PhD Talk Dirk Lorenser

ETH Zurich

Dirk Lorenser

Institute of Quantum Electronics

ETH Zurich, Switzerland

Picosecond VECSELswith repetition rates up to 50 GHz

Ph.D defense presentation - December 5, 2005

Motivation

-Optically Pumped

VECSELs

Mode Locking

VECSELs1 - 10 GHzVECSELs up to 50 GHz

Conclusion andOutlook

• Motivation• Optically-Pumped VECSELs

• Design, Growth, Processing• Thermal Management

• Mode Locking• Passive mode locking of VECSELs

• VECSELs with repetition rates of 1 - 10 GHz• High-power (up to 2 W) mode-locked VECSELs using QW

SESAMs

• VECSELs with repetition rates up to 50 GHz• Low-Fsat QD SESAMs for high-repetition-rate mode locking

• Towards wafer-scale integration of mode-locked VECSELs• 50-GHz ML VECSEL with 100 mW output power

• Conclusion and Outlook

OutlineOutline

MotivationMotivationDevelop a pulsed laser source with:• high average output powers Pavg (up to several watts)• good beam quality (TEM00)• high repetition rates (10s of GHz)• short pulses (τp 1 ps)≤• simple and compact setup

# : Kuznetsov et al., IEEE Photon. Technol. Lett., 9 (8), 1063 (1997)

Motivation

-Optically Pumped

VECSELs

Mode Locking



Optically-pumpedpassively mode-locked

Vertical External-Cavity Surface Emitting

Semiconductor Laser(VECSEL#)

Optical pumping

MotivationMotivation

• power scalability

Semiconductor

#: Keller et al., IEEE J. Sel. Top. Quant. Electron., 2 (3), 435 (1996)

• design wavelength• passive ML with SESAM#

• reduced QML tendency##

→ high repetition rates• broad gain bandwidth

→ short pulses

##: Hönninger et al., J. Opt. Soc. Am. B, 16 (1), 46 (1999)Grange et al., Appl.Phys.B, 80 , 151 (2005)

Motivation

-Optically Pumped

VECSELs

Mode Locking



Surface Emitter

External Cavity• good beam quality

• large-area homogenous inversion

ApplicationsApplications

Optical clocking

Telecommunications

Motivation

-Optically Pumped

VECSELs

Mode Locking



Frequency doublingIR→visible (RGB systems)

Gain Structure DesignGain Structure Designantireflective top section

active region

bottom mirror (DBR)

Motivation

-Optically Pumped

VECSELs

Mode Locking



Laser wavelength (950-970 nm):• angle of incidence 0-15º≈• bottom mirror: very highly reflecting (>99.9%)• AR top section: minimize subcavity resonances

Pump wavelength (808 nm):• angle of incidence = 45º• bottom mirror and AR top section: double-pass of pump light

Gain Structure Design: Active RegionGain Structure Design: Active Region

In0.13Ga0.87As quantum wells• placed in the maxima of the

standing wave pattern• compressively strained

GaAs0.94P0.06 layers• tensile strained• compensation of compressive

strain from In0.13Ga0.87As

active region

Motivation

-Optically Pumped

VECSELs

Mode Locking



Gain Structure Design: GDDGain Structure Design: GDD

Subcavity resonances between RAR and RHR

RHR > 99.9%

RAR < 1%

activeregion

heat sink

Motivation

-Optically Pumped

VECSELs

Mode Locking



Group-Delay Dispersion (GDD)

GDD of gain structure is the dominating source of dispersion in the cavity of a ML VECSEL (up to several ±1000 fs2)

ProcessingProcessing##

#: Häring et al., IEEE J. Quantum Electron., 38 (9), 1268 (2002)

Motivation

-Optically Pumped

VECSELs

Mode Locking



(≈ 7 μm thick)

ProcessingProcessing

2 mm

5 m

m

gain structureon copper heat spreader

gain structureon CVD diamondheat spreader

Motivation

-Optically Pumped

VECSELs

Mode Locking



VECSEL HeatingVECSEL Heating

€

∆T3D = Pheat

2πwκ3D

€

∆T1D =2Pheatd

πw2κ1D

808with 0.7

960

heat pump refl spont laser

spont QD rad thres thres

P P P P P

P P Pη η

= − − −

= ≈ × ×

Temperature rise in center:

