Fibre-compatible modelocked lasers at 1.5μm
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This thesis describes techniques for the generation of ultrashort optical pulses using semiconductor lasers. Active modelocking through gain modulation is applied to 1.?m InGaAsP semiconductor lasers which utilise microlensed optical fibres as external-cavity components, and tunable optical pulses of ~5ps duration have been generated. Particular attention has been given to suppression of the noisy subpulse features associated with low-frequency modelocked semiconductor lasers. This has led to two suppression methods applicable to symmetric (or balanced) and asymmetric cavity configurations, allowing the generation of clean, single feature short pulses of 7-10ps duration with peak powers in excess of 300mW. The physical phenomena relating to such processes will be outlined. Further amplification of these pulses using an erbium- doped fibre amplifier has resulted in output peak powers in the 5W range. A study of the phase-noise characteristics of these modelocked laser systems was undertaken using a high-speed photodetector and wide-band spectrum analyser arrangement. This characterisation allowed improvements in the rms pulse timing jitter from >2ps to below 300fs in the frequency range 50-5000Hz. Novel modelocked laser configurations were also constructed where the cavity components included both a semiconductor amplifier and an erbium-doped fibre amplifier. Linear and ring hybrid lasers were investigated with resultant pulse durations as short as 3ps with high peak power and excellent stability. CW and mechanical Q-switched doped fibre lasers were configured in diffraction grating tuned cavities with high output coupling (R=4%). Unprecedented output powers of up to 700mW tunable over ~100nm around 1.55mum, and 800mW tunable around 1.08mum were obtained. Also the value of 800W peak power for the 80ns output pulses from the Q-switched erbium-doped fibre laser amounts to the most intense pulse created thus far from any fibre laser device. Optimisation of the total tuning range attainable from erbium-doped fibre lasers via length tuning resulted in a variety of nonlinear phenomena, namely self Q-switching and optical bistability at the long wavelength tail of the tunable range.
Thesis, PhD Doctor of Philosophy
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