10-W mid-IR optical parametric oscillators based on ZnGeP2 elements pumped by a fibre-laser-pumped Ho : YAG Laser. Experimental and numerical study

The possibility of increasing the power and efficiency of 3.5–5.0 μm optical parametric oscillators (OPOs) based on one or two ZnGeP₂ crystals pumped by a repetitively pulsed Ho : YAG laser (λ = 2097 nm), which, in turn, is pumped by a thulium fibre laser (λ = 1908 nm), is studied. The OPO based on a tandem of ZnGeP₂ elements is demonstrated to generate high-quality beams with an average power up to 10 W in pulses with durations of 15–25 ns at a repetition rate of ~17 kHz. The total conversion efficiency of the fibre laser power to the mid-IR radiation reached 25%. A theoretical OPO model explaining a cluster spectrum and higher output power in the case of the ZnGeP₂ tandem than in the case of a single element is developed.