Bend sensing with long-period fiber gratings in capillaries embedded in structures

Bend responses of an LPG inscribed with a CO2 laser in a conventional single-mode fiber are investigated in an arrangement, where the LPG is inserted into a thin silica capillary bonded to a steel strip. The curvature of the strip, laid on two supports and bent with a micrometer driver in the middle between the supports, decreases linearly with distance from the center of bending to the supports. Experiments are done for a very large range of bend deflections with the LPG in three positions shifted with respect to the center of bending, and two distinct opposite rotational orientations with respect to the plane of bending. Responses obtained for the above positions and orientations are greatly different and some of them show features which have not been reported for LPGs bent with a constant curvature and smaller deflections. They are, particularly, maxima and minima of the central wavelength's shift along with reversals of its direction, or the reappearance of a previously vanished attenuation band with increasing magnitude of bending. Also, it is found that the responses depend not only on the average curvature of the bent LPG, but also on the particular dependence of the curvature along the LPG's length. Some of the results obtained cannot be satisfactorily explained at the present level of knowledge, and a more thorough theoretical analysis is needed.