1800Diamond

400Cu

45GaAs

κ (WK-1m-1)Material

Motivation

-Optically Pumped

VECSELs

Mode Locking



Model thermal lens as a thin gradient-index lens of thickness deff. Take the gain structure subcavity resonance as effective optical thickness:

Thermal lens: simple modelThermal lens: simple model

20 02 2 2eff

eff

c cd

n d n

λν λλ λ

∆ = ∆ = ⇒ =∆

For gain structures on high-thermal-conductivity heat spreaders: Gaussian transverse temperature distribution with ΔT ≈ ΔT1D which can be approximated with Taylor expansion to 2nd order:

ΔT = 40 K, w = 70 μmnb = 3.54 (GaAs)dn/dT = 2·10-4 K-1

2

2

2 2

210 22

4( )r

wb

dn dnn r n T e n TdT w dTn n r

− = + ∆ ⇒ = ∆≈ −

Motivation

-Optically Pumped

VECSELs

Mode Locking



Δλ = 35 nm, λ = 960 nm, n = 3.54 (GaAs)→ deff = 3.7 μm

Thermal lens: simple modelThermal lens: simple model

2

1effn d

f=

2

0

2 210 22( ) ; n

nn r n n r γ= − =

ray matrix for a GRIN duct of thickness deff:

0

1

0

cos( ) sin( )

sin( ) cos( )

eff effn

eff eff

d d

n d d

γγ γ

γ γ γ −

2 10

1 01 0

11eff fn dγ

≡ −−

for γdeff << 1 this is equivalent to a thin lens:

2

81 effd dnT

f w dT= ∆

( single pass )

for double-pass andGaussian profile:

1.8*17f (cm)

4530ΔT1D (K)

Hi-Rep(50 GHz)w = 70 μm

deff = 3.7 μm

dn/dT = 2·10-4

Hi-Power(4 GHz)w = 175 μm

deff = 3.7 μm

dn/dT = 2·10-4

*measured: 3.2 ± 0.3 cm

Motivation

-Optically Pumped

VECSELs

Mode Locking



• Semiconductor Saturable Absorber Mirror (SESAM)U. Keller et al., IEEE JSTQE 2, 435 (1996)

• Mode Locking Condition

VECSEL Mode LockingVECSEL Mode Locking

for QW SESAMs, typical mode area ratio Ag/Aa ≈ 10-40

, ,

, ,

1sat a sat a a

sat g sat g g

E F A

E F A= =

Motivation

-Optically Pumped

VECSELs

Mode Locking



Pulse-shaping mechanismPulse-shaping mechanism

→ Numerical simulations# of ML dynamics in VECSELs

Early ML VECSELs:• nearly transform-limited 3.2 ps pulses## (213 mW)• strongly chirped pulses up to 27 ps (1.9 W)

Key results• pulse shaping influenced by nonlinear phase change due to

dynamic saturation of gain and absorption• simulations yielded soliton-like pulse solutions for positive

GDD

#: Paschotta et al., Appl. Phys. B, 75 (4-5), 445 (2002)##: Hä ring et al., Electron. Lett., 37 (12), (2001)

Motivation

-Optically Pumped

VECSELs

Mode Locking



nearly transform-limited "quasi-soliton" pulses

Nearly transform-limited pulses atNearly transform-limited pulses at4 GHz4 GHz## and 10 GHz and 10 GHz####

Cavity• length: Lcav ≈ R• spot radius on gain structure:

wg ≈ wpump ≈ 175 μm• spot radius on SESAM:

wa ≈ 50 μm (4 GHz)wa ≈ 30 μm (10 GHz)

• output coupler: T = 2.5%R = 38 mm (4 GHz)R = 15 mm (10 GHz)

• cavity angle: 15°

Etalon

• 20 μm (4 GHz), 50 μm (10 GHz)

T = 2.5%

pum

p

SESAM

heat sinkgain structure

SESAM• 4 GHz: 8.5 nm In0.15Ga0.85As QW (LT-grown at 350 ° C with MBE), ∆R ≈

1%• 10 GHz: 5 nm In0.15Ga0.85As QW (grown with MOVPE), ∆R ≈ 1%

etalon

Motivation

-Optically Pumped

VECSELs

Mode Locking



##: Aschwanden et al., Opt. Lett., 30, 272-274 (2005)

#: Aschwanden et al., Appl. Phys. Lett., 86, 131102 (2005)

