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

Ultrafast laser oscillators: Ultrafast laser oscillators: perspectives from past to futures perspectives from past to futures 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 Physics Swiss Federal Institute of Technology Zürich

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) Ultrafast Laser Physics Swiss Federal Institute of Technology Zürich

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) Ultrafast Laser Physics Swiss Federal Institute of Technology Zürich

Mode locking 1 τ ≈ ∆ν ~ ~ I ( ω) I (t) I ( ω) I (t) + π + π 0 0 - π ~ φ (ω) φ ( t ) - π ~ φ (ω) φ ( t ) • axial modes in laser phase- locked • axial modes in laser not phase- locked • ultrashort pulse • noise • inverse proportional to phase- locked spectrum Ultrafast Laser Physics Swiss Federal Institute of Technology Zürich

Ultrashort pulse generation (Science 286, 1507, 1999) KLM First ML Laser Ti:Sapphire Chirped Mirror CEO control 1960 1970 1980 1990 2000 dye laser 10 ps 27 fs with ≈ 10 mW Ti:sapphire laser FWHM pulse width (sec) ≈ 5.5 fs with ≈ 200 mW 1 ps 100 fs 10 fs compressed 1 fs 1960 1970 1980 1990 2000 Year Ultrafast Laser Physics Swiss Federal Institute of Technology Zürich

Kerr Lens Modelocking (KLM) D. E. Spence, P. N. Kean, W. Sibbett, Opt. Lett. 16, 42, 1991 Nonlinear medium Aperture Incident Kerr lens beam Intense pulse Low intensity light Effective Saturable Absorber Fast Self-Amp. Modulation Loss Saturation fluence Gain Loss Pulse Time Pulse fluence on absorber Ultrafast Laser Physics Swiss Federal Institute of Technology Zürich

Passively modelocked solid-state lasers A. J. De Maria, D. A. Stetser, H. Heynau Appl. Phys. Lett. 8 , 174, 1966 Q-switching instabilities continued to be a problem until 1992 SESAM 200 ns/div First passively modelocked (diode-pumped) solid-state laser without Q-switching 50 ns/div U. Keller et al. Opt. Lett. 17 , 505, 1992 Nd:glass First passively modelocked laser KLM Q-switched modelocked Ti:Sapphire 1960 1970 1980 1990 2000 Flashlamp-pumped Diode-pumped solid-state lasers solid-state lasers (first demonstration 1963) Ultrafast Laser Physics Swiss Federal Institute of Technology Zürich

Enabling Technology: SESAM Semiconductor saturable absorber mirror (SESAM) Low-finesse D-SAM High-finesse Thin absorber A-FPSA, Saturable A-FPSA AR-coated SBR absorber and negative dispersion R ≈ 95 % R ≈ 30 % Saturable R ≈ 0 % R ≈ 30 % absorber Sat. abs. Sat. abs. Sat. abs. (Sat. abs.) R ≈ 100 % R ≈ 100 % R ≈ 100 % R ≈ 100 % April 92 Feb. 95 June/July 95 April 96 U. Keller et al., IEEE JSTQE 2, 435, 1996 Chapter 4 in Semiconductors and Semimetals, vol. 59, Academic Press, 1999 Ultrafast Laser Physics Swiss Federal Institute of Technology Zürich

Q-switched mode locking is avoided if... C. Hönninger, R. Paschotta, F. Morier-Genoud, M. Moser, and U. Keller, JOSA B 16 , 46 (1999) 2 > ∆ E E E R P sat,L sat,A 2 =   P = A sat,A ∆ F R intra   ∝ A eff,L eff,A   f σ rep em,L cw mode locking Q-switched mode locking Laser power Laser power 0 10 20 30 40 0 10 20 30 40 Time (multiples of round trip time) Time (multiples of round trip time) Ultrafast Laser Physics Swiss Federal Institute of Technology Zürich

