CEP stabilization system for PHAROS
Amplification of CEP stable pulses with a reproducible waveform under the pulse envelope is an enabling technology for a variety of strong-field control applications such as generation of isolated attosecond pulses or directional electron emission in above-threshold ionization. Femtosecond diode pumped ytterbium laser amplifiers present an interesting opportunity in this field as compared to Ti:Sapphire because of the average power and repetition rate scalability. Pursuing those applications Light Conversion has introduced active stabilization of the carrier envelope phase (CEP) to its main line products - Yb:KGW laser system PHAROS.
Light Conversion CEP stabilization setup is based on a f-2f interferometer carrier envelope phase detector and phase lock loop of carrier envelope phase signal to solid state pump diodes. It comprises two feedback loops for stabilization of oscillator and amplifier carrier envelope phases.
Fig. 1 Single side power spectral density of fceo phase noise. The right axis shows the integrated phase jitter.
The first stage of the CEP stabilization setup is built around PHAROS oscillator featuring output of sub 80fs with near transform-limited pulses and typical 1W output power at 75MHz repetition rates. Cavity dispersion of oscillator is balanced by chirped mirrors and finely tuned with an intracavity prism pair, which is also used to adjust the fceo offset frequency. Fraction of the oscillator output is fed into 5-cm-long piece of photonic crystal fiber (PCF) for spectral broadening. Afterwards broad spectra signal is directed into nonlinear f-2f interferometer (MenloSystems GmbH) for fceo detection. Using a phase lock loop, the carrier envelope offset frequency fceo is locked to a reference signal which is a quarter of the pulse repetition rate frep/4. Differential fceo and frep/4 electrical signal is supplied to oscillator solid state pump diodes completing the phase stabilization feedback loop. Notable that such arrangement is a significant advantage of an Yb DPSS Kerr-lens mode-locked oscillator over its Ti:Sapphire counterpart because of possibility to control the CEP by modulating the laser diode current with up to a MHz modulation frequency, i.e. much faster than the system memory effect determined by the rather long 0.6 ms upper state lifetime. Use of a specially designed electronic second-order active high-pass filter improves the amplitude and phase transfer function of the oscillator, extending bandwidth of the feedback loop up to 50 kHz, which greatly reduced the phase jitter. The integrated phase jitter in the bandwidth from 10 MHz to 30 Hz is 0.065 rad.
Phase stability of the amplified pulses is achieved by using additional slow-loop stabilization. A single-shot measurement of CE phase with and without slow loop is shown in Fig. 2(c-d). The r.m.s. width of the CE phase distribution (Fig. 2(e)) is 0.45 rad, corresponding to ~250 attosecond timing jitter.
Figure 2: CEP stability after amplification in RA. a) interference pattern of the f-2f spectrum of the amplified pulses measured using an out-of-loop f-2f interferometer, b) comparison of the phase calculated from the interference pattern to the set-point of the phase. A saw-tooth modulation was applied to distinguish between the intrinsic modulation in the supercontinuum spectrum and the CEP-sensitive interference fringes, c) single-shot measurement of CEP of the amplified with oscillator stabilization only, d) CEP with compensation of drift with a slow-loop, e) distribution of the
CEPs with slow-loop switched on. r.m.s width is σ=0.45 rad.
Tadas Balčiūnas, Oliver D. Mucke, Paulius Mišeikis, Giedrius Andriukaitis, Audrius Pugžlys, Linas Giniūnas, Romas Danielius, Ronald Holzwarth, and Andrius Baltuška, "Carrier envelope phase stabilization of a Yb:KGW laser amplifier," Opt. Lett. 36, 3242-3244 (2011),