Fiber optic intra-grating distributed strain sensor

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Abstract
Fiber-optic intracore Bragg gratings have been widely used as strain sensors. In most applications, the strain along a fiber grating is uniform and the strain is measured by the shift of the reflective wavelength of the grating. If the grating is under a gradient strain field, however, its reflective spectrum will be not only shifted but also distorted due to the chirp of grating periods. The shape of this spectrum depends on the strain distribution. Therefore the location and the shape of the reflective spectrum together can provide information on the strain distribution over a small region (sub-centimeter). We have calculated the reflective spectra of gratings by using the T-matrix formalism which allows us to simulate gratings with any distribution of the refractive-index modulation depth and the chirp of the period length. The calculation results show a strong dependence of the reflective spectrum of a grating on the distributions of its refractive-index modulation depth and period length. Experimental results have verified the T-matrix calculations. These results indicate that fiber-optic gratings can be used as quasi-distributed strain sensors to detect strain gradients that is very useful for monitoring and studying the presence and the evolutionary process of fiber sensor malfunction or material fringe/damage.

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