Fig. 1. Layout of the Yb:CALGO regenerative amplifier. TFP1, TFP2, thin film polarizers; HWP, half-wave plate; FR, Faraday rotator; QWP, quarter-wave plate; PC, dual BBO Pockels cell; M1, M4, flat cavity mirrors; M2, M3, curved cavity mirrors. Closeup, schematic of the double-pass pumping system. LD, laser diode; Cyl. lenses, a pair of cylindrical lenses to expand, shape and collimate the pump beam; CM, concave mirror with a radius of curvature of 400 mm.

Fig. 2. (a) Cavity mode distribution and arrangement of the optics. (b) Beam radius variation at 1/e² level with different focal lengths of the thermal lens (red solid curve), and stability parameter as a function of the thermal lens (blue curve). The red dashed line indicates that the evolution of intracavity beam radius converges at 270 μm when the thermal-effect-induced focal length is larger than 2500 mm.

Fig. 3. Spectra (left-hand column) and auto-correlation traces (right-hand column) of the broadband seed pulses with different energy. The seed pulse is measured by a fiber spectrometer and an SHG-FROG. The red curves are the retrieved spectra of the pulses, while the grey curves are the measured spectra by the fiber spectrometer.

Fig. 4. (a) CW output power versus absorbed pump power. (b) Amplified signal power at 50 kHz versus round trips. Inset: blue line, seed pulse amplification process inside the RA cavity; red line, output single-pulse of the RA. (c) Pulse energy (red line) and output power (blue line) at different repetition rates. (d) Beam quality at 50 kHz.

Fig. 5. (a) Measured spectra of the seed (black dotted curve) and compressed pulses (red solid curve) by the fiber spectrometer FX4000, and retrieved spectrum (black solid curve) and phase (blue solid curve) by the FROG (FROG error: 0.0077). (b) FROG trace and auto-correlation trace (cyan curve). (c) Measured long-term stability of the system. The system was started in a cold state. After several minutes of warming-up phase, the system stayed at 50.7 W with a noise value of 0.46% (NRMSD).

Fig. 6. Yb-doped bulk crystal-based RA parameter statistics.