The demands for ultra-violet (UV) laser sources are ever increasing for dermatopathy and spectroscopy. However, conventional UV laser sources, such as excimer lasers are expensive, bulky in size, and difficult to maintain because of toxic gases. Moreover, wavelengths are unable to be tuned. We fabricated a non-critically phase-matched UV optical parametric oscillator (OPO) based on a $LiB_3O_5$ crystal, and analysed the output characteristics of the OPO.
The UV OPO was pumped by 266 nm, which was obtained by two consecutive second harmonic generations of a Nd:YAG laser (1064 nm, 9 ns, 10 Hz) using a KDP and a CLBO crystals as second harmonic media, respectively. The phase matching condition for CLBO crystal for 4HG is type I ( Φ = 45.03 ˚, θ = 61.42 ˚) and the corresponding nonlinear optical coefficient is 0.822 pm/V. The UV OPO consisted of two plane cavity mirrors and a LBO crystal, and the cavity was a singly resonant. The input coupler mirror had transmittances of 0% near 314 nm and 92~93 at 266 nm. The output coupler mirror had transmittances of 92~93% at 266 nm and 95% near 1743 nm, and 10% near 314 nm. The phase matching condition for LBO OPO was type II non-critical phase matching ( Φ = 0 ˚, θ = 0 ˚) and the corresponding nonlinear optical coefficient was 0.98 pm/V. The LBO crystal length was 16 mm and the total cavity length was 20.6 mm. The threshold pump intensity of the OPO was 25.34 MW/ ㎠, which agrees very well with the theory. The conversion efficiency was 3.4% at the pump intensity of 32.9 MW/ ㎠. The temporal profile of the OPO signal was delayed. The rear part of the signal was much broadened as long as ~44 ns, which is a characteristic of pulsed OPOs.
