Doubly resonant mid-infrared PPMgLN optical parametric oscillator pumped by a 1908 nm Tm:YLF laser

A high-power mid-infrared (MIR) optical parametric oscillator (OPO) is reported, which was pumped by a diode-end-pumped, acousto-optically Q-switched Tm:YLF laser at 1908nm. Based on thermal simulations of the Tm:YLF crystal, the pump power range was rationally selected, yielding 12.88 W of 1908 nm laser output at the laser diode power of 70 W and repetition rate of 4.2 kHz, with a pulse width of 110.40 ns. Using a dual-pass doubly resonant cavity and a 50 mm-long PPMgLN crystal, the maximum MIR output power of 2.35 W was achieved at 130 °C, corresponding to the conversion efficiency of 18.25%. It is believed to be the highest average power, peak power, and single pulse energy reported to date for PPMgLN-based OPOs pumped at around 2 µm. The MIR OPO was tunable across the range of 3.58-4.10 µm. The output characteristics were systematically investigated under different output couplers, cavity geometries, and crystal lengths. The OPO exhibited good pulse stability with the MIR pulse width of 46.20 ns and a good beam profile (M_x² = 4.87, M_y² = 6.44). Additionally, analysis of parasitic nonlinear processes confirmed the presence of first-order quasi-phase-matching sum-frequency mixing, which provided a convenient approach for monitoring the MIR wavelength.