Sub-nanosecond blue laser at 443 nm using a multi-longitudinal-mode frequency-doubled optical parametric oscillator for oceanic high-spectral-resolution lidar.

We have demonstrated a sub-nanosecond frequency-doubled potassium titanyl phosphate optical parametric oscillator with a large laser longitudinal mode spacing at 443 nm. Pumped by a 532.2 nm pulsed laser with a pulse width of 0.74 ns and a maximum pulse energy of 4.08 mJ, the maximum output energy of a 576 µJ signal laser pulse at 886 nm was successfully obtained with a short cavity optical length of 8.1 mm; the corresponding conversion efficiency was up to 14.1%. The signal laser was frequency-doubled to a deep blue laser at 443 nm with a longitudinal mode spacing of around 19 GHz, corresponding to the water molecule Brillouin scattering frequency spacing excited by a 443 nm laser. The spectral linewidth, pulse width, and single pulse energy of the blue laser were 0.3 nm, 0.5 ns, and 70 µJ, respectively. This is the first demonstration, to the best of our knowledge, of a sub-nanosecond laser at 443 nm with such a large longitudinal mode spacing. It provides a reliable laser source for verifying the feasibility of the multi-longitudinal-mode laser technique in oceanic high-spectral-resolution lidar.