Ultrafast laser oscillators: perspectives from past to futures. New frontiers in all-solid-state lasers: High average power High pulse repetition rate

Transcription

1 Ultrafast laser oscillators: perspectives from past to futures New frontiers in all-solid-state lasers: High average power High pulse repetition rate Ursula Keller Swiss Federal Institute of Technology Ë Zürich, Switzerland

2 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)

3 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)

4 Mode locking τ 1 ν ~ I (ω) I (t) ~ I (ω) I (t) +π +π 0 0 ~ φ (ω) -π φ (t) ~ φ (ω) -π φ (t) axial modes in laser not phaselocked noise axial modes in laser phaselocked ultrashort pulse inverse proportional to phaselocked spectrum

5 Ultrashort pulse generation (Science 286, 1507, 1999) KLM First ML Laser Ti:Sapphire Chirped Mirror CEO control FWHM pulse width (sec) 10 ps 1 ps 100 fs 10 fs 1 fs dye laser 27 fs with 10 mw Ti:sapphire laser 5.5 fs with 200 mw Year compressed

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

7 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 Q-switched modelocked KLM Ti:Sapphire Flashlamp-pumped solid-state lasers Diode-pumped solid-state lasers (first demonstration 1963)

8 Enabling Technology: SESAM Semiconductor saturable absorber mirror (SESAM) High-finesse A-FPSA R 95 % Thin absorber AR-coated Low-finesse A-FPSA, SBR D-SAM Saturable absorber and negative dispersion Saturable absorber (Sat. abs.) R 0 % Sat. abs. R 30 % Sat. abs. R 30 % 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

9 Q-switched mode locking is avoided if... C. Hönninger, R. Paschotta, F. Morier-Genoud, M. Moser, and U. Keller, JOSA B 16, 46 (1999) = P intra f rep 2 cw mode locking E 2 > E E R P sat,l sat,a A eff,l σ em,l = A F R eff,a sat,a Q-switched mode locking Laser power Laser power Time (multiples of round trip time) Time (multiples of round trip time) 40

10 Saturation fluence and modulation depth C. Hönninger, R. Paschotta, F. Morier-Genoud, M. Moser, and U. Keller, JOSA B 16, 46 (1999) SESAM E 2 > E E R P sat,l sat,a Semiconductor saturable absorber mirror A F R eff,a sat,a F sat,a σ A Absorber σ [ A cm 2 ] ion-doped solidstate dye F sat, A Saturation fluence Reflectivity (%) 95 R ns Non-saturable losses R Modulation depth semiconductor Incident pulse fluence F p ( µj/cm 2 )

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

12 KLM vs. SESAM modelocking loss loss gain gain pulse pulse time Kerr lens modelocking (KLM) - fast/broadband saturable abs. - critical cavity adjustment: KLM better at cavity stability limit - typically not self-starting time SESAM modelocking - not so fast saturable absorber - absorber independent of cavity design - self-starting

13 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

14 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 Perturbation Theory: A(T,t) = Asech t τ T exp i Φ 0 T R { soliton + small perturbations { continuum only GVD & SAM Continuum Loss GDD spreading Continuum GDD Gain Gain Pulse Pulse Frequency Time Frequency domain Time domain Dispersion spreads continuum out where it sees more loss

15 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

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

17 Passively Mode-Locked Thin Disk Laser output coupler heat sink saturation parameter S := E p /(F sat,a A eff,a ) in our thin disk laser: S < 10 far below damage threshold (S > ) Brewster plate negative group delay dispersion generated with a GTI linear polarization enforced by Brewster plate GTI SEmiconductor Saturable Absorber Mirror wedged Yb:YAG disk on cooling finger R=1 m R=1.5 m R=0.5 m SESAM: F sat,a 100 µj/cm 2 R 0.5% R ns 0.3%

18 Passively ML Yb:YAG thin-disk laser J. Aus der Au et al., Opt. Lett. 25, 859, 2000 Autocorrelation trace P avg τ p Time delay (ps) = 16.2 W = 730 fs P peak 560 kw ν τ p = 0.32 τ p = 730 fs Spectral intensity (a.u.) f rep = 34.6 MHz E p 0.47 µj S 7 M 2 < nm Wavelength (nm) 1034 far away from SESAM damage (S > ) optical-to-optical efficiency: 28%

19 Power Scaling: How to Double the Output Power Thin disk laser head: SESAM: double pump power and mode area in gain medium double mode area on SESAM, 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

20 Passively ML Yb:KYW thin-disk laser F. Brunner et al., CLEO 2002, accepted Autocorrelation signal fs Time delay (ps) Spectral intensity (normalized) nm Wavelength (nm) P avg τ p f rep M = 22 W = 240 fs = 24.6 MHz P peak 3.3 MW E p 0.9 µj I peak = 2 x W/cm 2, 2 µm radius

