high average power new frontiers in all-solid-state lasers:...

Swiss Federal Institute of Technology ZürichUltrafast Laser Physics

New frontiers in all-solid-state lasers:High average power

High pulse repetition rate

Ursula Keller

Ultrafast Laser PhysicsSwiss Federal Institute of Technology Ë

Zürich, Switzerland

Ultrafast laser oscillators:perspectives from past to futures

Ultrafast laser oscillators:perspectives from past to futures


Research Group of Prof. KellerUltrafast diode-pumped solid-state lasers (R. Paschotta)

Sub-10-femtosecond pulse generation (G. Steinmeyer)

Novel materials: III-V/fluoride MBE (S. Schön)

Attosecond Science (J. Tisch, J. Biegert)


Current status in ultrafast lasersKerr-lens modelocked Ti:sapphire lasers

Pulse duration of about two optical cycles (≈≈≈≈ 5.5 fs)

Ultrafast diode-pumped solid-state lasersSESAM modelocking is becoming the “standardapproach”Compact reliable lasers commercially availableNew Frontier: High average powerfs lasers: 22 W, 240 fs, 25 MHz, 3.3.MW peak (Yb:KYW)ps lasers: 60 W, 6 - 24 ps, 34 MHz, 1.7 µJ (Yb:YAG)New Frontier: High pulse repetition rateUp to 157 GHz (Nd:Vanadate miniature laser)


Mode locking

I (ω)

φ (ω)

0

I (t)

+π

-π

~

~φ (t)

• axial modes in laser not phase- locked

• noise

I (ω) I (t)

φ (ω)

0

+π

-π

τ ≈ 1∆ν

φ (t)~

~

• axial modes in laser phase- locked

• ultrashort pulse

• inverse proportional to phase- locked spectrum


Ultrashort pulse generation (Science 286, 1507, 1999)

1960 1970 1980 1990 2000

First ML Laser Ti:Sapphire

KLM

Chirped Mirror

CEO control

FWH

M p

ulse

wid

th (s

ec)

20001990198019701960 Year

10 fs

100 fs

1 ps

1 fs

10 ps

Ti:sapphire laser≈5.5 fs with ≈200 mW

dye laser27 fs with ≈10 mW

compressed


D. E. Spence, P. N. Kean, W. Sibbett, Opt. Lett. 16, 42, 1991

Effective Saturable Absorber Fast Self-Amp. Modulation

Pulse

Gain

Loss

Time

Kerr Lens Modelocking (KLM)

Incident beam

Nonlinear mediumKerr lens

Low intensity light

Aperture

Intense pulse

Loss

Pulse fluence on absorber

Saturation fluence


Passively modelocked solid-state lasersA. J. De Maria, D. A. Stetser, H. HeynauAppl. Phys. Lett. 8, 174, 1966

200 ns/div

50 ns/div

1960 1970 1980 1990 2000

Nd:glassFirst passively modelocked laser

Q-switched modelockedTi:Sapphire

KLM

SESAM

First passively modelocked(diode-pumped) solid-state laserwithout Q-switching

U. Keller et al. Opt. Lett. 17, 505, 1992

Flashlamp-pumped solid-state lasers

Diode-pumped solid-state lasers(first demonstration 1963)

Q-switching instabilitiescontinued to be a problem until 1992


U. Keller et al., IEEE JSTQE 2, 435, 1996Chapter 4 in Semiconductors and Semimetals, vol. 59, Academic Press, 1999

R ≈ 0 %Saturableabsorber(Sat. abs.) Sat. abs.

R ≈ 95 %

R ≈ 30 %

High-finesseA-FPSA

Thin absorberAR-coated

Low-finesseA-FPSA,SBR

D-SAMSaturableabsorber and negativedispersion

Sat. abs. Sat. abs.R ≈ 30 %

April 92 Feb. 95 June/July 95 April 96

R ≈ 100 % R ≈ 100 % R ≈ 100 % R ≈ 100 %

Enabling Technology: SESAMSemiconductor saturable absorber mirror (SESAM)


∝Aeff,L

em,Lσ

= A F Reff,A sat,A∆=

P

fintra

rep

2E E E RP sat,L sat,A

2 > ∆

C. Hönninger, R. Paschotta, F. Morier-Genoud, M. Moser, and U. Keller,JOSA B 16, 46 (1999)

cw mode locking

Lase

r pow

er

403020100Time (multiples of round trip time)

Q-switched mode locking

Lase

r pow

er

403020100

Time (multiples of round trip time)

Q-switched mode locking is avoided if...


