Synchronously pumped optical parametric oscillation in barium borate and potassium titanyl phosphate
The synchronously pumped parametric oscillator is potentially a widely tunable source of picosecond optical radiation. The aim of this project was to assess whether continuous operation of such a device would be possible with the nonlinear materials and laser sources available at the time. Singly resonant operation, though it would result in a higher threshold, was felt to be a necessary prerequisite for the stable operation of the parametric oscillator required for it to become a viable research tool. To assess the feasibility of continuous operation, three synchronously pumped parametric oscillators were investigated. The first utilised the nonlinear medium barium borate and, as a pump source, a frequency-doubled, amplified, Q-switched, and mode-locked Nd:YAG laser. In this case a peak power, of the most energetic pulse in the Q-switched train, of ~3.5MW was required; a value several orders of magnitude higher than available from a c.w. pumped mode-locked Nd:YAG laser, as would be required to continuously pump a parametric oscillator. The threshold was lowered by around a factor of four by simply replacing the barium borate crystal with one of potassium titanyl phosphate, taking advantage of the higher nonlinearity offered by this material. The last stage of the investigation was carried out utilising a frequency doubled, amplified, mode-locked pump source to provide a long train of pulses. Using a tight focusing geometry, as allowed by the quasi-noncritical phase-matching offered by KTP for near degenerate operation, the intensity threshold was reduced by nearly three orders of magnitude to ~2.2kW. Extrapolation of this result to estimate what level of power would be required from a c.w. mode-locked Nd:YAG laser pump indicated powers in excess of those available from conventional sources, but achievable through the exploitation of pulse compression. A preliminary investigation of compression was therefore performed. While the necessary power levels were achieved, the unstable behaviour of the laser output power - resulting from unavoidable feedback - posed too great a risk of damage to the nonlinear crystal, and an experimental demonstration of optical parametric oscillation pumped by these short pulses was therefore not attempted. The final conclusion of this project was that parametric oscillation threshold should now be achievable with a c.w. diode-pumped Nd:YAG laser, using the short pulse output generated via the recently introduced technique of additive mode-locking.