Multiplexed fibre Bragg grating Fabry–Perot interferometers for measuring the coefficients of thermal expansion of anisotropic solids

This paper introduces a fibre-optic interferometric technique for experimental determination of the coefficients of thermal expansion of anisotropic solids. It is based on the use of three or more low-finesse fibre Bragg grating Fabry?Perot interferometers of different lengths. Expressions are derived for the coefficients of thermal expansion in terms of the phase shift per unit cavity length of the individual interferometers. Analysis of matrix-based simulated reflectance data of interferometers mounted on anisotropic metals confirmed the validity of the proposed procedure. Measurements of the coefficients of thermal expansion of a specific type of steel yielded values of 12.7 ? 10?6 ?C?1 and 7.5 ? 10?6 ?C?1 in the axial and the transverse directions respectively. The corresponding values for an aluminium alloy were 21.1 ? 10?6 ?C?1 and 20.9 ? 10?6 ?C?1 indicating that the anisotropy is very small for this material.