[PPT] - Ultrafast laser oscillators: Ultrafast laser oscillators: PowerPoint Presentation

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Swiss Federal Institute of Technology Zürich Ultrafast Laser Physics

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

Ursula Keller

Ultrafast Laser Physics Swiss Federal Institute of Technology Ë Zürich, Switzerland

Ultrafast laser oscillators: perspectives from past to futures Ultrafast laser oscillators: perspectives from past to futures

SLIDE 2

Swiss Federal Institute of Technology Zürich Ultrafast Laser Physics

Research Group of Prof. Keller

Ultrafast 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)

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Swiss Federal Institute of Technology Zürich Ultrafast Laser Physics

Current status in ultrafast lasers

Kerr-lens modelocked Ti:sapphire lasers

Pulse duration of about two optical cycles (≈

≈ ≈ ≈ 5.5 fs)

Ultrafast diode-pumped solid-state lasers

SESAM modelocking is becoming the “standard

approach”

Compact reliable lasers commercially available New Frontier: High average power

fs 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 rate

Up to 157 GHz (Nd:Vanadate miniature laser)

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Swiss Federal Institute of Technology Zürich Ultrafast Laser Physics

Mode locking

I (ω) φ (ω) I (t) +π

π

~ ~ φ (t)

axial modes in laser not phase-

locked

noise

I (ω) I (t) φ (ω) +π

π

τ ≈

1 ∆ν

φ (t) ~ ~

axial modes in laser phase-

locked

ultrashort pulse
inverse proportional to phase-

locked spectrum

SLIDE 5

Swiss Federal Institute of Technology Zürich Ultrafast Laser Physics

Ultrashort pulse generation (Science 286, 1507, 1999)

1960 1970 1980 1990 2000

First ML Laser Ti:Sapphire KLM Chirped Mirror CEO control

FWHM pulse width (sec) 2000 1990 1980 1970 1960 Year 10 fs 100 fs 1 ps 1 fs 10 ps

Ti:sapphire laser

≈5.5 fs with ≈200 mW

dye laser 27 fs with ≈10 mW compressed

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Swiss Federal Institute of Technology Zürich Ultrafast Laser Physics

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 medium Kerr lens Low intensity light Aperture Intense pulse

Loss Pulse fluence on absorber Saturation fluence

SLIDE 7

Swiss Federal Institute of Technology Zürich Ultrafast Laser Physics

Passively modelocked solid-state lasers

A. J. De Maria, D. A. Stetser, H. Heynau
Appl. Phys. Lett. 8, 174, 1966

200 ns/div 50 ns/div

1960 1970 1980 1990 2000

Nd:glass First passively modelocked laser Q-switched modelocked Ti:Sapphire KLM SESAM First passively modelocked (diode-pumped) solid-state laser without 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 instabilities continued to be a problem until 1992

SLIDE 8

Swiss Federal Institute of Technology Zürich Ultrafast Laser Physics

U. Keller et al., IEEE JSTQE 2, 435, 1996

Chapter 4 in Semiconductors and Semimetals, vol. 59, Academic Press, 1999

R ≈ 0 % Saturable absorber (Sat. abs.)

Sat. abs.

R ≈ 95 % R ≈ 30 % High-finesse A-FPSA Thin absorber AR-coated Low-finesse A-FPSA, SBR D-SAM Saturable absorber and negative dispersion

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: SESAM Semiconductor saturable absorber mirror (SESAM)

SLIDE 9

Swiss Federal Institute of Technology Zürich Ultrafast Laser Physics

∝ Aeff,L

em,L

σ = A F R

eff,A sat,A∆

=       P f

intra rep 2

E E E R

P 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

Laser power

40 30 20 10

Time (multiples of round trip time)

Q-switched mode locking

Laser power

40 30 20 10

Time (multiples of round trip time)

Q-switched mode locking is avoided if...

