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Determination of the coefficient of thermal expansion of an anisotropic material with Bragg grating interferometers

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 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. Measurements of the coefficients of thermal expansion of a specific type of rolled steel yielded values of 12.7 x 10-6 °C-1 and 7.5 x 10-6 °C-1 in the axial and the transverse direction respectively.