Investigations of a high power all-solid-state synchronously-pumped lithium triborate optical parametric oscillator
The work presented in this thesis describes the operation of a high power all-solid-state synchronously pumped optical parametric oscillator based on a Brewster-angled lithium triborate crystal. The OPO is pumped by a resonant frequency doubled, amplified, diode-pumped mode-locked laser. Performance characteristics of the individual "modules" in the overall system are presented. The work describes the production of 2.0 psec pulses from a diode-pumped Nd:YLF laser using the passive mode-locking technique of additive-pulse-modelocking (APM). This method was the most convenient available at the time of this research, and provides the shortest pulses from the Nd:YLF laser and hence, the greatest peak power. Average power levels of 540mW were obtained in pulses having a peak power of 2.5kW. The pulses were subsequently amplified in a simple end-pumped Nd:YLF amplifier to average power levels of ~1W with peak powers of ~4kW. At these peak power levels, efficient single-pass harmonic generation was still not possible with the non-linear crystals available at the time, so the technique of resonant enhancement was employed. This produced 660mW of green light in an optimised resonator with a peak power of ~3.1kW at a conversion efficiency approaching 80%. These three individual stages when taken together, constitute the pump source for the LBO optical parametric oscillator. Continuous synchronous pumping was achieved using a temperature-tuned noncritically phase-matched lithium triborate crystal as the nonlinear gain medium. An extensive tuning range from 0.65 to 2.7µm was obtained with average output powers for the signal (idler) as high as 200mW (110mW). With the high intra-cavity powers of the resonated signal wave we observed significant chirping on the output pulses via self-phase modulation. By providing dispersion compensation via intra-cavity prisms the chirp was removed and produced transform-limited pulses of duration 1.6psec with peak powers of ~800W. As a result of this work it has been possible to provide a complete design strategy for achieving reliable operation of an all-solid-state picosecond source with broad tunability in the near infrared.