Measurement of thermal expansion coefficients using an in-fibre Bragg-grating sensor

A simple method to measure the thermal expansion coefficient using a surface-mounted Bragg-grating sensor is presented. This method uses a single, uniform-pitch Bragg-grating fibre that is only partially glued onto the specimen. To prevent cross talk between the two Bragg wavelengths reflecting from glued and free sections, a preloading is applied to the fibre before it is partially glued onto the specimen. After release, two Bragg wavelengths are observed. The Bragg wavelength reflected from the sensor section not glued onto the specimen is used to measure temperature variations; the Bragg wavelength reflected from the sensor section glued onto the specimen is affected by variations in thermal strain. Therefore, the thermal expansion coefficient can be determined by calculating the spectral separation of the two Bragg wavelengths. The measured thermal expansion coefficient of an aluminium plate is (24.9 ± 2.1) × 10^-6°C^-1 for every 5°C in the range 35-75°C. It is in good agreement with the value of 25 × 10^-6°C^-1 for aluminium. However, the scheme will lead to incorrect results if there is a temperature gradient across the Bragg grating or if the material to be measured is inhomogeneous.