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  • Publication
    Accès libre
    Carrier-envelope offset frequency stabilization of a gigahertz semiconductor disk laser
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    Gürel, Kutan
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    Wittwer, Valentin J
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    Waldburger, Dominik
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    Keller, Ursula
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    Optical frequency combs based on ultrafast lasers have enabled numerous scientific breakthroughs. However, their use for commercial applications is limited by the complexity and cost of femtosecond laser technology. Ultrafast semiconductor lasers might change this issue as they can be mass produced in a cost-efficient way while providing large spectral coverage from a single technology. However, it has not been proven to date if ultrafast semiconductor lasers are suitable for stabilization of their carrier-envelope offset (CEO) frequency. Here we present what we believe to be the first CEO frequency stabilization of an ultrafast semiconductor disk laser (SDL). The optically pumped SDL is passively modelocked by a semiconductor saturable absorber mirror. It operates at a repetition rate of 1.8 GHz and a center wavelength of 1034 nm. The 273 fs pulses of the oscillator are amplified to an average power level of 6 W and temporally compressed down to 120 fs. A coherent octave-spanning supercontinuum spectrum is generated in a photonic crystal fiber. The CEO frequency is detected in a standard ƒ–to–2ƒ interferometer and phase locked to an external reference by feedback applied to the current of the SDL pump diode. This proof-of-principle demonstrates that ultrafast SDLs are suitable for CEO stabilization and constitutes a key step for further developments of this comb technology expected in the coming years.
  • Publication
    Accès libre
    Highly efficient Q-switched Yb:YAG channel waveguide laser with 5.6 W of average output power
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    Wittwer, Valentin J
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    Hasse, Kore
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    Kränkel, Christian
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    Südmeyer, Thomas. Laboratoire Temps-Fréquence, Université de Neuchâtel, Switzerland
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    Calmano, Thomas
    In this Letter, we present high-power continuous wave (CW) and Q-switched femtosecond laser-written Yb:YAG channel waveguide lasers. In Q-switched operation, obtained by a semiconductor saturable absorber mirror (SESAM), as well as in CW operation, the laser generates average output powers of more than 5.6 W and reaches slope efficiencies above 74%. The Q-switched laser operated at a maximum repetition rate of 5.4 MHz with a minimum pulse duration of 11 ns, and with a maximum pulse energy of 1 μJ. This laser has almost an order of magnitude higher average output power than previously reported Q-switched channel waveguide lasers.
  • Publication
    Accès libre
    Carrier envelope offset frequency detection and stabilization of a diode-pumped mode-locked Ti:sapphire laser
    Gürel, Kutan
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    Wittwer, Valentin J
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    We demonstrate the first diode-pumped Ti:sapphire laser frequency comb. It is pumped by two green laser diodes with a total pump power of 3 W. The Ti:sapphire laser generates 250 mW of average output power in 61-fs pulses at a repetition rate of 216 MHz. We generated an octave-spanning supercontinuum spectrum in a photonic-crystal fiber and detected the carrier envelope offset (CEO) frequency in a standard ƒ-to-2ƒ interferometer setup. We stabilized the CEO-frequency through direct current modulation of one of the green pump diodes with a feedback bandwidth of 55 kHz limited by the pump diode driver used in this experiment. We achieved a reduction of the CEO phase noise power spectral density by 140 dB at 1 Hz offset frequency. An advantage of diode pumping is the ability for high-bandwidth modulation of the pump power via direct current modulation. After this experiment, we studied the modulation capabilities and noise properties of green pump laser diodes with improved driver electronics. The current-to-output-power modulation transfer function shows a bandwidth larger than 1 MHz, which should be sufficient to fully exploit the modulation bandwidth of the Ti:sapphire gain for CEO stabilization in future experiments.
  • Publication
    Accès libre
    Carrier-envelope offset stabilization of a GHz repetition rate femtosecond laser using opto-optical modulation of a SESAM
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    Wittwer, Valentin J
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    Gürel, Kutan
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    Mayer, Aline S
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    We demonstrate, to the best of our knowledge, the first carrier-envelope offset (CEO) frequency stabilization of a GHz femtosecond laser based on opto-optical modulation (OOM) of a semiconductor saturable absorber mirror (SESAM). The 1.05-GHz laser is based on a Yb:CALGO gain crystal and emits sub-100-fs pulses with 2.1-W average power at a center wavelength of 1055 nm. The SESAM plays two key roles: it starts and stabilizes the mode-locking operation and is simultaneously used as an actuator to control the CEO frequency. This second functionality is implemented by pumping the SESAM with a continuous-wave 980-nm laser diode in order to slightly modify its nonlinear reflectivity. We use the standard ƒ-to-2ƒ method for detection of the CEO frequency, which is stabilized by applying a feedback signal to the current of the SESAM pump diode. We compare the SESAM-OOM stabilization with the traditional method of gain modulation via control of the pump power of the Yb:CALGO gain crystal. While the bandwidth for gain modulation is intrinsically limited to ∼250  kHz by the laser cavity dynamics, we show that the OOM provides a feedback bandwidth above 500 kHz. Hence, we were able to obtain a residual integrated phase noise of 430 mrad for the stabilized CEO beat, which represents an improvement of more than 30% compared to gain modulation stabilization.
