Ultrafast MHz-rate burst-mode pump-probe laser for the FLASH FEL facility based on nonlinear compression of ps-level pulses from an Yb-amplifier chain

TL;DR

This paper presents a novel ultrafast laser system for the FLASH FEL facility, achieving MHz-rate burst-mode operation with high power, spectral broadening, and femtosecond pulse durations, enhancing capabilities for ultrafast experiments.

Contribution

The work introduces a compact, efficient, and low-noise laser system based on nonlinear compression of Yb-amplifier pulses, synchronized with the FEL for high-repetition-rate pump-probe experiments.

Findings

Achieved 60 fs pulse duration at 1 MHz repetition rate.

Generated up to 50 μJ pulse energy with high stability.

Demonstrated successful synchronization with FEL for user experiments.

Abstract

The Free-Electron Laser (FEL) FLASH offers the worldwide still unique capability to study ultrafast processes with high-flux, high-repetition rate XUV and soft X-ray pulses. The vast majority of experiments at FLASH are of pump-probe type. Many of them rely on optical ultrafast lasers. Here, a novel FEL facility laser is reported which combines high average power output from Yb:YAG amplifiers with spectral broadening in a Herriott-type multi-pass cell and subsequent pulse compression to sub-100 fs durations. Compared to other facility lasers employing optical parametric amplification, the new system comes with significantly improved noise figures, compactness, simplicity and power efficiency. Like FLASH, the optical laser operates with 10 Hz burst repetition rate. The bursts consist of 800 $\mu$s long trains of up to 800 ultrashort pulses being synchronized to the FEL with femtosecond precision. In the experimental chamber, pulses with up to 50 $\mu$J energy, 60 fs FWHM duration and 1 MHz rate at 1.03 $\mu$m wavelength are available and can be adjusted by computer-control. Moreover, nonlinear polarization rotation is implemented to improve laser pulse contrast. First cross-correlation measurements with the FEL at the plane-grating monochromator photon beamline are demonstrated, exhibiting the suitability of the laser for user experiments at FLASH.