Abstract
M.Ing.
This thesis covers the theory and development of fused, biconically tapered, fibre-optic
couplers and their application in sensors. The optical coupling between fibres is described by means of the coupled-mode theory in the case of a weakly guiding coupler. Firstly, simplified expressions for the guided modes in an optical fibre are derived. This is followed by the derivation of the coupled-mode equations for a perturbed waveguide. The coupled-mode equations are then used, together with the expressions for the guided modes, to determine the coupling coefficient for a weakly guiding coupler. In the case where the coupler is not weakly guiding, a model based on a rectangular dielectric waveguide is presented. A process for the manufacture of fibre-optic couplers, based on the fusion and elongation technique, was developed. This process is described and special attention is paid to the influence which different process parameters has on the properties of the couplers. During the development, the coupling theory was utilized to optimize the process. Results obtained with couplers manufactured by this process, are presented. A specific application of couplers, namely a new type of fibre-optic coupler sensor, was developed. The operation of the sensor relies on the change of the coupling ratio with an axially applied mechanical strain. Analog signal processing of the two outputs yields a signal which is directly proportional to the strain and which is independent of fluctuations of the light source power. The sensor was investigated experimentally by manufacturing temperature and elongation sensors which work on this principle. The operation and features of these sensors are presented, together with very promising experimental results.