Free space optical coupling of reflection-based fiber Bragg grating sensors

In this paper, a robust free space optical coupling of reflection-based fiber Bragg grating (FBG) sensors is presented. Fiber Bragg grating (FBG) sensors have shown great utility for integrity management and environmental sensing of composite structures. However, they suffer from cumbersome and fragile techniques for bringing the sensing light into and out of the structure since the optical fiber must be routed through the surface of the composite (i.e. pigtailing). In our approach, 45-degree-angled mirrors integrated into fibers were used as an input and output coupling technique. With the difficulty of directly free space coupling to the tiny single mode fiber (SMF), a novel method of coupling to the sensor via splicing and fusing a multimode fiber (MMF) to the single mode FBG (SMFBG) was explored. Using this method, we have previously demonstrated free space optical coupling to transmission-based FBG sensors embedded inside a composite panel. In this paper, we are able to interrogate un-embedded sensors with reflected light using 45-defree-angled mirrors. We have also performed preliminary investigation on FBG sensors embedded in a composite panel. We conclude that this novel space coupling method can be used effectively couple the transmitted and reflected light for FBG sensors.