High average power UV sources based on SHG and SFG of copper vapour lasers

Ultraviolet sources based on non-linear second harmonic or sum frequency generation from the high average power output of the copper laser are of considerable interest as alternatives to uv excimer lasers for a variety of industrial applications. Early attempts at frequency doubling of the CVL resulted in derisory optical conversion efficiencies and uv output powers, however over the past five years dramatic increases in uv efficiencies and powers have been achieved. Peak optical conversion efficiencies as high as 10%, wall-plug efficiencies over 0.05% and uv powers up to 8W have all been reported recently. Ultimately the high gain and comparatively long cavity round-trip time for CVLs limit the extraction efficiency in low-divergence output for single CVL oscillators and high beam quality power scaling is achieved more efficiently for oscillator-amplifier arrangements. For example we can achieve some 6 Watts of near-diffraction limited output from a nominally 20W CVL with an M = 50 unstable resonator, but over 25W of near diffraction limited output from two such 20W CVLs in MOPA configuration. To date SHG and SFG powers (at 255 and 271 nm) over 1 watt have been obtained from a single CVL oscillator at optical conversion efficiencies (based on average powers) over 20%. For MOPA arrangements, stable SHG and SFG powers approaching 4W have been obtained for 18 W (λ=578nm) incident power. Our current investigations are aimed at minimising wavefront distortions in the CVL gain media. Optical components, and beam delivery paths, and thermal effects in the non-linear material.