1048-nm dual-crystal Yb:CALGO regenerative amplifier aiming for a VUV source at 1-MHz repetition rate

A high-power 1-MHz dual-crystal Yb:CALGO femtosecond regenerative amplifier (RA) is demonstrated with an average output power of 55 W, while featuring a carefully selected central wavelength of 1048 nm, whose third harmonic wave of 349 nm is expected to enable high-efficiency 10.6-eV vacuum ultraviolet (VUV) source generation. Here, spatial chirp is intentionally introduced in the Offner stretcher and employed for shaping the seed spectrum to compensate for gain narrowing and spectral blue-shift during regenerative amplification. After compression, 51 W output with a pulse duration of 146 fs was obtained, with beam quality factors of M²_x = 1.16 and M²_y = 1.16. Furthermore, 349-nm ultraviolet radiation with an output power of 3.1 W was obtained with two pieces of cascaded beta barium borate (BBO) crystals.