Theory of fiber optic strain and temperature sensors

Models were developed which are required for utilizing intrinsic and extrinsic Fabry-Perot and Bragg grating fiber optic sensors for measuring strains and temperature on the surface as well as inside a material. The starting point of the models is the demodulated light signal reflected from or transmitted through the sensor. First a model was developed which relates the strains and temperature inside the material to the light intensity (for Fabry-Perot sensors) or the wavelength (for Bragg sensors) reflected from or transmitted through the sensor. The expressions resulting from these models were then inverted providing the strains and temperature from the measured values of the light intensities or wavelengths. On the base of the models, a computer code SENSOR was written which can be used to calculate the strains and the temperature inside anisotropic, orthotropic, transversely isotropic, and isotropic materials. The validities of the model and the corresponding computer codes were assessed by comparing strains measured by intrinsic Fabry-Perot and Bragg grating sensors to results generated by the code. Good agreement was found between the measured and calculated strain values. The procedure for selecting the appropriate sensors for a given application was given and the sensor types most suitable for applications with composite materials were identified.