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Optical Fibre Distributed Corrosion Sensor

Optical fibre sensors have drawn strong research interest during the last two decades for health monitoring of military and civil structures. Corrosion is a pervasive and costly problem in many large structures. As corrosion is an electrochemical process, the detection of chemical changes in the environment might indicate conditions that could lead to corrosion, e.g. pH level, oxygen level, moisture content, the presence of salt contamination. Monitoring corrosion products (such as metal ions) can also be employed to detect the initiation of corrosion.

Optical fibre can be used to sense environmental effects in two different ways. The fibres may be used strictly as information carriers from any type of optical sensor to a receiving module like a detector. The second approach relies on the properties of the optical fibre itself to convert an environmental action into a modulation of the light. A common method to achieve this is based on the evanescent field radiation that extends beyond the fibre core into the cladding layer. The use of optical fibre cladding materials that are doped with a compound that fluoresces in the presence of the target chemical species provides a good opportunity to determine a range of parameters with high selectivity and sensitivity.

A detection system based on optical fibre sensor has significant potential for "distributed" sensing along the length of the fibre. A large area can potentially be monitored simultaneously because the whole length of the optical fibre can be made sensitive to a particular analyte in such a way that the presence of the analyte at any point along the fibre will modify its optical properties. Alternatively, the sensitized fibre can be interrogated by an optical equivalent of radar (optical time domain reflectometry, OTDR) to give a measure of the analyte concentration as a function of position over the extended area.

Swinburne University has experience in the development of a distributed temperature sensor (DTS) based on the phenomenon of Raman scattering the Sentor 101® system. In collaboration with the Defence Science and Technology Organization, the Fibre Optic Distributed Corrosion Sensor project aims to extend this experience into a new application area by developing a system to detect the initial stages of corrosion in large civil and military structures.