Saturation fluence and modulation depth C. Hönninger, R. Paschotta, F. Morier-Genoud, M. Moser, and U. Keller, JOSA B 16 , 46 (1999) 2 > ∆ E E E R P sat,L sat,A SESAM sat,A ∆ A F R Semiconductor saturable absorber mirror eff,A ∝ 1 F 100 σ sat,A F sat, A A ∆ R ns Saturation fluence [ ] σ A cm 2 Non-saturable losses Reflectivity (%) Absorber − − − 19 22 ion-doped solid- 0 10 95 ∆ R state Modulation depth − 0 16 dye − 0 14 90 semiconductor 0 50 100 150 200 250 300 Incident pulse fluence F p ( µ J/cm 2 ) Ultrafast Laser Physics Swiss Federal Institute of Technology Zürich

Recovery times in semiconductors R. Paschotta, U. Keller, Applied Physics B 73 , 653, 2001 Time Delay E Interband Absorption Recombination ≈ ns E LT grown materials: D Electron trapping ≈ ps - ns D Intraband Density of states D Thermalization ≈ 100 fs τ ≤ τ τ 10 to 30 p p A Density of states D Ultrafast Laser Physics Swiss Federal Institute of Technology Zürich

KLM vs. SESAM modelocking loss loss gain gain pulse pulse time time Kerr lens modelocking (KLM) SESAM modelocking - fast/broadband saturable abs. - “not so fast” saturable absorber - critical cavity adjustment: KLM - absorber independent of cavity better at cavity stability limit design - typically not self-starting - self-starting Ultrafast Laser Physics Swiss Federal Institute of Technology Zürich

Slow saturable absorber modelocking R. Paschotta, U. Keller, Appl. Phys. B submitted absorber delays pulse loss Fully saturated absorber: leading edge of pulse negligible loss for has significant loss from trailing edge of pulse the saturable absorber time 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 Ultrafast Laser Physics Swiss Federal Institute of Technology Zürich

fs domain: soliton modelocking 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   Soliton     A ( T , t ) = A sech t T    exp i Φ 0 + small perturbations      τ   Perturbation   TR   {   { “continuum” Theory: soliton only GVD & SAM spreading Loss Continuum GDD GDD Continuum Gain Gain Pulse Pulse Frequency Time Frequency domain Time domain Dispersion spreads continuum out where it sees more loss

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 Ultrafast Laser Physics Swiss Federal Institute of Technology Zürich

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 roof prism laser output heat sink with crystal in focal plane fiber coupled diode laser parabolic mirror collimating lens • efficient cooling nearly one-dimensional • high pump intensities possible longitudinal heat flow • very weak thermal lensing 16-pass arrangement efficient pump absorption • excellent thermal properties Yb:YAG as gain material • broad emission bandwidth Ultrafast Laser Physics Swiss Federal Institute of Technology Zürich

Passively Mode-Locked Thin Disk Laser output coupler wedged Yb:YAG disk on cooling finger R=1.5 m Brewster plate R=0.5 m heat GTI sink R=1 m SE miconductor SESAM: F sat,A ≈ 100 µJ/cm 2 S aturable ∆ R ≈ 0.5% A bsorber ∆ R ns ≈ 0.3% M irror ➤ saturation parameter S := E p /( F sat,A · A eff,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 Ultrafast Laser Physics Swiss Federal Institute of Technology Zürich

Passively ML Yb:YAG thin-disk laser J. Aus der Au et al., Opt. Lett. 25 , 859, 2000 Spectral intensity (a.u.) 1.0 1.0 Autocorrelation trace τ p = 730 fs 0.8 0.8 1.55 nm 0.6 0.6 0.4 0.4 0.2 0.2 0.0 0.0 -3 -2 -1 0 1 2 3 1026 1028 1030 1032 1034 Time delay (ps) Wavelength (nm) = 16.2 W f rep = 34.6 MHz P avg τ p = 730 fs E p ≈ 0.47 µJ far away from S ≈ 7 SESAM damage P peak ≈ 560 kW ( S > 100-200) ∆ν τ p = 0.32 M 2 < 1.5 optical-to-optical efficiency: 28% Ultrafast Laser Physics Swiss Federal Institute of Technology Zürich

Power Scaling: How to Double the Output Power Thin disk laser head: SESAM: double pump power and mode double mode area on SESAM, area in gain medium keep SESAM parameters unchanged • unchanged temperature rise (1-dim. heat flow) • unchanged intensities no SESAM damage • thermal lensing not increased • Q-switching tendency not increased Ultrafast Laser Physics Swiss Federal Institute of Technology Zürich