PhD Talk Dirk Lorenser

Dirk Lorenser Institute of Quantum Electronics ETH Zurich, Switzerland Picosecond VECSELs with repetition rates up to 50 GHz Ph.D defense presentation - December 5, 2005

Motivation Optically-Pumped VECSELs Design, Growth, Processing Thermal Management Mode Locking Passive mode locking of VECSELs VECSELs with repetition rates of 1 - 10 GHz High-power (up to 2 W) mode-locked VECSELs using QW SESAMs VECSELs with repetition rates up to 50 GHz Low-F sat QD SESAMs for high-repetition-rate mode locking Towards wafer-scale integration of mode-locked VECSELs 50-GHz ML VECSEL with 100 mW output power Conclusion and Outlook Outline Motivation Optically-Pumped VECSELs Mode Locking VECSELs 1 - 10 GHz VECSELs up to 50 GHz Conclusion and Outlook

Motivation Develop a pulsed laser source with: high average output powers P avg (up to several watts) good beam quality (TEM 00 ) high repetition rates (10s of GHz) short pulses (  p ≤ 1 ps) simple and compact setup # : Kuznetsov et al., IEEE Photon. Technol. Lett., 9 (8), 1063 (1997) Motivation Optically-Pumped VECSELs Mode Locking VECSELs 1 - 10 GHz VECSELs up to 50 GHz Conclusion and Outlook Optically-pumped passively mode-locked V ertical E xternal- C avity S urface E mitting Semiconductor L aser ( VECSEL # )

Optical pumping Motivation power scalability Semiconductor # : Keller et al., IEEE J. Sel. Top. Quant. Electron. , 2 (3), 435 (1996) design wavelength passive ML with SESAM # reduced QML tendency ## -> high repetition rates broad gain bandwidth -> short pulses ## : Hönninger et al., J. Opt. Soc. Am. B , 16 (1), 46 (1999) Grange et al., Appl.Phys.B, 80 , 151 (2005) Motivation Optically-Pumped VECSELs Mode Locking VECSELs 1 - 10 GHz VECSELs up to 50 GHz Conclusion and Outlook Surface Emitter External Cavity good beam quality large-area homogenous inversion

Applications Optical clocking Telecommunications Motivation Optically-Pumped VECSELs Mode Locking VECSELs 1 - 10 GHz VECSELs up to 50 GHz Conclusion and Outlook Frequency doubling IR ->visible (RGB systems)

Gain Structure Design antireflective top section active region bottom mirror (DBR) Motivation Optically-Pumped VECSELs Mode Locking VECSELs 1 - 10 GHz VECSELs up to 50 GHz Conclusion and Outlook Laser wavelength (950-970 nm): angle of incidence ≈ 0-15º bottom mirror: very highly reflecting (>99.9%) AR top section: minimize subcavity resonances Pump wavelength (808 nm): angle of incidence = 45º bottom mirror and AR top section: double-pass of pump light

Gain Structure Design: Active Region In 0.13 Ga 0.87 As quantum wells placed in the maxima of the standing wave pattern compressively strained GaAs 0.94 P 0.06 layers tensile strained compensation of compressive strain from In 0.13 Ga 0.87 As active region Motivation Optically-Pumped VECSELs Mode Locking VECSELs 1 - 10 GHz VECSELs up to 50 GHz Conclusion and Outlook

Gain Structure Design: GDD Subcavity resonances between R AR and R HR R HR > 99.9% R AR < 1% active region heat sink Motivation Optically-Pumped VECSELs Mode Locking VECSELs 1 - 10 GHz VECSELs up to 50 GHz Conclusion and Outlook Group-Delay Dispersion (GDD) GDD of gain structure is the dominating source of dispersion in the cavity of a ML VECSEL (up to several ±1000 fs 2 )

Processing # # : Häring et al., IEEE J. Quantum Electron., 38 (9), 1268 (2002) Motivation Optically-Pumped VECSELs Mode Locking VECSELs 1 - 10 GHz VECSELs up to 50 GHz Conclusion and Outlook (≈ 7 μ m thick)

Processing 2 mm 5 mm gain structure on copper heat spreader gain structure on CVD diamond heat spreader Motivation Optically-Pumped VECSELs Mode Locking VECSELs 1 - 10 GHz VECSELs up to 50 GHz Conclusion and Outlook

