
Ultrafast Physics Department, ETH Zurich ETH Zurich
Physics
nano-tera.ch Annual Meeting 12. 5. 11

Outline
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

Ultrafast Laser ETH Zurich
Physics

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

Multi-photon imaging

Frequency comb

Physics

The first VECSELs conference at Photonics West
Jan. 24 - 25, 2011

Physics

CW 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
pump output
coupler
thickness
laser

IEEE JQE 38, 1268 (2002)
gain structure
• Pump: high power diode bar
heat sink • External cavity
Ultrafast Laser
for diffraction-limited output
ETH Zurich
Physics

VECSEL gain structure
heat gain structure
sink

pump output
coupler pump energy
laser

gain structure

heat sink ETH Zurich
Ultrafast Laser
Physics

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

Physics

Semiconductor materials: bandgap engineering
Wavelength of interest 960 nm, 1.3 µm, and 1.5 µm

1.5 µm

Physics

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 ETH Zurich
Ultrafast Laser 3, 407 (2009)
Physics
updated by Jennifer Hastie, University of Strathclyde, group of Prof. Martin Dawson

Ultrafast VECSELs: Modelocking with SESAMs
cw
laser

SESAM modelocked
Semiconductor laser
Saturable
Absorber Mirror

output
pump coupler

SESAM

gain structure

heat sink

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

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

Passively modelocked VECSEL
vertical external cavity surface emitting laser

Review: Physics Reports 429, 67-120, 2006

SESAM

MIXSEL
modelocked integrated external-cavity surface emitting laser

Ultrafast Laser D. J. H. C. Maas et al., Appl. Phys.ETH88, 493, 2007
B Zurich
Physics

MIXSEL wafer scale integration

A. R. Bellancourt et al., “Modelocked integrated external-cavity surface emitting laser” ETH Zurich
Ultrafast Laser
IET Optoelectronics, vol. 3, Iss. 2, pp. 61-72, 2009 (invited paper)
Physics

Comparison of Ultrafast GHz Lasers

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

Optically 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
Physics

Resonant 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

growth error simulation:
layer thickness variations < 1%

Here: MIXSEL demonstration • tolerant to growth errors
with Ultrafast Laser
antiresonant design • low GDD - short pulses
ETH Zurich
Physics

MIXSEL: improved thermal management
heat thermal estimated pump/ temp. rise heat sink output
sink conductivity heating power laser (FE sim.) temperature power
material (W m-1K-1) (pump power) mode
radius
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

• exchange the copper with
CVD diamond
 reasonable temperatures
• leads to highest output power
from a ultrafast
Finite Element (FE) temperature simulations
Ultrafast Laser semiconductorZurich
ETH laser
Physics

High power MIXSEL

• Optical pumping 36.7 W at 808 nm • Cavity length: 60.8 mm  2.47 GHz
• Pump / laser spot radius: ~215 m • Output coupling: 0.7%
• Efficiency (opt-opt): 17.4 % • TBP: 1.35 (4.2 times sech2)

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


M. Hoffmann, O. D. Sieber, V. J. Wittwer, I. L. Kestnikov, D. A. Livshits, T. Südmeyer, U. Keller,
Ultrafast Laser
Opt. Express 19, 8108, 2011 ETH Zurich
Physics

Femtosecond all Quantum Dot VECSEL
modelocked Separate pump mirror
laser DBR separation tuning for maximum
pump absorption
 higher efficiency
Active region
chirped QD-layer positions
output • each layer stack resonant for
coupler QD-SESAM different laser wavelength
• according to absorption intensity
 broader gain
QD-gain AR section
CVD-diamond hybrid semiconductor / fused silica
structure
 reduction of the GDD

Physics

Femtosecond QD-VECSEL
modelocked
laser
heat sink: thinned QD gain structure on CVD substrate pump
output coupler: 100 mm

output coupler transmission: 2.5%
output
laser mode radius on QD-VECSEL: 115 µm coupler QD-SESAM

laser mode radius on QD-SESAM: 115 µm

heat sink temperature: -20°C CVD-diamond QD-gain
structure

pulse duration: 784 fs repetition rate: 5.4 GHz
output power: 1.05 W TBP: 1.3 sech2
center wavelength: 970 nm peak power: 219 W

M. Hoffmann, O. D. Sieber, V. J. Wittwer, I. L. Kestnikov, D. A. Livshits, T. Südmeyer, U. Keller, Opt. Express 19, 8108, 2011
Physics

2.62 W wafer fused VECSEL at 1550 nm
• Combine advantages of InP-based active
medium with GaAs/AlGaAs reflector
• Intra-cavity diamond for good heat dissipation

2.62 W cw

Ultrafast 21881-21886 (2008)
Opt. Express 16, Laser ETH Zurich
Physics

First 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)
Physics

Electrical vs. optical pumping
OP-VECSEL Output
EP-VECSEL
coupler

Pump laser

Top ring contact
Active
region ~ 50 μm

DBR

DBR

Heat spreader
Ultrafast Laser ETH Bottom
Zurich disk contact
Physics

ETH Zurich EP-VECSEL design

AR section
Suitable for modelocking
SiNx • Relatively low GDD: AR section
top contact
current spreading • Confined current injection for good beam profile
• 6 µm current spreading layer
layer
• bottom p-doped, top n-doping
n-DBR • small bottom disk p-contact
p-DBR
Power scalability
bottom contact • Wafer removal
active region
• Large apertures possible

SiNx Trade off between electrical and optical losses
CuW wafer • Optimized doping profile
• High doping → high free carrier absorption
SEM • Low doping → high resistivity
• Intermediate n-DBR for increased gain

Design guidelines:
P. Kreuter, B. Witzigmann, D.J.H.C. Maas,
Y. Barbarin, T. Südmeyer and U. Keller,
14 µm Appl. Phys. B, 91, 257, 2008
Physics
nano-tera.ch Annual Meeting 12. 5. 11 11

First EP-VECSEL results
 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)

Physics

EP-VECSEL cw results
40 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

Physics
nano-tera.ch Annual Meeting 12. 5. 11 17

A key application: optical frequency combs

offer
- Phase stable link between optical (100s THz)
and microwave frequencies (GHz)
- Counting of arbitrary optical frequencies
practicable for the first time

impact
- Fundamental physics
- Optical clocks
- Satellite navigation
- Large bandwidth telecommunication
- Spectroscopy
- Medical applications, noninvasive
diagnostics
Ultrafast Laser
Physics
www.faszination-uhrwerk.de
ETH Zurich


Femtosecond Er:Yb:glass laser

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

Moving of the laser from ETH to Neuchatel

Physics

Noise performance of DPSSLs and VECSELs
DPSSLs
+ high-Q cavity, low nonlinearities
⇒ extremely low intrinsic noise
+ convenient and robust

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

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

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

(right scale: Ultrafast Laser
relative frequency stability with respect to the optical carrier) ETH Zurich
Physics

VECSELs for Frequency Comb Generation
crucial for frequency comb stabilization:
detection of the carrier envelope offset frequency (fCEO)

fCEO detected with a DPSSL
targeted VECSEL
without pulse compression or amplification

278 fs p 200 fs
74 mW Pav 1W
75 MHz frep 1 GHz
3.1 kW Ppeak 4.4 kW
1550 nm λcenter 960 nm

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

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

Physics


Physics

Gantt 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)
Physics
• First EP-VECSEL in a12. 5. 11
nano-tera.ch Annual Meeting university,120 mW realized => 200 mW achievable in a next realization (Task 5.3)

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

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