6.2 kHz pulse repetition rate Ce³⁺: LiLuF₄ laser pumped by a frequency doubled copper vapour laser

A high beam quality copper vapour lasers (CVLs) master-oscillator/power-amplifier system consisting of a small scale (2 W) master oscillator and approx.20 W power amplifier (Oxford Lasers LS 20-10) has been frequency doubled for pumping of Ce:LiLuF. Using a dye amplifier green to yellow converter to boost the yellow output, uv powers at 289 nm of over 2.6 W were obtained at 6.2 kHz PRF, however due to high losses in uncoated coupling optics, and low input coupling mirror transmission, only 700 mW uv pump power was incident on the crystal. At 6.2 kHz PRF average Ce:LiLuF laser output powers (σ-polarised and at 327 nm) were up to 12 mW (based on twice the power contained in two of the four Fresnel reflected beams) corresponding to an output energy of 1.9 μJ, and 3.5% slope efficiency. Passive absorption in the unpumped length of the crystal was evident as strong fluorescence from the TEM₀₀ laser mode volume within the crystal. At low pump PRFs (reduced down to approx.30 Hz using a beam chopper), average Ce:LiLuF laser output energy increased to 4.4 μJ corresponding to 8% slope efficiency. Both thermal effects and the production of transient colour centres are thought to be responsible for the reduced output energies at 6.2 kHz PRF. However no permanent colour centres were observed for 289 nm pump energies up to the crystal damage threshold in marked contrast to those observed in the same crystal sample for KrF excimer (248 nm) pumping. Indeed, no further reduction in Ce:LiLuF laser output power was observed during continuous high PRF operation for several hours (>5 × 10⁷ pulses).