VECSEL Heating Temperature rise in center: Motivation Optically-Pumped VECSELs Mode Locking VECSELs 1 - 10 GHz VECSELs up to 50 GHz Conclusion and Outlook 1800 Diamond 400 Cu 45 GaAs  (WK -1 m -1 ) Material

Thermal lens: simple model Model thermal lens as a thin gradient-index lens of thickness d eff . Take the gain structure subcavity resonance as effective optical thickness: For gain structures on high-thermal-conductivity heat spreaders: Gaussian transverse temperature distribution with Δ T ≈ ΔT 1D which can be approximated with Taylor expansion to 2 nd order: Δ T = 40 K, w = 70 μ m n b = 3.54 (GaAs) dn/dT = 2·10 -4 K -1 Motivation Optically-Pumped VECSELs Mode Locking VECSELs 1 - 10 GHz VECSELs up to 50 GHz Conclusion and Outlook Δλ = 35 nm, λ = 960 nm, n = 3.54 (GaAs) -> d eff = 3.7 μ m

Thermal lens: simple model ray matrix for a GRIN duct of thickness d eff : for  d eff << 1 this is equivalent to a thin lens: ( single pass ) for double-pass and Gaussian profile: *measured: 3.2 ± 0.3 cm Motivation Optically-Pumped VECSELs Mode Locking VECSELs 1 - 10 GHz VECSELs up to 50 GHz Conclusion and Outlook 1.8* 17 f (cm) 45 30 Δ T 1D (K) Hi-Rep (50 GHz) w = 70 μ m d eff = 3.7 μ m dn/dT = 2·10 -4 Hi-Power (4 GHz) w = 175 μ m d eff = 3.7 μ m dn/dT = 2·10 -4

VECSEL Mode Locking Se miconductor S aturable A bsorber M irror ( SESAM ) U. Keller et al., IEEE JSTQE 2 , 435 (1996) Mode Locking Condition for QW SESAMs, typical mode area ratio A g /A a ≈ 10-40 Motivation Optically-Pumped VECSELs Mode Locking VECSELs 1 - 10 GHz VECSELs up to 50 GHz Conclusion and Outlook

Pulse-shaping mechanism -> Numerical simulations # of ML dynamics in VECSELs Early ML VECSELs: nearly transform-limited 3.2 ps pulses ## (213 mW) strongly chirped pulses up to 27 ps (1.9 W) Key results pulse shaping influenced by nonlinear phase change due to dynamic saturation of gain and absorption simulations yielded soliton-like pulse solutions for positive GDD # : Paschotta et al., Appl. Phys. B, 75 (4-5), 445 (2002) ## : Häring et al., Electron. Lett., 37 (12), (2001) Motivation Optically-Pumped VECSELs Mode Locking VECSELs 1 - 10 GHz VECSELs up to 50 GHz Conclusion and Outlook nearly transform-limited "quasi-soliton" pulses

Nearly transform-limited pulses at 4 GHz # and 10 GHz ## Cavity length: L cav ≈ R spot radius on gain structure: w g ≈ w pump ≈ 175 μ m spot radius on SESAM: w a ≈ 50 μ m (4 GHz) w a ≈ 30 μ m (10 GHz) output coupler: T = 2.5% R = 38 mm (4 GHz) R = 15 mm (10 GHz) cavity angle: 15° Etalon 20 μ m (4 GHz), 50 μ m (10 GHz) T = 2.5% pump SESAM heat sink gain structure SESAM 4 GHz: 8.5 nm In 0.15 Ga 0.85 As QW (LT-grown at 350 °C with MBE),  R  1% 10 GHz: 5 nm In 0.15 Ga 0.85 As QW (grown with MOVPE),  R  1% etalon Motivation Optically-Pumped VECSELs Mode Locking VECSELs 1 - 10 GHz VECSELs up to 50 GHz Conclusion and Outlook ## : Aschwanden et al., Opt. Lett., 30 , 272-274 (2005) # : Aschwanden et al., Appl. Phys. Lett., 86 , 131102 (2005)

2.1 W at 4 GHz autocorrelation optical spectrum 4.7 ps pulse duration FWHM spectral width: 0.25 nm (transform limit: 2.3 ps ) peak power 98 W pump power 18.9 W 0.25 nm Motivation Optically-Pumped VECSELs Mode Locking VECSELs 1 - 10 GHz VECSELs up to 50 GHz Conclusion and Outlook RF spectrum 1 MHz span 10 kHz RBW