2.1 W at 4 GHz2.1 W at 4 GHzautocorrelation optical spectrum

• 4.7 ps pulse duration

• FWHM spectral width: 0.25 nm (transform limit: 2.3 ps)

• peak power 98 W

• pump power 18.9 W

0.25 nmMotivation

-Optically Pumped

VECSELs

Mode Locking



RF spectrum 1 MHz span

10 kHz RBW

1.4 W at 10 GHz1.4 W at 10 GHzautocorrelation optical spectrum

• 6.1 ps FWHM pulse duration

• optical spectrum: 0.21 nm sech2 near 960 nm

• time bandwidth product ∆ν∆t ≈ 0.42

• pump power 17 W

Motivation

-Optically Pumped

VECSELs

Mode Locking



10 kHz RBW

RF spectrum 1 MHz span

Towards higher repetition rates

• Power level in experiments at 1-10 GHz was limited by thermal damage of SESAM

• SESAM heating becomes more critical at higher repetition rates

for a given intracavity power level Pint and saturation parameter S:

int2

, ,

p

sat a sat a a rep

E PS

E F w fπ= =

( )int 1 Sheat ns

RP P R e

S−∆ = ∆ + −

( )int ,

11

2S

a rep sat a ns

RT P f F S R e

Sκ−∆ ∆ = ∆ + −

2heat

a

a

PT

wπ κ∆ =

Motivation

-Optically Pumped

VECSELs

Mode Locking



when this goes up ... ... this must go down

Low-Fsat SESAMs for high-repetition-rate mode locking

Rdivergent-beam cavity

Aa << Ag

collimated-beam cavity

Aa ≈ Ag

• output coupler R ≈ Lcav

• close to stability limit → critical alignment

• tight focus on SESAM → thermal problems

high-Fsat

SESAMlow-Fsat

SESAM

• output coupler R >> Lcav

• far inside the stability zone → frep widely tunable

• large spot on SESAM → heating uncritical

Motivation

-Optically Pumped

VECSELs

Mode Locking



Integrated-absorber VECSELD. Lorenser et al.,

Appl. Phys. B 79, 927 (2004)

Towards Wafer-Scale Integration

Motivation

-Optically Pumped

VECSELs

Mode Locking



1:1 mode locking

Low-Fsat SESAMs for high-repetition-rate mode locking

# R. Grange et al., Appl. Phys. B 80, 151 (2005)

• single absorber layer of InAs QDs embedded in GaAs. QDs low-T MBE grown at 300 °C. Resonant at 955 nm.

• Fsat = 1.7 µJ/cm2

• ΔR ≈ 3%, ΔRns ≈ 0.3%

• presumably very fast recovery time < 1ps (measured with similar absorber in pump-probe experiment)

Characterization# of first high-repetition-rate low-Fsat QD SESAM

SESAMcharacterizationconditions:

λ = 960 nmτ = 290 fs

Motivation

-Optically Pumped

VECSELs

Mode Locking



25-mW, 30-GHz Mode-locked VECSEL25-mW, 30-GHz Mode-locked VECSEL

• Cavity length: 5 mm• Output coupler:

• T = 0.35%• R = 200 mm

• Same mode areas on gain and absorber: Ag = Aa

• Pump power: 2.9 W • Pump spot radius: ≈ 90 µm• Laser mode radius ≈ 90 μm• Heat sink T: 16 °C• Etalon: 20 µm• Pulse fluence on gain and

absorber ≈ 1 μJ/cm2

→ S ≈ 0.6

successfuldemonstration of1:1 mode lockingMotivation

-Optically Pumped

VECSELs

Mode Locking



25-mW, 30-GHz Mode-locked VECSEL25-mW, 30-GHz Mode-locked VECSEL

• center wavelength: 960 nm• FWHM spectral width: 0.31 nm • 4.7 ps pulse duration• time-bandwith product

∆ν∆τ ≈ 0.47

Optical spectrum

RF spectrum

Autocorrelation

Motivation

-Optically Pumped

VECSELs

Mode Locking



50-GHz VECSELfolded cavity with Lcav = 3 mm

top-down pump under 45º to maximize space in xy-plane

Motivation

-Optically Pumped

VECSELs

Mode Locking



50-GHz VECSEL: cavity

collimated-beam cavity with weakly curved or flat OC for 1:1 mode locking

Motivation

-Optically Pumped

VECSELs

Mode Locking



Thermal Lens Measurement

Before mode locking:Maximize TEM00 power extraction in CW operation

OC with R = 200 mm

• maximize mode size on gain structure and match to pump spot size

• thermal lens has a very strong influence

thermal lens was determined by measuring output beam diameter at a distance Lmeas from OC

