Comparison of a high-power 3.75 µm BaGaSe OPO based on a plane-parallel resonator and an unstable resonator with a Gaussian reflectivity mirror.

Using a homemade Nd:YAG laser with a pulse repetition frequency of 300 Hz as a pump source, we demonstrated a tunable (BGSe) optical parametric oscillator (OPO). Wavelength-tuning ranges of 1.42-1.59 µm and 3.21-4.22 µm were realized, and a maximum average output power of 1.03 W at 3746 nm was achieved in a plane-parallel resonator. To date, this is the highest output power at around 4 µm obtained from a BGSe OPO pumped by a 1 µm laser, to our knowledge. Moreover, the BGSe OPO employing a plano-convex Gaussian reflectivity mirror (GRM) as an output coupler (OC) was demonstrated for the first time, to our knowledge. At the same pump power, the corresponding output power and value of the idler at 3746 nm were 0.86 W and 10.2 ( direction) and 11.6 ( direction), respectively. The results indicate that the beam quality factor of the idler has an over 30% improvement in comparison with the BGSe OPO based on a conventional flat OC of a plane-parallel resonator. Limited by the beam quality of the pump source, there is little room for improvement in the beam quality of the BGSe OPO with a GRM OC, which could be improved in the future by optimizing the pump beam.