A continuous wave 10 W cryogenic fiber amplifier at 1015 nm and frequency quadrupling to 254 nm

A stable, continuous wave, single frequency fiber amplifier system at 1015 nm with 10 W output power is presented. It is based on a large mode double clad fiber cooled to liquid nitrogen temperature. The amplified light is frequency quadrupled to 254 nm and used for spectroscopy of the 61S → 63P transition in mercury.