Using Speckle Suppression Approach for Improving Interrogation of Step-index, Multimode Optical Fiber Bragg Gratings

Fiber Bragg grating (FBG) technology is one of the most developed and industrially integrated among the broader fiber optic sensors domain. FBGs inscribed in single-mode fibers are used in several applications due to their stable and reliable single-peak reflection spectrum. On the other hand, FBGs inscribed in multimode fibers (MMFs) demonstrate complex multipeak reflection spectrum (RS), deteriorating their applicability. Thanks to the formation of mode groups, the problem is not that complicated in the case of graded-index (GI) MMFs due to well separated and relatively stable peaks in the FBG reflection spectrum. Oppositely, multimode (MM) step-index (SI) FBGs demonstrate complex and unstable RS with highly overlapping peaks. Furthermore, the effects of intermodal interference and mode coupling cause significant RS modifications even when the FBG is isolated from external perturbations, and only the MMF of the interrogation scheme is disturbed (e.g. heated). The above render the use of MM-SI-FBGs impractical, especially, in the case of polymer fibers (which are highly-MM SI fibers in the most cases), where inscription of MM FBGs is the only available solution; the above outlines the problem of FBG stabilization being of high importance [1].