A pressure sensor based upon the transverse loading of a sub-section of an optical fibre Bragg grating

An experimental and theoretical study of locally transverse loaded fibre Bragg gratings ( FBGs) for pressure sensing purposes is presented. When a load is applied to a short section of an FBG, a spectral hole is generated in the reflection spectrum, which exhibits a redshift in wavelength as the load is increased. This effect has been modelled using Rouard's method and has been characterized experimentally. Two techniques for analysing the spectrum for pressure measurements at constant temperature are considered, one based on measurement of the wavelength shift of the spectral hole, and the other on measurement of peak reflectivities either side of the hole. Normalized pressure sensitivities of 3.30 +/- 7.93 x 10(-2) MPa-1 and 4.97 x 10(-4) +/- 1.37 x 10(-5) MPa-1 were obtained for the reflectivity and wavelength measurement, respectively. A pressure resolution of 1.3 kPa, over a range of 745 kPa, was achieved when using a tunable laser of 1 pm wavelength resolution to interrogate the sensor. For pressure measurements in the presence of temperature changes, a technique based on the measurement of the position of the spectral hole within the Bragg envelope is presented. This technique allows the opportunity to discriminate pressure and temperature using a single FBG sensor element.