E E E RP sat,L sat,A2 > ∆

C. Hönninger, R. Paschotta, F. Morier-Genoud, M. Moser, and U. Keller,JOSA B 16, 46 (1999)

Saturation fluence and modulation depth

100

95

90

Ref

lect

ivity

(%

)

300250200150100500

Incident pulse fluence Fp ( µJ/cm2)

∆R Modulation depth

Fsat, A Saturation fluence ∆R ns

Non-saturable losses

SESAM

Semiconductor saturable absorber mirror A F Reff,A sat,A∆F

Asat,A ∝

1σ

Absorber σ A cm2[ ]ion-doped solid-state

0 1019 22− −−

dye 0 16−

semiconductor 0 14−


Recovery times in semiconductors

Density of states D

D

E

IntrabandThermalization

≈ 100 fs

Density of states D

D

E

InterbandRecombination

≈ nsLT grown materials:

Electron trapping≈ ps - nsA

bsor

ptio

nTime Delay

τ τ τA p p≤ to 10 30

R. Paschotta, U. Keller, Applied Physics B 73, 653, 2001


KLM vs. SESAM modelocking

Kerr lens modelocking (KLM)

- fast/broadband saturable abs.

- critical cavity adjustment: KLM

better at cavity stability limit

- typically not self-starting

SESAM modelocking

- “not so fast” saturable absorber

- absorber independent of cavity

design

- self-starting

pulse

gain

loss

time time

loss

gain

pulse


time

loss

Slow saturable absorber modelockingR. Paschotta, U. Keller, Appl. Phys. B submitted

leading edge of pulse

has significant loss from

the saturable absorber

Fully saturated absorber:

negligible loss for

trailing edge of pulse

absorber delays pulse

Dominant stabilization process:

Picosecond domain: absorber delays pulse

The pulse is constantly moving backward and

can swallow any noise growing behind itself

Femtosecond domain: dispersion in soliton modelocking

{A(T , t ) = Asech tτ exp i Φ0 TTR +Soliton Perturbation Theory:Frequency domain Time domain

soliton

{

“continuum”only GVD & SAM

small perturbations

spreading

F. X. Kärtner, U. Keller, Optics Lett. 20, 16, 1995Invited Paper: F. X. Kärtner, I. D. Jung, U. Keller, IEEE JSTQE, 2, 540, 1996

fs domain: soliton modelocking

Dispersion spreads continuum out where it sees more loss

Continuum

Time

Pulse

Gain

Loss

GDD GDD

Frequency

Gain

Pulse

Continuum


Motivation for Mode-LockedHigh-Power Lasers

Multi-kW to MW peak powers, ≈ µJ pulse energiesApplications:

Material processingMedical applicationsNonlinear frequency conversione.g. with high-power optical parametric oscillators:

➔ RGB laser displays

➔ mid-infrared sources

➔ tunable femtosecond sources


16-pass arrangement

Thin-Disk Laser HeadS. Erhard, A. Giesen, M. Karszewski, T. Rupp, C. Stewen, I. Johannsen, and K. Contag,

in OSA Topical Meeting, Advanced Solid-State Lasers, 1999

efficient pump absorption

• efficient cooling• high pump intensities possible• very weak thermal lensing

• excellent thermal properties• broad emission bandwidth

nearly one-dimensional longitudinal heat flow

Yb:YAG as gain material

fiber coupleddiode laser

collimating lens

heat sink withcrystal in focal plane

laser output

parabolic mirror

roof prism


➤ saturation parameter S := Ep/(Fsat,A·Aeff,A) in our thin disk laser: S < 10 ⇒ far below damage threshold (S > 100-200) negative group delay dispersion generated with a GTI linear polarization enforced by Brewster plate

Passively Mode-Locked Thin Disk Laser

GTI

wedged Yb:YAG diskon cooling finger

R=1.5 m

output coupler

Brewster plate

R=0.5 m

SESAM: Fsat,A ≈ 100 µJ/cm2 ∆R ≈ 0.5% ∆Rns ≈ 0.3%

SEmiconductor Saturable

Absorber Mirror

R=1 mheat sink


1.0

0.8

0.6

0.4

0.2

0.0Aut

ocor

rela

tion

trac

e

-3 -2 -1 0 1 2 3

Time delay (ps)

τp = 730 fs

1.0

0.8

0.6

0.4

0.2

0.0Spe

ctra

l int

ensi

ty (

a.u.

