Studies of optical parametric oscillators for the ultraviolet and visible spectral regions

Abstract

The work described herein concerns the characterisation and development of optical parametric oscillators (OPOs) tunable in the ultraviolet, visible and near infrared regions. These devices were pumped by the 308nm output from line-narrowed Xenon Chloride excimer lasers of pulse energy up to 150mJ. The behaviour of Type 2 phase-matched Urea, and Type 1 phase-matched Barium Borate OPOs in terms of oscillation threshold and conversion efficiency, has been explored. The detrimental effects of pump beam walkoff on the threshold of the critically phase-matched Barium Borate OPO have been quantified. It was found that minimum 17ns pulse energies of 5mJ were required to reach threshold in a device based on a crystal of 20mm length. By contrast, noncritically phase-matched Urea OPOs using crystal lengths of 25mm were operated with as little as 0.6mJ pump energy. A deterioration in performance was observed in both cases with decreasing pump beam waist. Maximum pump depletions of 72% and 64% were observed in Urea and BBO respectively. The useful output from the urea device reached 65%, while higher absorption/scattering losses meant that the useful fraction in BBO was very much lower. Two different types of noncollinear phase-matching were studied in the BBO-OPO. The first recorded observation of operation of a Type 1 OPO at crystal angles beyond the degenerate wavelength point was made. The output took the form of two concentric rings and was attributed to simultaneous singly and doubly resonant operation. Finally, single longitudinal mode operation of the BBO-OPO was demonstrated using a dispersive cavity arrangement. The widely varying inherent linewidth of the device required that different strategies be adopted over different wavelength ranges. Encouraging performance in terms of threshold was observed using the dispersive cavity, and the feasibility of using this device as a low-power first stage for an oscillator/amplifier set-up was studied.