1.4 W at 10 GHz autocorrelation optical spectrum 6.1 ps FWHM pulse duration optical spectrum: 0.21 nm sech 2 near 960 nm time bandwidth product    t ≈ 0.42 pump power 17 W Motivation Optically-Pumped VECSELs Mode Locking VECSELs 1 - 10 GHz VECSELs up to 50 GHz Conclusion and Outlook 10 kHz RBW RF spectrum 1 MHz span

Towards higher repetition rates Power level in experiments at 1-10 GHz was limited by thermal damage of SESAM SESAM heating becomes more critical at higher repetition rates for a given intracavity power level P int and saturation parameter S : Motivation Optically-Pumped VECSELs Mode Locking VECSELs 1 - 10 GHz VECSELs up to 50 GHz Conclusion and Outlook when this goes up ... ... this must go down

Low-F sat SESAMs for high-repetition-rate mode locking R divergent-beam cavity A a << A g collimated-beam cavity A a ≈ A g output coupler R ≈ L cav close to stability limit -> critical alignment tight focus on SESAM -> thermal problems high-F sat SESAM low-F sat SESAM output coupler R >> L cav far inside the stability zone -> f rep widely tunable large spot on SESAM -> heating uncritical Motivation Optically-Pumped VECSELs Mode Locking VECSELs 1 - 10 GHz VECSELs up to 50 GHz Conclusion and Outlook

Integrated-absorber VECSEL Towards Wafer-Scale Integration D. Lorenser et al., Appl. Phys. B 79 , 927 (2004) Motivation Optically-Pumped VECSELs Mode Locking VECSELs 1 - 10 GHz VECSELs up to 50 GHz Conclusion and Outlook 1:1 mode locking

Low-F sat SESAMs for high-repetition-rate mode locking # R. Grange et al., Appl. Phys. B 80 , 151 (2005) single absorber layer of InAs QDs embedded in GaAs. QDs low-T MBE grown at 300 °C. Resonant at 955 nm. F sat = 1.7  J/cm 2 Δ R ≈ 3% , Δ R ns ≈ 0.3% presumably very fast recovery time < 1ps (measured with similar absorber in pump-probe experiment) Characterization # of first high-repetition-rate low-F sat QD SESAM SESAM characterization conditions: λ = 960 nm  = 290 fs Motivation Optically-Pumped VECSELs Mode Locking VECSELs 1 - 10 GHz VECSELs up to 50 GHz Conclusion and Outlook

25-mW, 30-GHz Mode-locked VECSEL Cavity length: 5 mm Output coupler: T = 0.35% R = 200 mm Same mode areas on gain and absorber: A g = A a Pump power: 2.9 W Pump spot radius: ≈ 90 µm Laser mode radius ≈ 90 μ m Heat sink T: 16 °C Etalon: 20  m Pulse fluence on gain and absorber ≈ 1 μ J/cm 2 -> S ≈ 0.6 successful demonstration of 1:1 mode locking Motivation Optically-Pumped VECSELs Mode Locking VECSELs 1 - 10 GHz VECSELs up to 50 GHz Conclusion and Outlook

25-mW, 30-GHz Mode-locked VECSEL center wavelength: 960 nm FWHM spectral width: 0.31 nm 4.7 ps pulse duration time-bandwith product  ≈ 0.47 Optical spectrum RF spectrum Autocorrelation Motivation Optically-Pumped VECSELs Mode Locking VECSELs 1 - 10 GHz VECSELs up to 50 GHz Conclusion and Outlook

50-GHz VECSEL folded cavity with L cav = 3 mm top-down pump under 45 º to maximize space in xy-plane Motivation Optically-Pumped VECSELs Mode Locking VECSELs 1 - 10 GHz VECSELs up to 50 GHz Conclusion and Outlook

50-GHz VECSEL: cavity collimated-beam cavity with weakly curved or flat OC for 1:1 mode locking Motivation Optically-Pumped VECSELs Mode Locking VECSELs 1 - 10 GHz VECSELs up to 50 GHz Conclusion and Outlook

Thermal Lens Measurement Before mode locking: Maximize TEM 00 power extraction in CW operation OC with R = 200 mm maximize mode size on gain structure and match to pump spot size thermal lens has a very strong influence thermal lens was determined by measuring output beam diameter at a distance L meas from OC Motivation Optically-Pumped VECSELs Mode Locking VECSELs 1 - 10 GHz VECSELs up to 50 GHz Conclusion and Outlook

