Thin-Disk Multipass Amplifier Delivering Radially Polarized Ultrafast Pulses with an Average Output Power of 1 kW

We present the amplification of ultrafast laser beams with radial and or azimuthal polarization using an Yb:YAG thin-disk multipass amplifier. The seed laser source emitted linearly polarized beams with an output power of 188 W at a repetition rate of 300 kHz and a pulse duration of 6.5 ps. Before amplification, the linear polarization was converted to radial polarization using a polarization converter composed of a segmented half-wave plate. The conversion efficiency was measured to be 85 %. Hence, the multipass amplifier was seeded with radially polarized laser pulses with an average power of 160 W. The amplifier gain medium is an Yb:YAG thin-disk mounted on a diamond heat sink. It has a thickness of 125 μm and a diameter of 20 mm. As pump cavity we used a 44-pass thin-disk pump module which allows a maximum pump spot size of 10 mm. The disk was pumped at the "zero-phonon-line" with stabilized diodes emitting at a wavelength of 969 nm. In order to extract the energy from the crystal, the seed pulses were folded 30 times over the disk using an array of 60 individual adjustable plane mirrors placed at a distance of 1.37 m in front of the disk [1]. With this approach, the seed beam diameter in the amplifier was oscillating between 3 mm and 8 mm over a total propagation length of approximately 150 m. Figure 1 (a) shows the amplifier's output power and efficiency as function of incident pump power. Up to 1002 W of average output power was obtained at a pump power of 2170 W. This corresponds to an amplification efficiency of 38.7 %. With the repetition rate of 300 kHz, the energy per pulse was 3.34 mJ. The measured autocorrelation trace exhibits a pulse duration of 7.5 ps by assuming a Gaussian temporal pulse shape. Figure 1 (b) depicts the far-field beam profile together with the qualitative polarization analysis. This is, to the best of our knowledge, the highest average output power and pulse energy reported so far for radially polarized ultrafast lasers.