Measurement of transverse strains with fiber Bragg gratings

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Abstract
In this paper, we present a method to measure two components of transverse strain in an optical fiber using a single Bragg grating written into high-birefringent, polarization- maintaining (PM) fiber. The reflected spectrum from this grating contains two peaks corresponding to the two orthogonal polarization modes of the fiber. If the axial strain and temperature in the fiber is known, then two components of transverse strain can be computed from the changes in wavelength of the two peaks. A Bragg grating written near 1300 nm in PM fiber was loaded in diametrical compression, and the changes in wavelength of the Bragg peaks were monitored using an optical spectrum analyzer. Transverse strains were computed from the changes in wavelength using available strain-optic coefficients for low-birefringent optical fiber. These strains are compared to finite element analysis predictions, and it is shown that the observed sensor response is greater than the response predicted by the low-birefringent analysis. A calibration factor is developed for the sensor to allow the determination of transverse strains from the measured wavelength shifts.

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