Compact, efficient and widely tunable 2-μm high-repetition-rate optical parametric oscillators

Compact efficient high-repetition-rate tunable optical parametric oscillators (OPOs) in the 2-μm region were investigated, based on nonlinear crystals of KTP and periodically polarized LiNbO3 (PPLN) intracavity pumped by a diode-end-pumped acousto-optical (AO) Q-switched Nd:YVO4 laser. The cavity was designed and optimized from the theoretically analysis on thermal stability and mode matching. Using walk-off compensated twin KTP-crystal pairs as the nonlinear media, the maximum output power of 4.15 W was obtained at 25 kHz and the tuning range spanned from 1923.22 nm to 2378.63 nm, pumped at the diode power of 18.41 W at 808 nm. The optical-optical conversion efficiency of 22.5% and the photon conversion efficiency of around 60% are both among the highest in intracavity pumped OPOs. To overcome the power degradation during angle tuning, a type-II quasi-phase matched (QPM) PPLN crystal with temperature tuning was also studied. With a 55-mm-long 14.1-μm-grating period PPLN crystal, the maximum output power was 3.41 W corresponding to the optical-optical conversion efficiency of 18.5%. Both the KTP and PPLN OPOs demonstrated good efficiency as well as temporal, spectrum and beam characteristics, providing excellent 2-μm laser source for applications of lidar, medical diagnosis, nonlinear frequency conversion to the long-wave infrared and terahertz range, etc.