SLIDE 10

Swiss Federal Institute of Technology Zürich Ultrafast Laser Physics

E E E R

P sat,L sat,A 2 >

∆

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

Reflectivity (%)

300 250 200 150 100 50

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 R

eff,A sat,A∆

F

A

sat,A

∝

1

σ

Absorber

σ A cm2 [ ]

ion-doped solid- state

10

19 22 − −

−

dye

0 16

−

semiconductor

0 14

−

SLIDE 11

Swiss Federal Institute of Technology Zürich Ultrafast Laser Physics

Recovery times in semiconductors

Density of states D D E Intraband Thermalization

≈ 100 fs

Density of states D D E Interband Recombination

≈ ns

LT grown materials: Electron trapping

≈ ps - ns

Absorption Time Delay

τ τ τ

A

p p

≤ to 10 30

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

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Swiss Federal Institute of Technology Zürich Ultrafast Laser Physics

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

SLIDE 13

Swiss Federal Institute of Technology Zürich Ultrafast Laser Physics

time loss

Slow saturable absorber modelocking

R. 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

SLIDE 14

{

A(T,t) = Asech t τ

            exp i Φ0

T TR

         

+

Soliton Perturbation Theory:

Frequency domain Time domain

soliton

{

“continuum”

nly GVD & SAM

small perturbations

spreading

F. X. Kärtner, U. Keller, Optics Lett. 20, 16, 1995

Invited 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

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Swiss Federal Institute of Technology Zürich Ultrafast Laser Physics

Motivation for Mode-Locked High-Power Lasers

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

Material processing Medical applications Nonlinear frequency conversion

e.g. with high-power optical parametric oscillators:

➔ RGB laser displays ➔ mid-infrared sources ➔ tunable femtosecond sources

SLIDE 16

Swiss Federal Institute of Technology Zürich Ultrafast Laser Physics

16-pass arrangement

Thin-Disk Laser Head

S. 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 coupled diode laser collimating lens heat sink with crystal in focal plane laser output parabolic mirror roof prism

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Swiss Federal Institute of Technology Zürich Ultrafast Laser Physics

➤ 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 disk

n cooling finger

R=1.5 m

utput 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 m heat sink

SLIDE 18

Swiss Federal Institute of Technology Zürich Ultrafast Laser Physics

1.0 0.8 0.6 0.4 0.2 0.0

Autocorrelation trace

3
2
1

1 2 3

Time delay (ps) τ

p = 730 fs

1.0 0.8 0.6 0.4 0.2 0.0

Spectral intensity (a.u.)

1034 1032 1030 1028 1026

Wavelength (nm) 1.55 nm

Passively ML Yb:YAG thin-disk laser

frep = 34.6 MHz Ep ≈ 0.47 µJ S ≈ 7 M 2 < 1.5 Pavg = 16.2 W τp = 730 fs Ppeak ≈ 560 kW ∆ν τp = 0.32

ptical-to-optical efficiency: 28%

far away from SESAM damage (S > 100-200)

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

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Swiss Federal Institute of Technology Zürich Ultrafast Laser Physics

Thin disk laser head: double pump power and mode area in gain medium SESAM: double mode area on SESAM, keep SESAM parameters unchanged

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

SLIDE 20

Swiss Federal Institute of Technology Zürich Ultrafast Laser Physics

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 MHz M 2 ≈ 1.1

F. Brunner et al., CLEO 2002, accepted

1.0 0.8 0.6 0.4 0.2 0.0 Spectral intensity (normalized) 1040 1030 1020 1010 Wavelength (nm)

6.9 nm

1.0 0.8 0.6 0.4 0.2 0.0 Autocorrelation signal

0.4

0.0 0.4 Time delay (ps)

240 fs

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Swiss Federal Institute of Technology Zürich Ultrafast Laser Physics

New frontiers: high pulse repetition rates

10 10

1

10

2

10

3

10

4

Average Output Power [mW]

