Towards CEP-stabilized, high-power, few-cycle pulses from a KLM Yb:YAG disk laser

O. Pronin, 1 * M. Seidel, 2 J. Brons, 2 F. Lücking, 1 C. Grasse, 3 I. Angelov, 2 V. Pervak, 2

G. Boehm, 3 M.-C. Amann, 3 V. L. Kalashnikov, 4 Th. Udem, 2 A. Apolonski 1, 2 and F. Krausz 1, 2

1Ludwig-Maximillians-Universität München, Garching, Germany

2 Max-Planck Institut für Quantenoptik, Garching, Germany 3 Walter Schottky Institut, Garching, Germany

4Institut für Photonik, TU Wien, Vienna, Austria *Corresponding author: oleg.pronin@mpq.mpg.de

The performance of a thin-disk oscillator in both anomalous and normal dispersion regimes is described. Spectral broadening and pulse compression leading to sub-30 fs pulses as well as first carrier envelope stabilization experiments are performed.

Introduction

The Thin-disk (TD) technology allowed reaching unprecedentedly high average powers and high energies with sub-ps pulses from oscillators [1,2,3]. Such oscillators can be considered an extremely attractive alternative to Ti:Sa oscillators, the working horse of the ultrafast community. Two more features are missing to make TD oscillators a compatible substitute to Ti:Sa oscillators: few cycle pulses (<10 fs) and carrier envelope phase (CEP) stabilization. Realization of latter two properties would pave the way to applications in attosecond science and XUV frequency combs. Anomalous (ADR) and normal dispersion regimes (NDR)

Our recent results with Kerr-lens mode locking (KLM) allowed to reach the emission gain bandwidth of Yb:YAG and open the route to generation emission bandwidth limited pulses directly from the oscillator at high energy and high average power. The key element in these experiments is KLM applied to the TD cavity geometry.

Fig. 1. a): Spectrum of the oscillator in ADR and NDR. b): Autocorrelation traces in ADR and NDR (after compression) c): summary of the results obtained with different cavity parameters in ADR and NDR; OC, output coupler; K, Kerr medium thickness.

The performance of the KLM TD oscillator operating at 40 MHz repetition rate in the anomalous and normal

dispersion regime is described in detail in [3,4]. The ADR is realized with -22000 fs2 total roundtrip negative group delay dispersion (GDD) introduced by 9 bounces on highly dispersive mirrors and a 1-mm-thick fused silica plate as Kerr medium. The same oscillator configuration was mode-locked in NDR by inserting a 6.4-mm-thick fused silica substrate as Kerr medium and introducing a roundtrip normal GDD of +3500 fs2 by 4 bounces on dispersive mirrors. The results achieved in both regimes and corresponding parameters are compared in Fig. 1. The ADR shows intensity fluctuations <0.3 % r.m.s. and can easily be started in the regimes of both soft or hard-aperture KLM. In contrast the NDR makes start-up difficult and was only realized in the presence of a weak semiconductor saturable absorber and shows stronger intensity fluctuations. Thus further experiments on CEP characterization and stabilization were done with the ADR oscillator. CEP control

Ultrafast Optics 2013 Conference paper We2.1.pdf

It is important to bring to mind the difference between standard bulk oscillators and our Yb:YAG KLM TD oscillator. The difference lies in a stronger influence of the TD water cooling geometry and much higher intra-cavity pulse energies (>10 µJ) as well as in the mode-locking mechanism (SESAM or KLM).

The output of the above described oscillator operating in ADR is coupled into a 35 𝜇m core diameter photonic crystal fiber (PCF) to provide spectral broadening. It was possible to couple in the whole power available from the oscillator, namely 45 W, with a coupling efficiency about 85 %. Compression of these pulses was accomplished by 8 bounces on two mirrors with GDD=-500 fs2 and without compensation of higher-order dispersion, leading to a pulse duration <30 fs. The spectrally broadened and compressed pulses were sent to an f-to-2f interferometer. The octave spanning spectrum was generated with a PCF (SC-3.7-975, NKT photonics) with 3.7-µm core diameter by launching about 500 mW and 30 fs pulses.

The CEO frequency sensitivity due to the variation of the pump current was investigated for our oscillator and found to be ≈4 MHz/W at 200-W pump power. This sensitivity could be reduced by operating the oscillator at slightly different pump power. In order to gain higher intrinsic CEO frequency stability, the intensity noise fluctuations of the oscillator were characterized and reduced. Furthermore a home-built frequency discriminator was used to characterize the roll-off of the fceo response to a modulation of the pump diodes. The roll-off sets on at around 3 kHz modulation frequency and corresponds to a 3 dB amplitude drop which agrees well with the relaxation time of the gain medium.

Fig. 2. Beat signal measured at 10 kHz resolution bandwidth. (a): free-running CEO frequency, 10 dB/div; (b): pre-stabilized CEO frequency, 5 dB/div. SNR is ≈40 dB, RBW=10 kHz in both measurements.

The locking scheme utilizes a digital phase detector and PID controller. The error signal from the PID controller was fed to the modulation input of the pump diodes. The CEO frequency was tuned to stay close to 11 MHz and then this signal was band passed filtered, amplified and sent to one of the inputs of the phase detector. The reference signal was obtained from a stable RF generator and fed to the second input of the phase detector. Locking of the CEO frequency could be achieved clearly indicating that the frequency range of the fceo fluctuations was drastically reduced. The free-running fceo fluctuates within the range of ≈2 MHz and the fluctuations of the locked fceo signal are reduced below 100 kHz range. One can compare the free-running beat signal in Fig. 2(a) (sweep time 0.5 s) and the locked beat signal in Fig. 2(b) (sweep time 3.5 s). Further steps towards a fully stabilized CEP include the improvement of the intrinsic oscillator CEP noise and increasing the control loop bandwidth.

Conclusion KLM applied to the TD geometry leads to intrinsically high stability of the oscillator and results in low noise operation. In conjunction with spectral broadening in PCF and post-compression it was possible to detect and lock the CEO frequency via the control of the pump-diode-current.

References 1. D. Bauer et al., "Mode-locked Yb:YAG thin-disk oscillator with 41 µJ pulse energy at 145 W average infrared

power and high power frequency conversion," Opt. Express 20, 9698 (2012). 2. C. Saraceno et al., "275 W average output power from a femtosecond thin disk oscillator operated in a vacuum

environment," Opt. Express 20, 23535 (2012). 3. O. Pronin et al., "High-power 200 fs Kerr-lens mode-locked Yb:YAG thin-disk oscillator, " Opt. Lett. 36, 4746 (2011) 4. O. Pronin et al., " High-power Kerr-lens mode-locked Yb:YAG thin-disk oscillator in the positive dispersion regime, " Opt. Lett. 37, 3543 (2012)

Ultrafast Optics 2013 Conference paper We2.1.pdf