TABLE OF CONTENTS The demand for high-capacity long-haul telecommunication systems is increasing at a steady rate, and is expected to accelerate in the next decade [ [1]]. At the same time, communication networks which cover long distances and serve large areas with a large information capacity are also in increasing demand [ [2]]. To satisfy the requirements on long distances, the communication channel must have a very low loss. On the other hand, a large information capacity can only be achieved with a wide system bandwidth which can support a high data bit rate (> Gbit/s) [ [3]]. Reducing the loss whilst increasing the bandwidth of the communication channels is therefore essential for future telecommunications systems.
Of the many different types of communication channels available, optical fibres have proved to be the most promising [ [4], [5]]. The first advantage of an optical fibre is its low attenuation. Typical values of attenuation factor in Modified Chemical Vapour Deposition (MCVD) optical fibres are plotted against wavelength of the electromagnetic carrier in Fig. 1.1 [ [6]]. At present, optical fibres with loss coefficients of less than 0.25 dB/km around emission wavelengths of 1.55 ?m are available [ [7]]. This remarkable progress in fibre manufacturing technology has led to wide applications of long distance optical fibre communications in recent years. Furthermore, optical fibres can also transmit signals over a wide bandwidth because the electromagnetic carrier in optical fibres has a frequency in the optical frequency region...
