|dc.description.abstract||Motivated by the industrial requirement for multi-100-W sub-ps pulses, this thesis describes the development of a high-average-power master-oscillator power-amplifier (MOPA) based on seeding an Yb:YAG planar waveguide amplifier using an Yb-based modelocked oscillator operating at 1030 nm. The scope of the research presented includes both the development of the seed oscillators and of the high-power amplifier.
Two end pumped Yb:KYW oscillators were demonstrated, one with Brewster-Brewster crystal geometry and another with a novel plane-Brewster crystal, permitting a simple pumping arrangement and which provided superior efficiency than Brewster-Brewster geometry. In both oscillators, one or more Gires-Tournois interferometer (GTI) mirrors were used in the cavity to compensate for the large amount of positive dispersion from the crystal. Both systems were modelocked using semiconductor saturable absorber mirrors. The oscillators demonstrated here produced amongst the highest average powers reported from end-pumped systems to date, generating up to 4.5 W in the nearinfra-red region in the form of 500-fs pulses, with a repetition frequency of 53 MHz.
The study of oscillator performance was extended to include a comparison between Yb:KYW and Yb:YAG lasers constructed in similar configurations, both based on plane-Brewster crystal geometries and dispersion compensated using GTI mirrors. The Yb:YAG system provided 700-fs pulses, compared to 500-fs pulses obtained using Yb:KYW, with the average power produced being 2.88 W for Yb:YAG, and 2.42 W for Yb:KYW, despite significantly better CW performance being observed with Yb:YAG. Due to their near-infrared wavelengths, high average powers and sub-ps pulse durations, both systems showed potential as seed lasers for high-power Yb:YAG amplifiers.
A MOPA system was developed around an Yb:YAG planar waveguide amplifier seeded by the Yb:KYW femtosecond laser based on a Brewster-Brewster crystal geometry and operating at 53 MHz repetition frequency. With single-sided pumping and five passes of the gain waveguide, the Yb:YAG amplifier provided 700-fs pulses with average powers of 50 W at 1030 nm. With the extension to double-sided pumping and the use of toroidal mirrors to achieve seven passes, the amplifier produced 780-fs pulses with average powers of 255 W. A numerical simulation of the amplifier identified gain narrowing as the dominant pulse-shaping mechanism||