Towards the Multi-kW Ultrafast Green Thin-Disk Laser

Ultrafast laser technology is a rapidly advancing field driven by the steadily growing demand for high average power, pulse energy and short pulse duration. All high-power (>1 kW) ultrafast lasers commonly emit in the NIR spectral range. Nonlinear processes are therefore commonly used to access the visible spectral range which offers attractive opportunities for a range of industrial and scientific applications [1] . While the average power of NIR ultrafast laser systems recently crossed the level of 10 kW [2] , so far no ultrafast laser system was reported to excess 1 kW of average power in the visible spectral range [3] , [4] . Here we present a frequency-doubled thin-disk laser delivering picosecond pulses at a wavelength of 515 nm with more than 1 kW of average power and close to diffraction-limited beam quality. The laser consists of two cascaded Yb:YAG-based thin-disk multipass amplifiers boosting the average power of a TruMicro5000-series laser. The amplified output beam was centered at a wavelength of 1030 nm, had an average power of 2050 W, a pulse energy of 6.83 mJ and $M_{x,y}^2 = 1.57/1.36$ . Subsequent frequency conversion in a type-I phase-matched 5 mm long lithium-triborate crystal with an aperture of 15x15 mm 2 enabled the generation of 1.4 kW of average power at a wavelength of 515 nm, corresponding to a SH conversion efficiency of 71%, a pulse energy of 4.87 mJ and $M_{x,y}^2 = 1.38/1.43$ . Figure 1a ) depicts the generated second-harmonic power and conversion efficiency as a function of the incident fundamental power. A measurement of the second-harmonic beams' caustic at maximum output power is shown in Fig. 1b ). Figure 1c ) shows the temporal evolution of the second-harmonic power when operated at the maximum infrared power.