)

10341032103010281026

Wavelength (nm)

1.55 nm

Passively ML Yb:YAG thin-disk laser

frep = 34.6 MHz

Ep ≈ 0.47 µJ

S ≈ 7M 2 < 1.5

Pavg = 16.2 W

τp = 730 fs

Ppeak ≈ 560 kW

∆ν τp = 0.32

optical-to-optical efficiency: 28%

far away fromSESAM damage(S > 100-200)

J. Aus der Au et al., Opt. Lett. 25, 859, 2000


Thin disk laser head:

double pump power and modearea in gain medium

SESAM:

double mode area on SESAM,keep SESAM parametersunchanged

Power Scaling:How to Double the Output Power

• unchanged temperature rise (1-dim. heat flow)• unchanged intensities no SESAM damage• thermal lensing not increased• Q-switching tendency not increased


Passively ML Yb:KYW thin-disk laser

Ppeak ≈ 3.3 MW

Ep ≈ 0.9 µJ

Ipeak = 2 x 1014 W/cm2 , 2 µm radius

Pavg = 22 W

τp = 240 fs

frep = 24.6 MHzM 2 ≈ 1.1

F. Brunner et al., CLEO 2002, accepted

1.0

0.8

0.6

0.4

0.2

0.0Spe

ctra

l int

ensi

ty (

norm

aliz

ed)

1040103010201010Wavelength (nm)

6.9 nm

1.0

0.8

0.6

0.4

0.2

0.0

Aut

ocor

rela

tion

sign

al

-0.4 0.0 0.4Time delay (ps)

240 fs


New frontiers: high pulse repetition rates

100

101

102

103

104

Aver

age

Out

put P

ower

[mW

]

1 10 100 1000Repetition Rate [GHz]

Nd:BEL

Nd:YLF

Cr:YAG

Ti:sapphire

Miniature Nd:YVO4

Fiber lasersSemicon. lasers

Semicon. lasers

Er:Yb:glass

High Power Nd:YVO4

VECSEL

Passive ML Active ML


Quasi-Monolithic Cavity Setup

Crystal lengths: 0.9 - 2.3 mm (FSR ~ 77 - 29 GHz)

Nd:YVO4 doping: 3 % (90 µm absoption length)

L. Krainer et al., Electron. Lett. 35, 1160, 1999 (29 GHz)APL 77, 2104, 2000 (up to 59 GHz), Electron. Lett. 36, 1846, 2000 (77 GHz)

4


Passively modelocked Nd:VanadateAppl. Phys. Lett., 77, 14, (2000)

39 GHzCrystal length = 1.76 mm

ττττP = 5 psEp = 1.5 pJ

Pout = 60 mW

ττττP = 2.7 psEp = 0.8 pJ

Pout = 65 mW

Electron. Lett., submitted


Electron. Lett., 34, 14, (1999)


ττττP = 6.8 psEp = 2.8 pJ

Pout = 81 mW

Aut

ocor

rela

tion

-40 -20 0 20 40

Time, ps

34 ps

Aut

ocor

rela

tion

-20 -10 0 10 20Time, ps

26 ps

Opt

ical

spe

ctru

m

1064.41064.01063.61063.2Wavelength, nm

Aut

ocor

rela

tion

-20 -10 0 10 20Time, ps

13 ps

Opt

ical

Spe

ctru

m

1064.41064.01063.61063.2Wavelength, nm

Opt

ical

spe

ctru

m

1064.51064.01063.5Wavelength, nm


150 GHz Nd:Vanadate Laser

Autocorrelation trace of the ≈157 GHz pulse train.The pulses are about 6.4 ps apart.

L. Krainer et al., CLEO 2002

1.0

0.5

0.0

s.h

. in

ten

sity

, a

.u.

-20 0 20

time, ps


10 GHz Er:Yb:glass laserL. Krainer et al., Electron. Lett., to be published March 1, 2002

10

8

6

4

2

0

P out a

t QM

L th

resh

old

(mW

)

15701560155015401530

Wavelength (nm)

10

8

6

4

2

0

Pulse duration (ps)

-80

-60

-40

-20

0

Phot

o de

tect

or s

igna

l (dB

c)

10.52610.52410.522Frequency (GHz)

span: 5 MHzres. bw.: 30 kHz

0.01

0.1

1

Aut

ocor

rela

tion

sign

al

-10 0 10Time delay (ps)

measured

sech2 fit

τp

= 3.8 ps


What about diode-pumped semiconductor lasers?

Edge emitting lasersStripe width limited by beam quality requirementsFacet damage limits peak power

Surface emitting deviceExternal cavity needed (repetition rate: 1–100 GHz)Electrical pumping: ring electrode limits sizeOptical pumping: large area with homogeneousinversion

Optical pumped Vertical-External-CavitySurface-Emitting Laser (VECSEL)*

* M. Kuznetsov, F. Hakimi, R. Sprague, and A. Mooradian, JSTQE 2, 435-453 (1996)


Optically pumped VECSEL

First demonstration of passively modelocked optically pumped VECSEL:

S. Hoogland et al., IEEE Photon. Technol. Lett. 12, 1135 (2000).

Simple cavityfiber coupled diode arraylarge pump diametercurved output couplerspot size smaller on SESAMthan on gain structure

time

loss

gain

pulse


Autocorrelation at 530 mW

Pulses with low chirpSESAM absorber: 8 nm In0.15Ga 0.85As (∆∆∆∆R ≈≈≈≈ 1.5%)

Gaussian pulse shape3.9 ps FWHM durationonly 1.5 times over Fourier limit

1.0

0.8

0.6

0.4

0.2

0.0Aut

ocor

rela

tion

sign

al (a

.u.)

