Comparison and optimization of two typical optical parametric oscillator configurations for multi-longitudinal-mode oceanic high-spectral-resolution lidar at 486.1  nm

A multi-longitudinal-mode (MLM) laser technique based on an optical parametric oscillator (OPO) provides a possibility for constructing high-spectral-resolution lidar (HSRL) at an arbitrary wavelength. In this paper, two typical OPO configurations capable of emitting 486.1 nm blue-pulsed laser for oceanic MLM-HSRL are analyzed mainly in terms of the signal laser conversion efficiency: an OPO using beta barium borate (BBO) crystal and a frequency-doubled OPO using potassium titanyl phosphate (KTP) crystal. The single-pass cavity optical length and the longitudinal mode spacing of both configurations are set to 8.9 mm and 16.8 GHz, corresponding to the frequency offset spacing of the seawater Brillouin scattering signal at 486.1 nm. Simulation results show that even under conditions favorable to BBO-based OPO, its signal laser conversion efficiency only reaches 74.8% of that of KTP-based OPO. Furthermore, the optimization of pump pulse width is critical to achieving high conversion efficiency, and the KTP-based OPO has a high tolerance for deviations in pump pulse width. When the pump pulse width deviates from the optimized value to 4 ns, the signal laser conversion efficiency of the KTP-based OPO still remains at 84.8% of that at 1 ns under a pump energy of 50 mJ. In contrast, this value is only 64.0% for the BBO-based OPO under the same condition. In conclusion, we propose that the KTP-based OPO configuration is the better choice for the development of oceanic MLM-HSRL at 486.1 nm.