An experimental investigation of Raman and erbium doped fibre amplifiers for use in optical communication systems

The main aim of the work described in this PhD thesis has been to investigate the performance of Raman and erbium-doped fibre amplifiers for use in optical communication systems, especially for applications related to the development of future multi-wavelength lightwave systems.

In the first stage of this research a tunable, narrow linewidth signal source was developed, suitable to be used as a signal source for both types of optical amplifiers. An external cavity laser was constructed with a tuning range in excess of 200 nm which sufficiently covers both transmission windows of the optical fibre. A signal linewidth in the kHz range has been achieved. To measure such narrow linewidths, a new linewidth measurement system has been developed based on a Michelson type self-homodyne interferometer configuration.

A Raman fibre amplifier was then constructed and its performance was measured, for different types of single-mode fibres. An extensive study has been carried out to evaluate the performance of this amplifier when the pump was partially depleted by different levels of stimulated Brillouin scattering in the amplifier's fibre. The effect of the amplifier phase noise on signal source linewidth has also been studied, thus testing the suitability of Raman fibre amplifiers for use in coherent lightwave systems.

A detailed study was then carried out on the properties of Er-doped fibre amplifiers (EDFAs), namely gain, noise figure, and saturation characteristics for different amplifier configurations. The suitability of these amplifiers for use in coherent communication systems has been investigated. A novel EDFA configuration with an integral band-pass filter has been developed. This amplifier structure is particularly suitable for use as preamplifier to the receiver. To appreciate the effect of temperature variation on the performance of EDFAs, several Er-fibres with different composition and core diameters have been tested. Finally the type of broadening in EDFAs has been determined using the spectral hole-burning technique. Valuable results have been obtained from this research as the level of homogeneous broadening sets fundamental limits on channel separation in multi-wavelength optical communication systems. New operating conditions for EDFAs have been proposed to solve gain equalisation and gain cross-saturation problems in systems employing EDFAs.