1 kHz, 10 mJ, sub-200 fs regenerative amplifier utilizing a dual-crystal configuration of Yb:CaGdAlO₄ featuring exceptional beam quality

In order to boost the energy of the femtosecond regenerative amplifier (RA), we adopt the chirped pulse amplification technique to stretch the seed pulses through the Martinez stretcher and inject them into our specially designed dual-crystal regenerative cavity based on the Yb:CaGdAlO₄ (Yb:CGA) crystals. To avoid damaging the component coating, we meticulously regulate the size of the cavity mode on the surface of each component within the cavity to ensure that the energy density remains below the damage threshold. The final output of 11.3 mJ pulse energy was obtained at a 1 kHz repetition rate with a power stability of 0.35% over 1 hour, which is the highest energy we know of for the regenerative output of Yb:CGA crystals. Additionally, leveraging the wide gain spectrum of the Yb:CGA crystal, we achieve a spectrum full width at half maximum (FWHM) of 9 nm along with a compressed pulse duration of 198 fs. The combination of the dual-crystal setup, superior thermal properties of the Yb:CGA crystals, quasi-continuous wave pumping approach, and the thermal-insensitive design of the regenerative cavity effectively minimize the thermal impact on the crystals. The output beam exhibits near-diffraction-limited performance, with an M² value of only 1.05 × 1.06.