CW measurements copper heat spreader w p ≈ 65 μ m slope efficiency: ≈ 12% threshold: ≈ 556 mW max. TEM 00 output power: ≈ 115 mW Motivation Optically-Pumped VECSELs Mode Locking VECSELs 1 - 10 GHz VECSELs up to 50 GHz Conclusion and Outlook output coupler R = 200 mm , T out = 0.8%

CW measurements CVDD heat spreader w p ≈ 65 μ m slope efficiency: ≈ 14% threshold: ≈ 480 mW max. TEM 00 output power: ≈ 100 mW Motivation Optically-Pumped VECSELs Mode Locking VECSELs 1 - 10 GHz VECSELs up to 50 GHz Conclusion and Outlook output coupler R = 200 mm , T out = 0.8%

50-GHz cavity with flat output coupler use flat output coupler to maximize laser mode size two important conclusions from thermal lens measurements: f thermal < R/2 (f thermal ≈ 3-5 cm vs. f(R) = R/2 = 10 cm) -> laser mode is confined mainly by thermal lens gain structure on CVD diamond heatspreader shows better performance (slope and thermal lens) Motivation Optically-Pumped VECSELs Mode Locking VECSELs 1 - 10 GHz VECSELs up to 50 GHz Conclusion and Outlook

CW measurements flat OC gain structure on CVDD heat spreader w p ≈ 70 μ m T out = 1.6% slope efficiency: ≈ 22% extrap. threshold: ≈ 620 mW max. TEM 00 output power: ≈ 370 mW Motivation Optically-Pumped VECSELs Mode Locking VECSELs 1 - 10 GHz VECSELs up to 50 GHz Conclusion and Outlook

ML Results: 102 mW at 50 GHz Pump Pump spot radius: ≈ 70 µm Pump power 3.7 W Cavity Gain structure on CVD diamond heat spreader Flat output coupler, T = 1.6% T heatsink 5 ºC Etalon: 25 μ m Laser mode radius ≈ 62 μ m f thermal ≈ 3.2 ± 0.3 cm Pulse fluence ≈ 1.1 μ J/cm 2 SESAM single absorber layer of InAs QDs embedded in GaAs. QDs low-T MBE grown at 360 °C. Resonant at 940 nm. Δ R ≈ 1% F sat on the order of ≈ 1  J/cm 2 (not yet measured, but probably similar to SESAM used in 30-GHz 1:1 mode locking experiment) presumably very fast recovery time < 1ps (measured with similar absorber in pump-probe experiment) 1:1 mode locking A g ≈ A a Motivation Optically-Pumped VECSELs Mode Locking VECSELs 1 - 10 GHz VECSELs up to 50 GHz Conclusion and Outlook

ML Results: 102 mW at 50 GHz 3.3 ps pulse duration FWHM spectral width: 0.36 nm center wavelength: 958.5 nm time-bandwith product  ≈ 0.39 Autocorrelation Optical Spectrum RF Spectrum Motivation Optically-Pumped VECSELs Mode Locking VECSELs 1 - 10 GHz VECSELs up to 50 GHz Conclusion and Outlook

Conclusion and Outlook High-power ML VECSELs (up to 2.1 W at 4 GHz ) High peak power (100 W) due to short pulses (few ps) Applications: frequency doubling in RGB systems High-repetition-rate ML VECSELs (up to 100 mW at 50 GHz ) Much higher average output power directly from oscillator than ML edge-emitting semiconductor lasers Demonstration of 1:1 mode locking proves the feasibility of integrating absorber and gain in same structure Applications: optical clocking of integrated circuits Future work Integration of absorber into gain structure Electrical pumping Motivation Optically-Pumped VECSELs Mode Locking VECSELs 1 - 10 GHz VECSELs up to 50 GHz Conclusion and Outlook

Acknowledgement FIRST Silke Schön Emilio Gini Dirk Ebling Martin Ebnöther Otte Homan Physics Department Hansruedi Scherrer Harald Hediger Jean-Pierre Stucki University of Southampton Anne C. Tropper ULP Group Ursula Keller Heiko Unold Rüdiger Paschotta Alex Aschwanden Deran Maas Aude-Reine Bellancourt Benjamin Rudin Rachel Grange Markus Haiml Reto Häring Industry Partners

PhD Talk Dirk Lorenser

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