Research On Fiber Optic Sensors Based On Multi-core Structured Fiber

Many structures exist that should ideally be rigid, but whose performance is limited by deformation due to loading. Often, corrective action can be taken if the deformation is known. For example "smart structures" have demonstrated the use of actuators to compensate for unwanted shape changes. In continuous structures, changes in length due to uniform strain and thermal expansion can be significant. However, most of the variation in the overall shape is due to bending, torsion creating a need for sensors specially optimized for the measurement of bending and twisting. Bending a beam produces a linear gradient in strain through the thickness of the beam. Local curvature can then be derived from the difference in strain between two strain gauges fixed to opposite sides of the beam; this is the basis type of electrical goniometer. They can be made by adhering two fiber optic strain sensors to either side of a flexible member, or embedding them both in the member.The objective of this work is to describe a multi-parameters relationship between a segment four-core fiber's curvature, twisting angle as well as ambient temperature gradient and the far-field interferogram of the fiber. The four-core fiber act as a four-beam interferometer in which phase differences is a function of curvature, twisting angle, and temperature-gradient in the plane containing the cores results in the shift or rotation of the far-field interferometric grid pattern.Based on the foreigner research results of multi-core optical fiber, the multi-core optical fiber structures and the characteristic as well as its applications in fiber optic sensors have been investigated and demonstrated. Three kinds fiber optic sensors based on the multi-core fiber have been proposed and designed include fiber optic temperature gradient sensor, bending or strain sensor and fiber optic twisting angle measuring sensor. Each sensor's sensing theoretical model were established based on the far-field interference fringe pattern. Experimental results are also confirmed this.In the present work, a series novel multi-core fiber has been designed and its geometry parameters of these special fibers such as core diameter, separate distances between neighbor cores are measured. The far-field interferogram characteristics of the multi-core fibers are investigated in detail. The first case is a fiber optic temperature gradient sensor based the four-core fiber are setup and studied. The second one is a multi-core fiber-bending sensor to monitoring the structures curvature or strain. The third is the multi-core fiber optic twisting angle-detecting sensor.According to the sensing theoretical model, a series experiments were conducted in our laboratory. The experimental results shown that the performances of all the three sensors are working in a linear manner. At the range of 90 degree, the twisting sensor not only twisting angle, but also the twisting direction can be measured. Comparing with the two-core fiber sensor function as the same twisting detection, it is found that the two-core fiber results in fringe pattern much easy to recognized then the grid pattern given by the four-core fiber.