Comparison of a high-power 3.75 µm BaGa4Se7 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 BaGa4Se7 (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 ð'€2 value of the idler at 3746 nm were 0.86 W and 10.2 (ð'¥ direction) and 11.6 (ð'¦ direction), respectively. The results indicate that the beam quality factor ð'€2 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.