Abstract

The boron-oxygen compound crystal LiB₃O₅ (LBO) has a broad transparency range, high optical damage threshold, wide acceptance angle,¹ and high conversion efficiency for third harmonic generation (THG) of a pulsed Nd:YAG laser.² It is thus a promising material for optical parametric oscillators (OPOs). We report here the operation of an OPO using LBO pumped at 355 nm. The crystal used in this experiment was grown in the Fujian Institute of Research on the Structure of Matter by the high temperature flux method. It was 6×6 mm² × 16.4 mm long and cut for type I phase matching at θ = 90°, ϕ = 45.4°. The uncoated LBO was placed between two plane mirrors 25 mm apart. The pump pulse beam with a diameter of 1.8 mm and duration of 8 ns was obtained from the third harmonic of a Q-switched Nd:YAG laser. The OPO tunable range observed in this experiment was 0.54-1.03 μm that was limited by the aperture of the LBO crystal. The tuning curves measured by a 0.5-m grating monochromator and a 0.01° precision crystal rotation table agree with the calculated tuning curves based on the Sell- meier equations reported in Ref. 1. A maximum total energy conversion efficiency of 14% was obtained at output wavelengths of 674 and 750 nm with a 20.3-mJ/pulse pump energy, and the linewidth of 674-nm output light was ~6 nm. The wide linewidth was caused by the large acceptance angle of LBO. When OPO operated at 674 and 750 nm, the effective nonlinear coefficient, the acceptance angle, and the walkoff angle of LBO were 2.4 × 10⁻⁹ esu, 4.3 mrad, and 19.5 mrad, respectively. With an output mirror of ~0.5% transmission for 673 nm and 4.7% for 750 nm, a double resonant oscillation threshold value of ~2.4 mJ/pulse with a pump beam of about four longitudinal modes, corresponding to an intensity of ~12 MW / cm², was measured at the above wavelength. This agrees with the calculated value of 5.4 MW/cm². The damage threshold of the LBO crystal measured with a lens to focus the pump beam was more than 700 MW/cm² at 355 nm, 8 ns.

© 1990 Optical Society of America