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This thesis describes new signal processing techniques applicable to miniature low finesse Fabry-Perot interferometric sensors. The principle of operation behind the techniques presented resides in the use of the axial modes from a single multimode laser diode to produce a series of phrase shifted interferometric outputs in conjunction with the path imbalance of the sensor microactivity.

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.

In their service life, fibre reinforced plastics (FRP) face a variety of environmental conditions resulting from natural or artificial factors. These include variable temperature and humidity conditions, energetic radiations such as ultraviolet rays from the sun, and diverse chemical reactants such as liquid in storage tanks and pipes. These factors are always combined and negatively affect the material properties over the time. Therefore, optimized utilization of FRP material requires reliable methods for quantifying, controlling, and predicting environmental effects. This allows for optimal handling of issues related to component design, economic assessment and safety considerations, as well as the technical problems relating to equipment maintenance. Efforts worldwide are devoted to the modelling of FRP environmental degradation. However, modelling efforts have been hindered by the complexity of the process. This analysis presents a comprehensive model of the environmental degradation of FRP and a prediction method. The modelling method consists of a theoretical demonstration based on material science theories. An analytical approach is proposed. It resolves the complexity of the process into only three components: the chemical degradation, the physical degradation, and the stress state modification. A method to represent the real service environment as a constant environment in laboratory is also introduced. Then, the comprehensive model is expressed as a dynamic constitutive equation resulting from the combination of the historical variation in chemical link density and cohesive forces and the stress history of the material. It is shown that: • The average of the chemical and physical degradation as well as its upper and lower limits can be determined in a laboratory, in a constant environment, as exponential functions of the degradation time. • The environmental degradation can be comprehensively measured as a stress relaxation. • Acceleration of the predictive test can be obtained from a modified time temperature shift principle.