Development of the synchronously pumped optical parametric oscillator

Abstract

An investigation was made into the feasibility of using the synchronously pumped optical parametric oscillator (SPOPO) as a source of subnanosecond pulses tunable across the near- and mid-infrared. Two devices were developed, based on relatively new nonlinear crystals that had not previously been used as gain media in SPOPOs. Optical parametric oscillation, synchronously pumped by the amplified and then frequency doubled output from an actively mode locked and Q-switched Nd:YAG laser, was first achieved in beta-barium borate (BBO) and then in potassium titanyl phosphate (KTP). The BBO oscillator produced broadly tunable pulses covering ~0.68 to 2.4 µm, but its large double refraction walk-off angle, and critical phase matching, precluded tight pump beam focusing in the crystal, and this led to a high threshold peak pump intensity of ~2.2 GWcm^-2 . The XTP crystal, used in the second SPOPO, allowed for non-critical phase matching, and had a higher nonlinear coefficient and smaller walk-off than BBO, and this gave a much lower threshold of ~650 MWcm^-2 (but limited tuning from ~1.04 to 1.09 µm). Both devices converted up to ~ 30% of the pump energy to generate ~ 75 ps parametric pulses.

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