High average power 258nm generation in a nanosecond fiber MOPA system

Quasi-cw UV light sources are of interest for replacing frequency-doubled Ar-Ion lasers in several applications. Our target application in semiconductor inspection requires a narrow bandwidth cw or quasi-cw source at 258nm, which cannot be achieved by frequency converting the output of a (modelocked) Neodymium-based laser. We developed a fiber MOPA system which operates at a high repetition rate of 5MHz and generates 1ns long pulses. The system consists of a low power oscillator and four consecutive amplifier stages. which boost the average power to 40W at 1031nm. The IR output of the fiber system is frequency doubled and quadrupled using LBO and CLBO crystals for SHG and FHG, respectively. We achieved SHG conversion efficiencies of up to 82% and a UV power of up to 14W.