Compact fibre-laser-pumped Ho:YLF oscillator-amplifier system

Ho:YLF is an attractive laser material for 2 µm high energy sources since it has a much longer upper laser level lifetime (∼14 ms) and higher emission cross section than Ho:YAG. In addition, the very weak thermal lens on the σ-polarisation helps to deliver diffraction limited beams even under intense end-pumping. However, Ho:YLF has a somewhat stronger quasi-three-level nature, which implies that in order to reach transparency at the 2065 nm line, 22% of the Ho ions need to be pumped into the upper laser level (at room temperature), but it already reaches transparency at the 1940 nm pump wavelength with only 56% of the Ho ions in the upper laser level. In addition, the pump absorption cross section at 1940 nm is relatively low and strongly polarised. Therefore, the laser design requires a trade-off between efficient pump absorption and low laser threshold.