Guiding Light: Optical Fiber

Abstract

Optical communications can be divided into Line of Sight, LOS, systems, and guided light methods. In this chapter we shall consider guiding light through silica-based optical fibres which represents by far the majority of optical communications systems. This chapter defines single and multimode fibre construction and operation and the attempts to minimize dispersion which is the limiting factor for information carrying capacity for all optical fibres. Dispersion limits the bandwidth by smearing out the digital optical pulses in time so that the sharp edges are lost as different components of the signal arrive at different times and so eventually the pulses merge into each other. Chromatic and modal dispersion are the two main culprits although there are subdivisions to these two headings which need to be defined. Singlemode fibre effectively removes modal dispersion but all fibres suffer from chromatic dispersion though we will see that for modern wavelength division multiplexing systems some level of chromatic dispersion is actually built in. Chromatic dispersion primarily comes from material and waveguide dispersion and it is these which can be ‘tuned’ to provide a range of ITU-defined fibres to cover a range of telecommunications applications.