Highly sensitive optical fibre Bragg grating contact pressure sensor embedded in a polymer layer: Modelling and experimental validation

Abstract

This paper discusses mathematical modelling and experimental validation of a highly sensitive optical fibre Bragg grating (FBG) contact pressure sensor developed for healthcare applications. Bare FBGs are not very sensitive to pressure (∼3 × 10−3 nm/MPa) but this can be increased by embedding the FBG in a polymer layer which acts as transducer to convert transverse load (pressure) applied to an axial strain, measured by the FBG sensor. The pressure sensitivity of the FBG sensor depends on the mechanical and physical properties such as Young’s modulus, shape and size of the polymer. A finite element analysis (FEA) model is developed to optimise the design parameters of the FBG sensor in order to achieve a high sensitivity. A transfer matrix mathematical formulism is then used to relate the reflection spectrum of the FBG to the strain experienced. Three different shapes, three different sizes and three different polymer materials with different Young’s moduli have been simulated and their wavelength sensitivities related to the transverse pressure. According to the simulation results, the pressure sensitivity of a bare FBG can be increased ∼ 270 times (0.8179 nm/MPa) by selecting an FBG of 3 mm length, embedding it at the horizontal centre of a polymer layer of Young’s modulus of 20 MPa, in the shape of a circular disc with a diameter 5.5 mm and thickness of 1 mm.