Thin-disk multi-pass amplifier delivering azimuthally polarized ultra-short pulses with an average power of 1.74 kW

We present the amplification of ultra-short laser pulses with radial or azimuthal polarization using an Yb:YAG thin-disk multi-pass amplifier. The seed was a TruMicro laser, which emitted linearly polarized pulses with an average power of 105 W at a repetition rate of 300 kHz and a pulse duration of 6.5 ps. These pulses were directly amplified in our home-built front-end amplifier up to an average power of 425 W [1] . Before the further amplification in our thin-disk multi-pass booster, the linear polarization state was converted to an azimuthal polarization state using a polarization converter composed of segmented half-wave plates. The power conversion efficiency was measured to be 79 %. Hence, the multi-pass booster was seeded with azimuthally polarized pulses with an average power of 337 W. The quality of the doughnut-shaped beam was measured to be M 2 < 2.5. We used an Yb:YAG thin-disk mounted on a diamond heat sink as the gain medium of the multi-pass booster. The disk is 125 μm thick and has a diameter of 20 mm. A 44-pass thin-disk pump module, which allows a maximum pump spot size of 10 mm, was used for pumping. The disk was pumped at the "zero-phonon-line" with spectrally stabilized diodes emitting at a wavelength of 969 nm. In order to extract the energy from the crystal, the beam to be amplified folded 30 times over the disk using an array of 60 individually adjustable plane mirrors placed at a distance of 1.37 m in front of the disk [2] . With this approach, the diameter of the amplified beam was oscillating between 3.8 mm and 5.7 mm on its way over a total propagation length of approximately 150 m through the amplifier. Figure 1 (a) shows the amplifier's output power and efficiency as a function of incident pump power. Up to 1741 W of average power was obtained at a pump power of 3048 W. This corresponds to an amplification efficiency of 46 %. With the repetition rate of 300 kHz, the energy per pulse was 5.8 mJ. The measured autocorrelation trace exhibits a pulse duration of 7.8 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 azimuthally polarized ultrafast lasers.