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We report on the status of the Allegra laser beamline currently being commissioned at the ELI-Beamlines facility. The beamline will be used to drive X-ray sources for research in material science, molecular physics, and biophysics, where a high repetition rate is essential. Designed for user operation, significant attention is given to stability, reliability, and automation of the system. A photo of the beamline is shown in Fig. 1(a).

A number of methods for the carrier-envelope phase (CEP) measurement employing a temporal or spectral domain interferometry and stabilization via feedback controlled modulation of the pump power have been developed in recent years. Locking of the carrier-envelope phase offset in mode-locked Ti:Sapph oscillators using feedback schemes has been demonstrated more than a decade ago [1,2], however the CEP-locked solid-state oscillators based on the ytterbium doped active elements appeared much later [3,4]. The results on an optimized CEP stabilization of the all-solid-state Yb:KGW laser system based on the Kerr lens mode-locking oscillator and regenerative chirped pulse amplifier (CPA) operating at repetition rates up to 1 MHz as well as different OPA setups pumped by second harmonic of this laser are presented. The oscillator has been phase-locked to sub-100 mrad values. The CEP drift of the whole CPA system was compensated down to 130 mrad (see Fig. 1 (a)) by implementing additional slow feedback loop to the oscillator pump. To our knowledge, it is the best result for ytterbium based amplifier systems. The CEP-stable laser pulses were used to pump the BBO-based OPA and produce the seed for it through continuum generation. The phase perturbations arising in this OPA were compensated to large extent by measuring the CEP at the output of the OPA and sending the feedback signal to the pump diodes of the oscillator. In this case the phase noise of 220 mrad has been measured (Fig. 1 (b)). This CEP stabilization approach is expected to be promising for the minimizing of the phase drift in CEP-stable multistage high energy OPCPA systems. The passive OPA idler pulse CEP stabilization has been performed in case of seeding the OPA with continuum generated by the laser second harmonic. The phase variation due to the changes in environment conditions was minimized by the implementation of the slow feedback loop to control the temporal delay between the pump and seed pulses. As a result, the long-term CEP-stable OPA operation with phase error as low as 70 mrad over 6 hours was demonstrated.

Here we demonstrate a direct method to stabilize the CEP of an Yb:KGW MOP A laser system, resulting in sub-100 mrad stability. To the best of our knowledge, this value surpasses most previously measured CEP jitter for Ti:sapphire CPA systems as well as any previous attempt to stabilize Yb-based oscillator and amplifier systems. This is a crucial step in widening the utility of amplifier femtosecond Yb sources, which are easily scalable in average power and pulse energy but could not be meaningfully employed in attosecond optics until now, given the difficulty of their CEP control.

In this work, the authors have introduced a method for picosecond pulse envelope shaping that is based on pulse temporal profile transformations during cascade second harmonic (SH) generation. In a SH generation process the temporal profile of the fundamental harmonic (FH) pulse undergoes significant changes that are governed by the input pulse profile and frequency conversion efficiency.