Motivation

-Optically Pumped

VECSELs

Mode Locking



CW measurements copper heat spreaderwp ≈ 65 μm

slope efficiency:≈ 12%

threshold:≈ 556 mW

max. TEM00

output power:≈ 115 mW

Motivation

-Optically Pumped

VECSELs

Mode Locking



output coupler R = 200 mm, Tout = 0.8%

CW measurements CVDD heat spreaderwp ≈ 65 μm

slope efficiency:≈ 14%

threshold:≈ 480 mW

max. TEM00

output power:≈ 100 mW

Motivation

-Optically Pumped

VECSELs

Mode Locking



output coupler R = 200 mm, Tout = 0.8%

50-GHz cavity with flat output coupler

use flat output coupler to maximize laser

mode size

two important conclusions from thermal lens measurements:

• fthermal < R/2 (fthermal ≈ 3-5 cm vs. f(R) = R/2 = 10 cm)→ laser mode is confined mainly by thermal lens

• gain structure on CVD diamond heatspreader shows better performance (slope and thermal lens)

Motivation

-Optically Pumped

VECSELs

Mode Locking



CW measurements flat OCgain structure on CVDD heat spreader

wp ≈ 70 μm

Tout = 1.6%

slope efficiency: ≈ 22%

extrap. threshold: ≈ 620 mW

max. TEM00 output power:≈ 370 mW

Motivation

-Optically Pumped

VECSELs

Mode Locking



ML Results: 102 mW at 50 GHz

Pump• Pump spot radius: ≈ 70 µm • Pump power 3.7 W

Cavity• Gain structure on CVD diamond heat

spreader• Flat output coupler, T = 1.6%• Theatsink 5 ºC• Etalon: 25 μm• Laser mode radius ≈ 62 μm• fthermal ≈ 3.2 ± 0.3 cm• Pulse fluence ≈ 1.1 μJ/cm2

SESAM• single absorber layer of InAs QDs embedded in GaAs. QDs low-T

MBE grown at 360 °C. Resonant at 940 nm.• ΔR ≈ 1%• Fsat on the order of ≈ 1 µJ/cm2 (not yet measured, but probably

similar to SESAM used in 30-GHz 1:1 mode locking experiment)• presumably very fast recovery time < 1ps (measured with

similar absorber in pump-probe experiment)

1:1 mode lockingAg ≈ Aa

Motivation

-Optically Pumped

VECSELs

Mode Locking



ML Results: 102 mW at 50 GHz

• 3.3 ps pulse duration• FWHM spectral width:0.36 nm • center wavelength: 958.5 nm• time-bandwith product

∆ν∆τ ≈ 0.39

Autocorrelation Optical Spectrum

RF Spectrum

Motivation

-Optically Pumped

VECSELs

Mode Locking



Conclusion and OutlookConclusion and Outlook

High-power ML VECSELs(up to 2.1 W at 4 GHz)

• High peak power (100 W) due to short pulses (few ps)• Applications: frequency doubling in RGB systems

High-repetition-rate ML VECSELs(up to 100 mW at 50 GHz)

• Much higher average output power directly from oscillator than ML edge-emitting semiconductor lasers

• Demonstration of 1:1 mode locking proves the feasibility of integrating absorber and gain in same structure

• Applications: optical clocking of integrated circuits

Future work• Integration of absorber into gain structure• Electrical pumping

Motivation

-Optically Pumped

VECSELs

Mode Locking



FIRSTSilke SchönEmilio GiniDirk Ebling Martin EbnötherOtte Homan

Physics DepartmentHansruedi ScherrerHarald HedigerJean-Pierre Stucki

University of SouthamptonAnne C. Tropper

ULP GroupUrsula KellerHeiko UnoldRüdiger PaschottaAlex AschwandenDeran MaasAude-Reine BellancourtBenjamin RudinRachel GrangeMarkus HaimlReto Hä ring

Acknowledgement

Industry Partners

Technology

PhD Talk Dirk Lorenser