Advanced high-power optical parametric oscillators synchronously pumped by ultrafast fibre-based sources

摘要

This thesis is concerned with investigating the generation of ultra short, tunable pulses at high average power and / or high pulse energy using synchronously pumped optical parametric oscillators (OPO) and appropriate power-scalable fibre-amplifier pump sources. Two types of pump sources with average powers up to ~100W are considered: (1) a picosecond, all-fiberised, high-power, variable-repetition-rate, Yb:fibre-amplified, gain-switched laser diode system and (2) a femtosecond, high-power, chirped-pulse amplification Yb:fibre laser system incorporating a pulse shaper module. Such OPO systems find applications in fields as diverse as materials processing and nonlinear microscopy / spectroscopy. Two OPOs based on periodically poled lithium niobate (PPLN) and pumped at 1060nm from the first pump source are demonstrated. With 20ps-long pump pulses, average powers of up to 7.3W (3.1W) for the signal (idler) are generated at variable repetition rates of ~0.1 . . . 1GHz. With longer 100ps pump pulses at a repetition rate of 8MHz and an intracavity fibre as an OPO feedback component for compactness, combined pulse energies approaching 1µJ are obtained, which are the highest reported to date. Tuning ranges from 1.4µm to 1.7µm (signal) and from 2.8µm to 4.4µm (idler) are typical for both OPOs. Including frequency-doubling of the 1060 nm light, the first pump source, operating at 230MHz and 20ps, is used for two green-pumped OPOs tunable in the near-infrared range from 650 nm to 1040nm (plus the idler from 1.08µm to 2.9µm). The first OPO uses MgO-doped PPLN and up to 270mW of combined output power with 1W of pump power is obtained. Operation at higher power is complicated due to damage effects such as photo-refraction, nonlinear or induced absorption in lithium niobate. The second OPO uses lithium triborate (LBO) and compressed 4.4ps green pump pulses to generate record average powers of up to 3.7W of signal and 1.8W of idler. Using the second pump source with a liquid crystal spatial light modulator pulse shaper, the demonstration of energy-scalable output pulses from 0.1µJ to 2µJ at a centre wavelength of 1045nm with typical pulse durations between 300fs and 600fs is reported. Pump pulses with a duration of ~ 600 fs at a repetition rate of 50MHz from this pump source are used for a PPLN OPO producing ~ 1.9W (~ 1.2W) of signal (idler) power at 1.5µm (3.6µm).