Development of miniature load sensors for smart materials and structures by using birefringent fibers

Mode couplings between x and y-polarized modes in a birefringent optical fiber are strongly influenced by transverse loads. Then, it is considered to develop miniature load sensors by using a birefringent optical fiber. This sensor can be integrated into a serial-multiplexed fiber optic sensor system. Some fundamental experiments to obtain the design concept of load sensors were conducted. Experimental results showed that a relation between a mode coupling and an transverse load was almost linear on a logarithmic graph. It was shown that the linear elastic coating was necessary for reproducible measurements. The mode couplings strongly depended upon the loading angle and the loading section length. From these experimental results, the design concept of stable, reproducible and quantitative sensors was obtained. A module model of load sensor was developed under this concept and demonstrated for verification of the concept. It was found that the sensor module had good performance for quantitative measurement of transverse loads. For practical use, three types of miniature load sensor modules, whose size and shape of the load sensor were improved for integration into a smart structure system, were developed. It was demonstrated that these sensors had capability of quantitative measurement.