We have recently reported an efficient source of tunable coherent ultraviolet radiation around 385 nm which is based on a lithium triborate optical parametric oscillator pumped by a XeQ laser at 308 nm.1 The device used a noncritical phase match geometry, was operated singly resonant at the ultraviolet wavelength, and could be both angle and temperature tuned. Internal conversion efficiencies (deduced from the degree of pump depletion) of up to 35% were observed for pumping at around four times oscillation threshold. However, the external con-version efficiency (defined as the ratio of the total useful output energy at both signal (385 nm) and idler (1.54 μm) wavelengths to the energy input in the pump) under such conditions was observed to be only around 18%. We report here a study of how parasitic losses lead to this discrepancy, and describe a configuration where the external efficiency approaches the internal efficiency.

© 1991 Optical Society of America

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