Active neodymium and erbium doped fibre devices
In this thesis a number of rare-earth-doped fibre devices are described including fluorescent and superfluorescent sources as well as several laser configurations. The laser configurations are all-fibre and include a neodymium-doped ring laser and recirculating delay line, a novel tunable neodymium-doped fibre laser and a single-frequency travelling-wave erbium-doped ring laser. The latter device has been the first description of a travelling-wave fibre laser device. Theory describing general fibre amplifier and laser devices is incorporated. A novel lumped element approach to fibre laser theory has been given applicable to 3 and 4-level laser devices which, under certain conditions, allows single pass gain of a fibre device to be described simply by the absorbed pump power. Numerical modelling of the erbium-doped fibre amplifier has been described which allows for analysis of a general device showing pump excited-state absorption. Results from the analysis have shown a difference in gain characteristics between co-propagating and counter-propagating signal/pump schemes when subject to pump excited-state absorption. In addition, the effect of pump direction on the noise figure is characterised in both small and large signal operating regimes. Characterisation of neodymium-doped fibres has shown a number of effects which will affect their use in amplifier and oscillator configurations. These include observation of sensitivity of the fluorescence characteristics to pump wavelength, observation of excited state absorption and polarisation of fluorescence. Additionally, the spectral gain-saturation characteristics have been investigated.