π-Phase-Shifted Fiber Bragg Grating for Strain Measurement With High Spatial Resolution

Because the static strain measured by a uniform fiber Bragg grating is a mean value along the grating length, it is difficult to precisely measure a local strain when encountering nonuniformity. It is found that phase-shifted fiber Bragg gratings have short effective grating length and reflectivity approaching one at the central peak; these properties are beneficial for improving the spatial resolution. These advantages have been validated by calculating the light power distribution along the grating and by spectrum simulation using the transfer-matrix method integrated with finite element simulation. An interrogation system integrated with a newly designed algorithm has been used to automatically track the wavelength shift in the central peak. A set of comparison experiments that mimicked symmetrically parabolic strain and linearly increasing strain were conducted to demonstrate the advantage of the phase-shifted fiber Bragg grating over the conventional fiber Bragg grating in static measurements.