Free space optical coupling of fiber optic sensors integrated with composite structures

In the past decades, optical fiber has proven useful for smart structures and structural health monitoring. 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 the lack of a robust, non-"pigtail" technique for coupling to the embedded FBG sensors. In order to solve this problem, novel methods of free space passive coupling of light into and out of FBG sensors were investigated.;For sensing application in small structures, an angled 45-degree mirror integrated directly into the fiber can be used as an input coupling technique. This approach was implemented on both single- and multimode glass fibers containing FBG's. For multimode FBG's (MMFBG's), the grating's uniformity across the fiber diameter and its effect on normal free space coupling were studied. In single mode investigations, a novel method of coupling to the sensor via splicing a multimode fiber (MMF) to a single mode FBG (SMFBG) was developed. With mirror machined onto the MMF, free space coupling to embedded FBG sensors was demonstrated. Finally, free space coupling to an embedded SMFBG was employed to measure the tensile strain. When the strain is uniform, excellent agreement was found between the FBG and conventional electrical resistance strain gauges. When the strain was non-uniform, the excellent agreement still existed when the strain was below 2500 muepsilon.;The intensity of the optical signal coupled into the embedded fiber via machined mirror, however, was fairly low because the surface of the machined mirror was relatively rough. Using this endmill technique, it was not possible to demonstrate dual ended, free space coupling to embedded sensors due to the cumulative optical losses of the input and output mirrors. In order to improve the quality of the mirror, a minimally-invasive hand polishing technique was developed, which results in considerably better mirrors that enable dual-ended interrogation. It is also demonstrated that this simultaneous free space in & out coupling technique was applied in a tensile strain test and a possible non-uniform strain profile was revealed along the length of the grating. In addition, with the significantly improved mirror, interrogation of the embedded FBG sensor with reflection measurement is also demonstrated.;However, this coupling technique cannot be directly applied to the monitoring of bridges and other large structures since it is impractical to machine or polish the angle after embedding the fiber into the structure, and it is difficult to maintain the rotational position of the fiber if the mirror is polished before embedding. Therefore rotationally symmetric concave conical optical couplers have been developed, which couple light independent of the rotational orientation of the fiber with respect to the outside of the structure and the optical source. In the preliminary experimental attempt, conical couplers machined into 1-mm and 3-mm plastic optical fiber (POF) are achieved. Free space coupling into un-embedded FBG with a conical coupler is also demonstrated.