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Studies have been made of optical waveguides produced in GaAs and InP epitaxial layers. Of the possible waveguiding mechanisms present in these devices the contribution from the photo-elastic effect (strain-induced refractive index changes) dominates. Stresses in evaporated metal films and their control have been investigated. Strain-induced waveguides have been used to produce a novel directional-coupler structure with a short coupling length (~2mm). In GaAs bias has been applied to control the amount of light at the output of each of the two waveguides forming these couplers and it has been possible to isolate the light in either the excited or the coupled waveguide. A new theoretical model, based on finite difference techniques, has been developed and used to analyse strain-induced, slab and rib waveguide structures. Results obtained have been compared with those from other methods. Theoretical predictions of guiding properties in GaAs strain-induced waveguides give good agreement with experimental results in all cases. Optical waveguiding in InP layers using the same photoelastic mechanisms, assessed experimentally, indicates that the refractive index changes are similar to those in GaAs but slightly larger. One of the first measurements of the nonzero electro-optic coefficient, r41, of InP is described. Guiding properties vary little with time in both InP and GaAs. The reflection of light guided in a single-mode photoelastic waveguide into a second perpendicular guide using a vertical etched facet running at 450 to the direction of propagation is proposed for providing bending with negligible loss and some experimental results are reported.

Digital pulse position modulation (PPM) is a transmission format that can be used to exchange excess channel bandwidth for improved receiver sensitivity. This thesis is concerned with the performance of digital PPM for use in optical fibre systems such that increased single-span transmission distances can be achieved. It includes a literature survey and a thorough theoretical investigation into uncoded directly detected and coherently detected digital PPM, along with an evaluation of forward error correction coded PPM. Original contributions are described for digital PPM systems employing PIN-BJT transimpedance and PIN -FET high impedance pre-amplifiers. In the PIN -BJT case, a new analysis is presented that accounts for the band-limiting effect of the preamplifier and the non-white noise power spectral density, resulting from equalisation. For the PIN-FET, a causal noise-whitening filter is considered and the factors that limit the maximum bit-rate determined. By using an upper Chernoff Bound to the average binary error probability, results are calculated for APD-BJT and APD-FET PPM systems. The practical implementation of the optimal PPM filter is considered and two novel techniques are described for automating the realisation process. Coherent PPM is investigated and new work is presented for Reed-Solomon coded homodyne PPM. The influence of the Reed-Solomon code rate on receiver sensitivity is studied and it is shown that a 3/4 code rate leads to optimum sensitivity. Digital PPM employing soliton pulses is examined and it is shown that, under ideal conditions, a higher bit-rate than PCM can be achieved. The influence of timing jitter, associated with soliton pulse energy fluctuations and the Gordon-Haus effect, on the performance of digital PPM is investigated. The simplification of the PPM pre-detection filter is studied and a range of suboptimum filters investigated. The calculations demonstrate that the optimal predetection filter can be replaced by a simple 3rd order filter without incurring a severe sensitivity penalty. The sensitivity results of the various digital PPM systems investigated in this thesis are compared to those of equivalent PCM systems. It is illustrated that digital PPM offers an improvement of between 5-10 dB depending upon which coding format/ detection technique is employed. This represents an increase in transmission distance of 25-50 km and demonstrates that digital PPM is a promising modulation format and has potential for future high performance telecommunication routes.

This study presents the results of an investigation to develop a novel sensor which would give a direct indication of the extent of impact damage in a composite. This was achieved by using glass reinforcing fibres to produce a light-guide, which was embedded within a composite laminate. The laminate was then subjected to impact events or bending stresses of sufficient magnitude to cause damage. The impact energies used in this study varied between 2 and 10 Joules, and the indentation depths varied between 0.125 and 1 mm, allowing damage propagation to be monitored. The fall-off in the transmitted light was used to monitor the level of damage, along with C-scanning and sectioning to provide reference data. The use of reinforcing fibres to produce the sensor meant that the strains required to cause failure in the fibres was realistically close to those of the composite constituents. Changes in the transmission characteristics of the sensor were found to correspond to real failure events occurring during impact.

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