21 New frontiers: high pulse repetition rates 10 4 High Power Nd:YVO 4 Passive ML Active ML Average Output Power [mw] Ti:sapphire VECSEL Nd:YLF Miniature Nd:YVO 4 Er:Yb:glass Cr:YAG Nd:BEL Semicon. lasers Fiber lasers Semicon. lasers Repetition Rate [GHz]

22 Quasi-Monolithic Cavity Setup 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 Crystal lengths: mm (FSR ~ GHz) Nd:YVO 4 doping: 3 % (90 µm absoption length)

23 Passively modelocked Nd:Vanadate Electron. Lett., 34, 14, (1999) 29 GHz Crystal length = 2.31 mm Appl. Phys. Lett., 77, 14, (2000) 39 GHz Crystal length = 1.76 mm Electron. Lett., submitted 77 GHz Crystal length = 0.9 mm Autocorrelation 34 ps Autocorrelation 26 ps Autocorrelation 13 ps Time, ps Optical spectrum τ P = 6.8 ps E p = 2.8 pj P out = 81 mw Wavelength, nm Optical Spectrum Time, ps τ P = 5 ps E p = 1.5 pj P out = 60 mw Optical spectrum Time, ps τ P = 2.7 ps E p = 0.8 pj P out = 65 mw Wavelength, nm Wavelength, nm

24 150 GHz Nd:Vanadate Laser L. Krainer et al., CLEO 2002 Autocorrelation trace of the 157 GHz pulse train. The pulses are about 6.4 ps apart. s.h. intensity, a.u time, ps

25 10 GHz Er:Yb:glass laser L. Krainer et al., Electron. Lett., to be published March 1, 2002 Photo detector signal (dbc) Autocorrelation signal span: 5 MHz res. bw.: 30 khz τ p = 3.8 ps Frequency (GHz) measured sech 2 fit P out at QML threshold (mw) Wavelength (nm) Pulse duration (ps) Time delay (ps)

26 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: 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, (1996)

27 Optically pumped VECSEL Simple cavity fiber coupled diode array large pump diameter curved output coupler spot size smaller on SESAM than on gain structure loss gain pulse time First demonstration of passively modelocked optically pumped VECSEL: S. Hoogland et al., IEEE Photon. Technol. Lett. 12, 1135 (2000).

28 Autocorrelation at 530 mw Autocorrelation signal (a.u.) Pulses with low chirp SESAM absorber: 8 nm In 0.15 Ga 0.85 As ( R 1.5%) Gaussian pulse shape 3.9 ps FWHM duration only 1.5 times over Fourier limit measured 3.9 ps gaussian 0.5 nm Wavelength (nm) Delay time (ps) Optical density (a.u.)

29 Microwave Frequency at 530 mw RF power density (dbc) Frequency (GHz) Frequency (GHz) Stable mode-locking Resolution 300 khz Noise free to -55 dbc Repetition rate = GHz Polarized: >100:1 nearly diffraction limited M 2 < W pump power 300 µm pump diameter 3 C heat sink temperature

30 Autocorrelation at 950 mw Autocorrelation signal (a.u.) measured 15.3 ps sech 2 1 nm Wavelength (nm) Delay time (ps) Optical density (a.u.) Higher power / longer pulse sech 2 shape, 15.3 ps FWHM duration 1 nm optical bandwidth chirp continuous wave: 2.2 W

31 Gain structure Refractive index Mirror QWs AR Position (nm)

32 Gain structure 100 Refractive index Mirror QWs AR Position (nm) Reflectivity (%) Wavelength (nm) R > 99.95% for 950 nm R 97% for 805 nm, 45 double pass pump light

33 Gain structure Reflectivity (%) Refractive index Wavelength (nm) R > 99.95% for 950 nm R 97% for 805 nm, 45 double pass pump light Mirror QWs AR Position (nm) Reflectivity (%) Wavelength (nm) 1000 R < 1% for 950 nm R 10% for 805 nm, 45

34 Gain structure Refractive index Mirror QWs AR Position (nm) Reflectivity (%) Wavelength (nm) Reflectivity (%) Wavelength (nm) 1000 R > 99.95% for 950 nm R 97% for 805 nm, 45 double pass pump light 5 InGaAs Quantum wells Spacer absorbs pump, carrier trapped in QWs R < 1% for 950 nm R 10% for 805 nm, 45

35 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

36 Thermal impedance Check of validity T 3d model Simulation T (K) T 1d model w crit constant intensity varied pump spot copper heat sink Radius (µm) Critical radius heat sink and semiconductor contribute equally

37 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

38 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 (1.3 µm and 1.5 µm, telecom application)