Novel fibre-optic-based interferometric sensors exploiting coherent and low-coherence signal processing techniques

The work presented in this thesis is concerned with the introduction of several novel fibreoptic-based interferometric sensors. The main objectives have been to design practical sensors which are remote, passive, insensitive to environmental perturbations and capable of reinitialisation when it is switched on. A sensor for temperature or strain measurement is demonstrated using a short coherence length light source. A novel signal processing technique, based on either tracking the point of maximum visibility (zero path imbalance) or the first quadrature point, has been introduced. The advantages of both techniques are that the value of the measurand can be recovered when the sensor is 'powered up' and the accuracy of the sensor is nearly independent of drift in the source wavelength. The potential of using multi-mode laser diodes, as alternatives to the low coherence length sources such as LED' s or SLD ' s commonly used in 'white light' fibre-optic interferometric sensors has been studied. It is shown that the main advantage gained in using such sources is improved resolution due to the significant increase in the launched optical power. The use of such sources however is subject to certain restrictions. A novel form of sensor in which the sensing element is a miniature hemispherical air cavity Fabry Perot interferometer has been introduced. The properties of the cavity are theoretically studied and then verified experimentally. This new cavity design has been exploited for two different types of thermometers. A novel accelerometer, in which the sensing element is a weighted diaphragm, has also been introduced. The displacement of the diaphragm produced by acceleration is measured using a similar miniature hemispherical air-spaced Fabry-Perot interferometer, of which one mirror is mounted on the diaphragm. The design of the accelerometer has been developed to have minimal sensitivity to environmental and source frequency drift by exploiting the very high common mode rejection ratio achieved using two miniature hemispherical cavities, constructed either side of the diaphragm. Two laser diodes, with distinct wavelengths, are used to illuminate the system in such 111 a way as to increase its unambiguous dynamic range. The system outputs are processed differentially such that the detrimental effects caused by the optical sources frequency jitter as well as environmental perturbations are strongly minimised.