28 W, 217-fs regenerative bulk amplifier based on Yb:CAlGdO4

Yb:CaAlGdO4 (Yb:CALGO) is a very promising material for high power ultrashort pulse generation, due to its broad emission bandwidth and good thermal properties. Here we report, to the best of our knowledge, the highest power and shorter pulses ever demonstrated from a Yb:CALGO-based regenerative amplifier. The system layout consists of a Yb:CALGO oscillator seeding a Yb:CALGO regenerative amplifier followed by a folded grating compressor. The Yb:CALGO oscillator provides approximately 650 mW output power in a 63 MHz repetition rate pulse train of 92-fs long pulses. The related spectrum is 12.5 nm wide (FWHM) and centered around 1050 nm. Average output powers as high as 36 W at 500 kHz are achieved out of the regenerative amplifier while pumping with 116 W at approximately 980 nm. A small roll-over in the regenerative output power is observed at maximum pump power. This is mostly due to a drift of the pump wavelength away from the maximum crystal absorption peak with increasing pump current. After compression, we obtained 28 W in a train of 217-fs long pulses, corresponding to a pulse energy higher than 50 µJ per pulse and a peak power above 0.25 GW. The pulse spectrum is centered at 1046 nm and is approximately 11 nm wide, corresponding to a time bandwidth product of 0.69. The beam quality factor stays below M2=1.15 up to the maximum output power level, confirming the outstanding thermal performances of the Yb:CALGO material. Experiments on further power up-scaling are in progress.