An investigation of laser action in single transverse mode fibre lasers
This thesis deals primarily with the study of pulsed output operation in single transverse mode fibre lasers resulting from active modulation of the laser inversion and the intracavity field. Sinusoidal modulation of the pump power offers an efficient method for producing energetic output pulses of a few microseconds duration. For certain ranges of pump parameters the fibre laser exhibits a highly nonlinear dynamic response to the pump signal and can even become chaotic. Modelocked operation has been demonstrated using both active amplitude and phase modulation schemes. Bandwidth-limited modelocked pulses with a sech2 profile and 20 psec duration are reported. These are the shortest duration pulses yet observed in a doped fibre laser system. The effects of self phase modulation and group velocity dispersion are shown to have a major influence on the modelocking process, resulting in significant performance differences between AM modelocked and FM modelocked fibre lasers. FM modelocking appears most favourable. Detuning the phase modulation frequency has allowed investigation of continuous-wave FM operation in fibre lasers. Laser spectra and temporal behaviour are characteristic of a single FM oscillation. Modelocking has also been demonstrated in a system where the phase modulation occurs within the fibre itself, induced by a transverse acousto-optic standing wave. The laser oscillates over a 500 GHz bandwidth capable of supporting sub-picosecond duration modelocked pulses. Incomplete locking of the axial modes leads to output pulses of 60 psec duration. The low loss resonator configuration allows efficient end-pumping by a low power laser diode. This system appears a likely candidate for enhanced modelocking through nonlinear mode-coupling techniques. We also discuss schemes for narrowing the fibre laser bandwidth, either by intracavity etaloning or by extracavity frequency-selective feedback.