Optical

Frequency

Microwave Output

Frequency comb:

Allows connection

between optical and

microwave frequencies

1 mW

10 mW

100 mW

1 W

10 W

aver

age

outp

ut p

ower

100 fs 1 ps 10 ps 100 ps

pulse duration

QW-VECSEL with SESAM QD-VECSEL with SESAM MIXSEL

Ultrafast vertical external cavity surface-emitting laser (VECSEL)!A. S. Mayer, M. Mangold, C. A. Zaugg, S. M. Link, M. Golling, B. W. Tilma, and U. Keller!!ETH Zurich, Institute for Quantum Electronics, Ultrafast Laser Physics

MIXSEL2 RTD 2012

VECSEL: potential compact frequency comb source with

gigahertz repetition rate

Phase shift by saturation !(Kramers-Kronig)!

saturation + positive GDD!➠ quasi-solitons [1]

40302010

0-10-20ph

ase

chan

ge (m

rad)

-4 -2 0 2 4time (ps)

pulse power (a.u.) gain _ = 3 sesam _ = 2 total

experimentally verified in picosecond regime [2]

[1] ! R. Paschotta, R Häring, A Garnache, S Hoogland, A.C Tropper, U Keller, Appl. Phys. B 75 (2002), 445 ! [2] ! M. Hoffmann, O.D. Sieber, D.J.H.C. Maas, V.J. Wittwer, M.Golling, T. Südmeyer and U.Keller, Opt Express 18 (2010), 10143

Ultrafast VECSELs

Motivation

distributed Bragg reflectors for lasing/pump wavelength

active region QWs or QDs

AR section

laser

pump

heat

spr

eade

r

heat

spr

eade

r

pump

output coupler

SESAM

gain

chi

p

Pulse Formation Mechanism

frequency

high freplow!frep

➠ easier to access to individual lines!

➠ higher power per mode!

➠ increased S/N ratio

Requirements!• Broad spectrum!• Fast measurement speed!• Spectral stability!• Mechanical compactness

SESAM:passive loss-modulation

100.0

99.8

99.6

99.4

99.2

99.0

refle

ctiv

ity (%

)

1 10 100 1'000 10'000pulse fluence (µJ/cm2)

ΔRns

ΔR

3.2

3.0

2.8

2.6

2.4

2.2

2.0

-3 -2 -1 0 1 2 3

gain

loss

pulse

Current Status Conclusion & Outlook

compact self-referenceable frequency comb from a

VECSEL

low noise Sub-200-fs pulses combined with high average power

Requirements!• Compact system!• Gigahertz repetition

rates

Multiphoton microscopy:!➠ high penetration depth!➠ high sensitivity!➠ flexibility

(fluorescence plus second harmonic or third harmonic image)Requirements!

• Pulsed IR-source!• Cheap (compared to currently used Ti:Sapphire systems)!

Spectroscopy Metrology Applications

Frequency Combs

Biomedical Imaging

• Optical clock!• Precise frequency

calibration

• Gas spectroscopy!• Process monitoring

and control

Requirements for self-referenceable frequency comb!• Low noise!• Sub-200-fs pulses combined with

high average power

Sub-300 femtosecond QW-VECSEL!➠ 273-fs-pulses at 605 mW of average

output power in a 1.8 GHz-cavity!!

wavelength filter

saturation noise

losses gain SESAM gain GDD

output coupler

[5] O. D. Sieber, V. J. Wittwer, M. Mangold, M. Hoffmann, M. Golling, T. Südmeyer, U. Keller, Optics! Express 19 (2011), 3538, 2011

Chip Design GoalsExperimental Results

First Watt-level femtosecond QD-VECSEL [3]!!➠ 784-fs-pulses with >1 W output power in a

5.4 GHz V-cavity

First CEO-frequency detection of a SESAM-modelocked VECSEL [4]!!➠ amplified and recompressed 240-fs

pulses from a 100-mW VECSEL[3] M. Hoffmann et. al., Optics Express (2011) vol. 19, 8108-8116!![4] C.A. Zaugg et. al., submitted to 6th EPS-QEOD Europhoton Conference

Theoretical Investigations & Simulations

• Iterative application of cavity elements on the pulse !

• Macroscopic measurable parameters as input parameters

correct prediction of output power and pulse duration!

+!design guidelines

high gain saturation fluences for short pulse

generation and high output power

high small signal gain for

high power operation

low saturation fluences of SESAM for short pulse generation!

low and slightly positive GDD for

femtosecond pulses

saturation fluence gain GDD saturation fluence

MIXSEL!!Modelocked Integrated External-Cavity Surface Emitting Laser

VECSEL Chip Structure

Cavity Design

• Distributed Bragg reflector (AlAs/GaAs)-pairs grown on GaAs for laser reflection!

• Active region: Laser light amplification in quantum wells (QW) or quantum dots (QD)!

• Antireflection-coating: Minimizing losses and tuning group delay dispersion (GDD)

• V- or Z-shape cavity!• Pumping under 45°!• SESAM: SEmiconductor Saturable Absorber

Mirror to obtain self-starting pulsed operation

1:1 mode-locking!(spot size on SESAM

= spot size on VECSEL)

VECSEL: compact & cheap pulsed laser source in the IR

spectral range

Dynamic gain saturation!Losses decrease faster than gain!➠ net gain window

VECSELSESAM

optical frequencies

RF beat note

Integration of the absorber

SESAMVECSEL

Cavity Design Goals

-100

-80

-60

-40

-20

int.

(dB

c)

1.51.00.50.0 frequency (GHz)

RBW 100 kHz

fCEO 2fCEO 1

frep=1.75 GHz

-80

-60

-40

-20

0

int.

(dB

c)

1600140012001000800600 wavelength (nm)

680

nm

1360

nm

signal background

RBW 5 nm