In this chapter, the principles behind subcarrier multiplexing (SCM), the factors affecting component and system performance and its application potential are discussed. SCM makes efficient use of available bandwidth and, even when techniques to reduce bandwidth in purely digital systems are considered, SCM remains less complex and expensive. The RF/microwave components required are often readily available from other systems (e.g. radio, radar, satellite) again reducing costs. The performance requirements for SCM systems are different to those for typical digital optical communications systems. Generally, these are more stringent regarding noise and nonlinearities, especially for multichannel applications. However, the correct choice of components and system design, perhaps with the use of compensation techniques, does allow these requirements to be met. The compatibility of SCM with many other types of RF/microwave systems and its flexibility makes it useful in a variety of applications, from antenna remoting and CATV to local-area networks. Probably the most significant feature of this flexibility, for the near-term application of SCM techniques, is its ability to allow an evolutionary development of existing networks into broadband-ISDN with low initial installed costs.

The widespread deployment of analogue optical fibre links started in the late 1980s with their incorporation as fibre trunk lines into coaxial cable-based CATV networks, particularly in North America. These coaxial cable television networks provide the multichannel amplitude modulated-vestigial sideband (AM-VSB) modulation over the frequency range from 50 to 88 MHz and from 120 to 550 MHz, whilst the band from 88 to 120 MHz is reserved for frequency modulated (FM) radio broadcast. Vestigial sideband amplitude modulation was employed within the CATV networks as it exhibited the good low-frequency baseband characteristics of double-sideband amplitude modulation whilst conserving bandwidth, and hence it has been widely utilised for the electrical transmission of TV and similar analogue signals.

The phenomenon of a train of optical fibre solitons, modulated in some way by impressed disturbances, is increasingly being identified as a new type of dynamical system, with interesting dynamical behaviour which can be studied both experimentally and mathematically. This perception leads rather naturally to an investigation of the behaviour of this system when subjected to various forms of pulse-analogue modulation, to which this chapter is devoted. The principal motivation for the study of solitons in pulse position modulation (PPM) arises from the fact that PPM with short pulses permits a greater transmission rate than longer pulses, and intense pulses exhibit higher signal-to-noise ratio than weak pulses. Consequently, intense short pulses are desirable for PPM, but fibre dispersion limits the minimum width an individual pulse can have after linear transmission. This consideration leads naturally to speculation on solitons in PPM because the soliton does not suffer dispersion. However, this is only one example of a general interest which is developing in the understanding of dynamical phenomena involving solitons, to which this work is addressed.

The work presented here is concerned with an investigation of the impact of source nonlinearity on the performance of multichannel subcarrier multiplexed optical fibre systems. There has been much interest recently in the capabilities of subcarrier multiplexing (SCM) as a means of realising economical multichannel systems which can be deployed in the short to medium term to support a wide range of analogue and digital services. The attractive feature of SCM is that it provides a way of exploiting the multi-gigahertz bandwidth potential of high speed lasers using conventional and established microwave techniques. Also it is very flexible being capable of simultaneously transmitting conventional baseband and microwave signals with the same fibre and detector. Moreover, SCM systems can be combined with coherent techniques and wavelength division multiplexing to utilise fully the tens of terahertz capacity of single-mode fibre.

Future mobile communications is expected to make use of mm-wave frequencies, spectral congestion at lower frequencies making these effectively unavailable for broadband applications. The potential of optical fibre techniques to support the generation and remote delivery of mm-wave based mobile broadband services has been outlined, with particular reference to developments effected within the context of the EU RACE programme by project MODAL. It has been shown that such systems can be developed using optical and microwave/mm-wave technology which is readily available today. By employing optical techniques the flexibility for systems deployment is greatly increased, with optical fibre allowing signal transport over tens of kilometers, effecting benefits in base station interconnection and antenna siting and so contributing to low construction, installation and maintenance costs. The remainder of this introductory section will outline possible electrical methods for distributed mm wave generation and briefly outline some of the advantages that could be enjoyed if an optically-based method for the generation and distribution of mm-waves were to be adopted.

Optical amplifiers have become a major element in the design of advanced optical systems. Their main characteristic is that they provide direct optical gain over a very wide bandwidth. In addition, they can support very high output powers and so are ideal for a number of analogue applications, such as CATV distribution. The wide bandwidth enables the simultaneous amplification of many signals at different wavelengths, making them ideal components to use in wavelength division multiplexed systems