A study of techniques for optical fibre sensors
This thesis reports on work performed within the field of optical fibre sensors. The topics studied cover three main areas of sensor research; fibre-remoted gas detection, the optical actuation of a resonant quartz device, and the assessment of compound glasses and fibres for current measurement applications. The gas detection techniques described have the common advantage of highly-selective measurement, and are applied to a number of industrial and environmental gases. Selectivity is achieved by effectively monitoring the spectral overlap between the gas sample to be measured, and that of a reference gas. Three techniques were employed to vary the spectral overlap, including Stark, pressure, and frequency modulation. For quantitative gas detection at atmospheric pressure, the modulation techniques are demonstrated here for the first time with fibre-remoted sampling cells. Practical examples of sensors for detection of methane, ammonia and carbon dioxide were demonstrated, and gas sensitivities down to 50ppm by volume were achieved. The optical powering and readout of a quartz resonator was demonstrated. An open-ended tuning fork device was used, and a novel interferometric technique was employed to minimise the optical drive power required. Threshold optical power levels of less than 25µW were achieved. The configuration was capable of operation with only a single fibre link to the sensor head, to carry both optical drive and readout signals, and a single optical source was employed. Optical fibre current monitors employing the Faraday effect can suffer measurement bandwidth restrictions if a long fibre is employed in order to attain good sensitivity. As the Faraday effect in many compound glasses is considerably stronger than for a standard silica fibre, then an equivalent sensitivity can be achieved for a shorter fibre coil by the use of compound glasses. The sensitivity and bandwidth of a typical current monitor configuration are analyzed for a number of compound glasses for the first time. The Verdet constant of a trial fibre fabricated from one promising compound glass was measured.