Narrow-linewidth erbium-doped fibre lasers
This thesis describes research towards narrow-linewidth and single-longitudinal-mode erbium-doped fibre lasers operating on the 1.55µm region. The work is concerned with the physics and technology of narrow-linewidth fibre lasers. Factors which affect the spectral width and number of modes oscillating in an erbium-doped fibre laser are described, with particular reference to spatial holeburning, the primary cause of multi-mode oscillation. Novel resonator configurations for narrow-linewidth operation are described, in both standing-wave and travelling-wave arrangements, with linewidths as narrow as 10Hz being achieved. The development of narrow-bandwidth distributed Bragg reflectors is discussed. Resonator configurations discussed include short fibre lasers with narrow bandwidth reflectors, polarimetric coupled-cavity fibre lasers, travelling-wave fibre ring lasers and travelling-wave fibre loop lasers. Travelling-wave operation is demonstrated to be a powerful technique for realising single-frequency fibre laser operation. Linewidths as narrow as 10kHz are demonstrated in long fibre laser resonators. The potential for high efficiency and broad tuning ranges are illustrated. The statistics of phase noise in erbium-doped fibre amplifiers are characterised, by measurements of spectral broadening of a signal passing through the amplifier. The process of spectral broadening is modelled using a rotating vector approach. The implications for amplification of narrow bandwidth signals are discussed.