Darby, Matthew Thomas; (1999) Modelling spurious signals in fibre networks; applied to HFR systems. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

The project work undertaken in this thesis has been concerned with the characterisation and modelling of spurious optical signals which may be generated in optical fibre communication networks. In particular the aim of the modelling work was to develop software tools which could be easily manipulated to represent different fibre systems and be used in the design, development and analysis of any desired network structure. The work can be divided into two studies, that of feedback travelling backwards through the fibre network, and of spurious signals propagating forwards and arriving with the signal at the detector, causing the generation of interferometric noise. Hybrid fibre radio (HFR) systems have Provided the focus for the work, both for analysis and as illustrative examples of simulation techniques. In order to simulate optical feedback effects a series of modules were developed for use within the Signal Processing Worksystem simulation environment. The Package and developed tools are graphically interactive, providing the user with an ergonomic windowed environment for system design, re-configuration and optimisation. The developed software includes a model for a semiconductor laser and EDFA, together with feedback generating modules which simulate the effects of back-reflections and Rayleigh back-scatter from a fibre network. Some illustrative results using the software are also presented. A mathematical analysis of interferometric noise (IN) in heterodyning HFR systems is presented. The analysis considers two frequency displaced optical carriers, one of which is on-off keyed. The signal is detected together with an interfering sum of time-delayed and attenuated reflectors which cause unwanted beat signals between signal and crosstalk components. The moment generating functions for the noise distributions on one and zero symbols are evaluated and the modified Chernoff bound and a Gaussian approximation used to determine the impact on the system error rate. Simulation results confirm the analytical predictions and go on, through a series of system studies, to show that angle modulation schemes are more resilient to IN generation.

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