High-average-power, high-beam-quality vis-UV sources based on kinetically enhanced copper vapour lasers

Investigations of the factors that limit average power scaling of elemental copper vapour lasers (CVLs) have demonstrated that decay of the electron density in the interpulse period is critical in restricting pulse repetition rate and laser aperture scaling. We have recently developed the `kinetic enhancement' (or KE) technique to overcome these limitations, whereby optimal plasma conditions are engineered using low concentrations of HCl/H <sub>2</sub> additive gases in the Ne buffer. Dissociative electron attachment of HCl and subsequent mutual neutralization of Cl <sup>-</sup> and Cu <sup>+</sup> promote rapid plasma relaxation and fast recovery of Cu densities, permitting operation at elevated Cu densities and pulse rates for given apertures. Using this approach, we have demonstrated increases in output power and efficiency of a factor of 2 or higher over conventional CVLs of the same size. For a 38 mm-bore KE-CVL, output powers up to 150 W have been achieved at 22 kHz, corresponding to record specific powers (80 mW/cm <sup>3</sup>) for such a `small/medium-scale' device. In addition, kinetic enhancement significantly extends the gain duration and restores gain on-axis, even for high pulse rates, thereby promoting substantial increases (5-10×) in high-beam-quality power levels when operating with unstable resonators. This has enabled us to achieve much higher powers in second-harmonic generation from the visible copper laser output to the ultraviolet (eg 5 W at 255 nm from a small-scale KE-CVL). Our approach to developing KE-CVLs including computer modelling and experimental studies will be reviewed, and most recent results in pulse rate scaling and scaling of high-beam-quality power using oscillator-amplifier configurations, will be presented.