Efficient, high-power, all-buIk spectral broadening in a quasi-waveguide

Summary form only given. The bandwidth of gain materials for high-power femtosecond lasers, mainly Yb:YAG, is fairly limited, allowing only > 100 fs long pulses to be generated efficiently in oscillators or near ps pulses from amplifiers. Therefore, additional pulse-compression, commonly employing solid or gas-filled fibres, is necessary to permit e.g. compact, MHz XUV sources or brilliant mid-infrared (MIR) sources for spectroscopy applications. The use of bulk materials offers some distinct advantages over fiber-based spectral broadening such as insensitivity to coupling or alignment as well as low cost. One of the drawbacks of bulk-based spectral broadening is the effect of self-focusing that spoils the beamprofile and compressibility. However, when the bulk material is set up for multiple passes of the pulse, emulating a waveguide, deterioration of the beam-profile can be suppressed and an effective spatial mixing of the spectrum can yield nearly perfectly compressible pulses. In our experiment the 140 fs long, 60 MW peakpower pulses from a 16 MHz, Yb:YAG KLM thin-disk oscillator were spectrally broadened and compressed to 32 fs in a very simple and compact setup. The nonlinear waveguide, consisted of spherical, curved HR-mirrors arranged as a stable waveguide with 9 passes through 6 mm fused silica windows. A chirped-mirror compressor with -4000 fs2 GDD reduced the pulse-duration to 60 fs upon which the pulse was re-imaged and returned for a second pass through the same waveguide. The spectrally broadened pulses were finally compressed and retrieved via second-harmonic FROG to yield 32 fs pulse-duration. To the best of our knowledge this is the shortest pulse-duration achieved in a high-average-power, bulk-based compressor. The power transmission through the whole setup was 95 % and the total average power of 136 W combined with the retrieved pulseshape were indicative of 230 MW peak-power. The beam had an M²_x,y quality of 1.3 and 1.1 respectively with good focusability. The spatial-spectral distribution was uniform, allowing good overall compressibility despite the relatively high nonlinear phase-shifts of≈ 0.2 and 0.4 π per window-pass. The exceptionally good throughput efficiency as well as good agreement with simulations make us confident that this type of compression can lead to high-quality pulses in the few-cycle regime, rivalling the efficiency of fibers. The average-power scalability, robustness and low cost make it a very attractive approach for building long-term stable, high-power, femtosecond laser-systems in different wavelength ranges.