1 10 100 1000

Repetition Rate [GHz]

Nd:BEL Nd:YLF Cr:YAG Ti:sapphire Miniature Nd:YVO4 Fiber lasers

Semicon. lasers
Semicon. lasers

Er:Yb:glass High Power Nd:YVO

4

VECSEL

Passive ML Active ML

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Swiss Federal Institute of Technology Zürich Ultrafast Laser Physics

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

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Swiss Federal Institute of Technology Zürich Ultrafast Laser Physics

Passively modelocked Nd:Vanadate

Appl. Phys. Lett., 77, 14, (2000)

39 GHz

Crystal length = 1.76 mm

τ τ τ τP = 5 ps Ep = 1.5 pJ Pout = 60 mW τ τ τ τP = 2.7 ps Ep = 0.8 pJ Pout = 65 mW

Electron. Lett., submitted

77 GHz

Crystal length = 0.9 mm

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

29 GHz

Crystal length = 2.31 mm

τ τ τ τP = 6.8 ps Ep = 2.8 pJ Pout = 81 mW

Autocorrelation

40
20

20 40

Time, ps 34 ps

Autocorrelation

20
10

10 20

Time, ps

26 ps

Optical spectrum 1064.4 1064.0 1063.6 1063.2 Wavelength, nm

Autocorrelation

20
10

10 20

Time, ps

13 ps Optical Spectrum

1064.4 1064.0 1063.6 1063.2

Wavelength, nm Optical spectrum

1064.5 1064.0 1063.5

Wavelength, nm

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Swiss Federal Institute of Technology Zürich Ultrafast Laser Physics

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. intensity, a.u.

20

20 time, ps

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Swiss Federal Institute of Technology Zürich Ultrafast Laser Physics

10 GHz Er:Yb:glass laser

L. Krainer et al., Electron. Lett., to be published March 1, 2002

10 8 6 4 2 Pout at QML threshold (mW) 1570 1560 1550 1540 1530 Wavelength (nm) 10 8 6 4 2 Pulse duration (ps)

80
60
40
20

Photo detector signal (dBc) 10.526 10.524 10.522 Frequency (GHz)

span: 5 MHz

res. bw.: 30 kHz

0.01 0.1 1 Autocorrelation signal

10

10 Time delay (ps)

measured sech

2 fit

τ

p

= 3.8 ps

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Swiss Federal Institute of Technology Zürich Ultrafast Laser Physics

What about diode-pumped semiconductor lasers? Edge emitting lasers

Stripe width limited by beam quality requirements Facet damage limits peak power

Surface emitting device

External cavity needed (repetition rate: 1–100 GHz) Electrical pumping: ring electrode limits size Optical pumping: large area with homogeneous

inversion

Optical pumped Vertical-External-Cavity Surface-Emitting Laser (VECSEL)*

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

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Swiss Federal Institute of Technology Zürich Ultrafast Laser Physics

Optically pumped VECSEL

First demonstration of passively modelocked optically pumped VECSEL:

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

Simple cavity

fiber coupled diode array large pump diameter curved output coupler spot size smaller on SESAM

than on gain structure

time loss gain pulse

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Swiss Federal Institute of Technology Zürich Ultrafast Laser Physics

Autocorrelation at 530 mW

Pulses with low chirp

SESAM absorber: 8 nm In0.15Ga 0.85As (∆

∆ ∆ ∆R ≈ ≈ ≈ ≈ 1.5%)

Gaussian pulse shape 3.9 ps FWHM duration

nly 1.5 times over Fourier limit

1.0 0.8 0.6 0.4 0.2 0.0 Autocorrelation signal (a.u.)

