Sensors based on optical fibre materials will be required to tolerate a restrictive combination of physical and environmental parameters for several remote monitoring applications at AWE. These include changes in atmospheric pressure, temperature, humidity, vibration, shock and acceleration, with the sensors being required to operate reliably for periods of up to 30 years with minimal intervention for maintenance. In addition, it is necessary that the sensors can function in the presence of ionising radiation. The sensors are being developed for a number of challenging in-situ physical and chemical measurements. These include remote gas composition analysis, monitoring shape change in compliant materials and the movement of metallic and polymeric components using sensors based on fibre Bragg gratings and interferometric techniques. Reliability issues include the long-term mechanical and optical performance of standard and novel glasses, optical fibres and cables, connectors, couplers, optical switches and Bragg gratings. The durability of materials used in the construction of fibre optic sensing components also requires to be assessed in addition to the epoxy and metallic coatings used to bond these components to a variety of material substrates.
We describe an optical system to monitor small long-term changes in the shape of a surface by using a network of optical fibre Bragg grating strain gauges, for applications in which space does not permit the use of techniques such as photogrammetry or structured light methods. Gratings are bonded to copper beryllium strips held under tension in contact with the test surface. The copper beryllium strips enable sufficient force to be transferred to the optical fibre from the compliant surface. Shape changes are revealed as strain changes in the sensor strips, inferred from wavelength shifts in the Bragg peaks. The optical signals are obtained in reflection by illuminating the sensor fibres with a broadband source and using a scanning Fabry-Perot filter to generate the spectrum with a wavelength resolution of 0.3 pm over the range 1530 to 1570 nm. Laboratory tests show that a strain resolution of 8 microstrain can be achieved with temperature compensation over the range 20 to 50 C, with a multiplexing capability of between 11 and 16 temperature - strain sensor pairs, depending on temperature gradients on the test surface. We present experimental measurements on a cylindrical test object subject to diametral loading, and show a comparison with a finite element model.