  • Publication
    Accès libre
    Full stabilization and characterization of an optical frequency comb from a diode-pumped solid-state laser with GHz repetition rate
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    Wittwer, Valentin J
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    Gürel, Kutan
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    Mayer, Aline S
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    Keller, Ursula
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    We demonstrate the first self-referenced full stabilization of a diode-pumped solid-state laser (DPSSL) frequency comb with a GHz repetition rate. The Yb:CALGO DPSSL delivers an average output power of up to 2.1 W with a typical pulse duration of 96 fs and a center wavelength of 1055 nm. A carrier-envelope offset (CEO) beat with a signal-to-noise ratio of 40 dB (in 10-kHz resolution bandwidth) is detected after supercontinuum generation and ƒ-to-2ƒ interferometry directly from the output of the oscillator, without any external amplification or pulse compression. The repetition rate is stabilized to a reference synthesizer with a residual integrated timing jitter of 249 fs [10 Hz – 1 MHz] and a relative frequency stability of 10−12/s. The CEO frequency is phase-locked to an external reference via pump current feedback using home-built modulation electronics. It achieves a loop bandwidth of ∼150 kHz, which results in a tight CEO lock with a residual integrated phase noise of 680 mrad [1 Hz – 1 MHz]. We present a detailed characterization of the GHz frequency comb that combines a noise analysis of the repetition rate ƒrep, of the CEO frequency ƒCEO, and of an optical comb line at 1030 nm obtained from a virtual beat with a narrow-linewidth laser at 1557 nm using a transfer oscillator. An optical comb linewidth of about 800 kHz is assessed at 1-s observation time, for which the dominant noise sources of ƒrep and ƒCEO are identified.
  • Publication
    Accès libre
    Green-diode-pumped femtosecond Ti:Sapphire laser with up to 450 mW average power
    Gürel, Kutan
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    Wittwer, Valentin J
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    Saraceno, Clara J
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    Resan, B
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    Rohrbacher, A
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    Weingarten, K
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    We investigate power-scaling of green-diode-pumped Ti:Sapphire lasers in continuous-wave (CW) and mode-locked operation. In a first configuration with a total pump power of up to 2 W incident onto the crystal, we achieved a CW power of up to 440 mW and self-starting mode-locking with up to 200 mW average power in 68-fs pulses using semiconductor saturable absorber mirror (SESAM) as saturable absorber. In a second configuration with up to 3 W of pump power incident onto the crystal, we achieved up to 650 mW in CW operation and up to 450 mW in 58-fs pulses using Kerr-lens mode-locking (KLM). The shortest pulse duration was 39 fs, which was achieved at 350 mW average power using KLM. The mode-locked laser generates a pulse train at repetition rates around 400 MHz. No complex cooling system is required: neither the SESAM nor the Ti:Sapphire crystal is actively cooled, only air cooling is applied to the pump diodes using a small fan. Because of mass production for laser displays, we expect that prices for green laser diodes will become very favorable in the near future, opening the door for low-cost Ti:Sapphire lasers. This will be highly attractive for potential mass applications such as biomedical imaging and sensing.
  • Publication
    Accès libre
    First investigation of the noise and modulation properties of the carrier-envelope offset in a modelocked semiconductor laser
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    Wittwer, Valentin J
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    Waldburger, Dominik
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    Link, Sandro M
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    Alfieri, Cesare G. E
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    Golling, Matthias
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    Morel, Jacques
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    Keller, Ursula
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    Südmeyer, Thomas. Laboratoire Temps-Fréquence, Université de Neuchâtel, Switzerland
    We present the first characterization of the noise properties and modulation response of the carrier-envelope offset (CEO) frequency in a semiconductor modelocked laser. The CEO beat of an optically-pumped vertical external-cavity surface-emitting laser (VECSEL) at 1030 nm was characterized without standard ƒ-to-2ƒ interferometry. Instead, we used an appropriate combination of signals obtained from the modelocked oscillator and an auxiliary continuous-wave laser to extract information about the CEO signal. The estimated linewidth of the free-running CEO beat is approximately 1.5 MHz at 1-s observation time, and the feedback bandwidth to enable a tight CEO phase lock to be achieved in a future stabilization loop is in the order of 300 kHz. We also characterized the amplitude and phase of the pump current to CEO-frequency transfer function, which showed a 3-dB bandwidth of ∼300 kHz for the CEO frequency modulation. This fulfills the estimated required bandwidth and indicates that the first self-referenced phase-stabilization of a modelocked semiconductor laser should be feasible in the near future.