Temperature-compensated flat-pack fiber optic strain gage: design and fabrication

Recent advances in fiber optic sensors have shown the great potential for Bragg gratings to be used as strain sensors. Optical fiber Bragg gratings offer significant advantages over traditional resistance foil strain gages, including a high degree of multipexibility, compact size, immunity to electromagnetic noise interference, and resistance to most chemicals. However, Bragg grating strain sensors have met only limited success in real-world applications. Two reasons for their limited presence is the inherent temperature sensitivity of the dual-parameter Bragg grating and the lack of experience of engineers with fiber optic sensors. This paper describes the development of a fiber Bragg grating strain sensor that attempts to address both of these issues. The flat-pack strain sensor incorporates a pair of Bragg gratings into a single package. One grating is bonded tightly to the pack and acts as a combined thermal-mechanical strain sensor. The second Bragg grating is packaged loosely within the sensor and is used to measure only temperature, which can then be subtracted from the tight grating, providing a temperature-compensated strain reading. By packaging the two gratings into a configuration that is similar to resistance strain gages, we expect that many of the technical and practical implementation issues of optical sensor technology will be overcome. This paper describes the details of the design and experimental testing of prototype sensor packages to validate the functionality of the fiber optic strain gage.