Efficient Ho:YAG laser pumped by a cladding-pumped tunable Tm:silica-fibre laser

Highly efficient room-temperature operation of a Ho:YAG laser end pumped by a cladding-pumped tunable Tm-doped silica-fibre laser operating at 1905 nm is reported. Using a simple two-mirror resonator configuration, we obtained 6.4 W of unpolarized output at 2097 nm in a diffraction-limited TEM00 beam (with M-2 approximate to 1.1) for 9.6 W of incident pump power, corresponding to a slope efficiency with respect to incident pump power of 80% and an optical-to-optical efficiency of 67%. Using a folded-resonator configuration incorporating a Brewster-angled glass plate, we obtained 5.8 W of linearly polarized TEM00 output, corresponding to an optical-to-optical efficiency of similar to 60%. Due to low quantum defect heating, the depolarization loss due to thermally induced birefringence in the Ho: YAG crystal was < 0.14% at the maximum available pump power, indicating that there is scope for further power scaling.