10
5

5 10 Delay time (ps) measured 3.9 ps gaussian

1.0 0.5 0.0 Optical density (a.u.) 954 953 952 951 Wavelength (nm) 0.5 nm

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Swiss Federal Institute of Technology Zürich Ultrafast Laser Physics

Microwave Frequency at 530 mW

Stable mode-locking

Resolution 300 kHz Noise free to -55 dBc Repetition rate = 5.9533 GHz

Polarized: >100:1 nearly diffraction limited

M2 < 1.05

18 W pump power

300 µm pump diameter 3°C heat sink temperature

60
50
40
30
20
10

RF power density (dBc) 5.97 5.96 5.95 5.94 Frequency (GHz)

60
40
20

15 10 5 Frequency (GHz)

SLIDE 30

Swiss Federal Institute of Technology Zürich Ultrafast Laser Physics

1.0 0.8 0.6 0.4 0.2 0.0 Autocorrelation signal (a.u.)

40
20

20 40 Delay time (ps)

measured 15.3 ps sech2

1.0 0.5 0.0 Optical density (a.u.) 958 957 956 955 Wavelength (nm)

1 nm

Autocorrelation at 950 mW

Higher power / longer pulse

sech2 shape, 15.3 ps FWHM duration 1 nm optical bandwidth ⇒

⇒ ⇒ ⇒ chirp continuous wave: 2.2 W

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Swiss Federal Institute of Technology Zürich Ultrafast Laser Physics

4 3 2 1 Refractive index 6000 4000 2000 Position (nm)

Mirror AR QWs

Gain structure

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Swiss Federal Institute of Technology Zürich Ultrafast Laser Physics

4 3 2 1 Refractive index 6000 4000 2000 Position (nm)

Mirror AR QWs

Gain structure

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

100 50 Reflectivity (%) 1000 950 900 850 800 Wavelength (nm)

SLIDE 33

Swiss Federal Institute of Technology Zürich Ultrafast Laser Physics

4 3 2 1 Refractive index 6000 4000 2000 Position (nm)

Mirror AR QWs

Gain structure

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

R > 99.95% for 950 nm R ≈ 97% for 805 nm, 45° double pass pump light 100 50 Reflectivity (%) 1000 950 900 850 800 Wavelength (nm)

100 50 Reflectivity (%) 1000 950 900 850 800 Wavelength (nm)

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Swiss Federal Institute of Technology Zürich Ultrafast Laser Physics

4 3 2 1 Refractive index 6000 4000 2000 Position (nm)

Mirror AR QWs

Gain structure

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

5 InGaAs Quantum wells Spacer absorbs pump, carrier trapped in QWs

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

100 50 Reflectivity (%) 1000 950 900 850 800 Wavelength (nm) 100 50 Reflectivity (%) 1000 950 900 850 800 Wavelength (nm)

SLIDE 35

Swiss Federal Institute of Technology Zürich Ultrafast Laser Physics

Thermal impedance: Idea

Consider epitaxial lift-off structure

(substrate replaced with a heat sink)

heat source is a thin sheet

d ≈ 1 µm, Ø ≈ 500 µm

1-dimensional heat flow in vicinity of source power scalable approach

e.g. double pump spot, keep pump intensity constant ⇒ temperature is unchanged, output power doubled

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Swiss Federal Institute of Technology Zürich Ultrafast Laser Physics

Thermal impedance

Check of validity Simulation

constant intensity varied pump spot copper heat sink

Critical radius

heat sink and

semiconductor contribute equally

100 80 60 40 20 ∆T (K)

4 6 810 2 4 6 8

100

2 4 6 8

Radius (µm) wcrit ∆T1d model ∆T3d model

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Swiss Federal Institute of Technology Zürich Ultrafast Laser Physics

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

SLIDE 38

Swiss Federal Institute of Technology Zürich Ultrafast Laser Physics

Hot topics in the near future

Ultrafast diode-pumped solid-state lasers

High average power in the 100 W regime for picosecond

to sub-100-fs pulse durations

Very simple (“single-pass”) and efficient nonlinear

frequency conversion (SHG, OPG, fiber OPO, ….)

Many 10 GHz pulse repetition rates at longer wavelength