Free space optical coupling to completely embedded fiber Bragg grating sensors

Abstract

ABSTRACT A novel method of free space passive optical coupling to co mpletely embedded fiber Bragg grating (FBG) sensors has been investigated. Fiber optical sensors have attracted intense research and comme rcial interest. Specifically, fiber Bragg grating (FBG) sensors have shown great utility for integrity management and environmental sensing of composite structures. One major drawback of FBG sensors, however, is that they suffer from cumbersome and fragile techniques for bringing the sensing light into and out of the structures. In our previous publication, we have demonstrated surface normal free space coupling into embedded FBG sensors with 45-degree-angled mirrors integrated into fibers as the coupling technique. These previous studies, however, required physical coupling to detect and analyze the light transmitted through the FBG. In this paper, we successfully couple free space light into and out of an un-embedded FBG sensor using multimode fiber (MMF) spliced to SMFBG with 45-degree-angled mirrors fabricated onto both MMF and SMFBG on a polishing wheel. We also present a manual po lishing technique to integrate mirrors onto MMF and SMF (without Bragg grating) embedded in a composite material, and successfully measure the co upling into and through these embedded optical fibers. With this approach, no pigtailing is needed, and the FBG sensors can be completely embedded inside the structures, greatly increasing system simplicity and robustness. Keywords : fiber optical sensors, reflection, fiber Bragg grating (FBG), free space coupling, embedded optical fiber, smart structures, structural health monitoring