Picosecond deep ultraviolet pulses generated by a 100 kHz thin-disk laser system

We report on the generation of picosecond pulses at 515 nm and deep ultraviolet pulses at 257 nm and 206 nm. They are generated as second, fourth and fifth harmonic frequencies of the high power diode-pumped Yb:YAG thin-disk laser at the fundamental wavelength of 1030 nm. The laser at the fundamental wavelength is based on a chirped-pulse amplification of oscillator pulses in two-stage regenerative amplifier based on thin-disks as active media. The diode pumping at the zero phonon line is used. The pulses are produced at ~100 kHz repetition frequency, 2-10 ps pulse duration and ≤2 mJ pulse energy. The fundamental beam is doubled in an LBO crystal at an efficiency of 42%. Subsequently the fourth harmonic frequency (257 nm) is produced by frequency doubling of the second harmonic frequency in a CLBO crystal. The unconverted part of the fundamental beam after SHG is used with the fourth harmonic beam as 1ω+4ω frequency sum in a further CLBO crystal for the fifth harmonic frequency (206 nm) generation. Two issues are important in the efficient optical conversion: nonlinear absorption given mainly by two-photon absorption (TPA) and proper timing of interacting pulses. In our simulations we take into account TPA and study the consequences of different timing of 1ω- and 4ω-pulses on the CLBO when generating the 5th harmonic. It was found that 1.4 ps delay of the 1ω-pulse after the 4ω-pulse is necessary to get the highest 5ω-output. Also the 5ω-pulse duration is affected by the timing.