-10 -5 0 5 10Delay time (ps)

measured3.9 ps gaussian

1.0

0.5

0.0O

ptical density (a.u.)954953952951Wavelength (nm)

0.5 nm


Microwave Frequency at 530 mW

Stable mode-lockingResolution 300 kHzNoise free to -55 dBcRepetition rate = 5.9533 GHz

Polarized: >100:1

nearly diffraction limitedM2 < 1.05

18 W pump power300 µm pump diameter3°C heat sink temperature

-60

-50

-40

-30

-20

-10

0

RF

pow

er d

ensi

ty (d

Bc)

5.975.965.955.94Frequency (GHz)

-60

-40

-20

151050Frequency (GHz)


1.0

0.8

0.6

0.4

0.2

0.0Aut

ocor

rela

tion

sign

al (a

.u.)

-40 -20 0 20 40Delay time (ps)

measured15.3 ps sech2

1.0

0.5

0.0O

ptical density (a.u.)958957956955Wavelength (nm)

1 nm

Autocorrelation at 950 mW

Higher power / longer pulsesech2 shape, 15.3 ps FWHM duration

1 nm optical bandwidth ⇒⇒⇒⇒ chirp

continuous wave: 2.2 W


4

3

2

1

0Ref

ract

ive

inde

x

6000 4000 2000 0Position (nm)

Mirror ARQWs

Gain structure


4

3

2

1

0Ref

ract

ive

inde

x

6000 4000 2000 0Position (nm)

Mirror ARQWs

Gain structure

R > 99.95% for 950 nmR ≈ 97% for 805 nm, 45°double pass pump light

100

50

0

Ref

lect

ivity

(%)

1000950900850800Wavelength (nm)


4

3

2

1

0Ref

ract

ive

inde

x

6000 4000 2000 0Position (nm)

Mirror ARQWs

Gain structure

R < 1% for 950 nmR ≈ 10% for 805 nm, 45°R > 99.95% for 950 nmR ≈ 97% for 805 nm, 45°

double pass pump light

100

50

0

Ref

lect

ivity

(%)

1000950900850800Wavelength (nm)

100

50

0

Ref

lect

ivity

(%)

1000950900850800Wavelength (nm)


4

3

2

1

0Ref

ract

ive

inde

x

6000 4000 2000 0Position (nm)

Mirror ARQWs

Gain structure

R < 1% for 950 nmR ≈ 10% for 805 nm, 45°

5 InGaAs Quantum wellsSpacer absorbs pump,carrier trapped in QWs

R > 99.95% for 950 nmR ≈ 97% for 805 nm, 45°double pass pump light

100

50

0

Ref

lect

ivity

(%)

1000950900850800Wavelength (nm)

100

50

0

Ref

lect

ivity

(%)

1000950900850800Wavelength (nm)


Thermal impedance: Idea

Consider epitaxial lift-off structure(substrate replaced with a heat sink)

heat source is a thin sheetd ≈ 1 µm, Ø ≈ 500 µm

1-dimensional heat flow in vicinity of source

power scalable approache.g. double pump spot, keep pump intensity constant

⇒ temperature is unchanged, output power doubled


Thermal impedance

Check of validity

Simulationconstant intensityvaried pump spotcopper heat sink

Critical radiusheat sink andsemiconductorcontribute equally

100

80

60

40

20

0

∆T (K

)

4 6 810

2 4 6 8100

2 4 6 8

Radius (µm)

wcrit ∆T

1d model

∆T3d

model


Success story is base on ... Transition from dye to solid-state lasers– Kerr lens modelocking– Ti:sapphire laser produces shorter pulses and more

average power

Diode-pumped solid-state lasers– development of high-power and high-brightness diode

lasers for direct pumping of solid-state lasers– efficient, compact and reliable sources

Semiconductor saturable absorbers– stable passive modelocking of diode-pumped solid-state

lasers (self-starting and no Q-switching instabilities)– many different parameter regimes such as laser

wavelength, pulse duration and power levels– engineering of linear and nonlinear optical response


Hot topics in the near future

Ultrafast diode-pumped solid-state lasers

High average power in the 100 W regime for picosecondto sub-100-fs pulse durations

Very simple (“single-pass”) and efficient nonlinearfrequency conversion (SHG, OPG, fiber OPO, ….)

Many 10 GHz pulse repetition rates at longer wavelength(1.3 µm and 1.5 µm, telecom application)

high average power new frontiers in all-solid-state lasers:...

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