Fiber optic strain sensor: comparison of HiBi fibers

The fundamental mode which propagates in a single mode fiber is actually a degenerate combination of two orthogonally polarized components. In standard single mode fibers, these components travel with the same velocity and so environmental disturbances can cause energy to couple from one component to the other, with the result that the polarization-state of the light varies unpredictably. High birefringence (HiBi) optical fibers are single mode fibers designed to maintain the polarization of the light launched into them to a high degree. This is achieved by introducing birefringence in the core of the optical fiber by prestressing the core or by fabricating the core with an asymmetry. Birefringence causes the two polarized components to travel with different velocities and thus prevents transfer of optical power from one to the other. If linearly polarized light is launched into these fibers along one of the principal stress axes, the state of polarization (SOP) is maintained. However, when linearly polarized light is launched at an angle with the principal axis, the SOP will periodically change from linear to elliptical to linear over a length (beat length) characteristic of the degree of birefringence. In recent years HiBi fibers are finding application in a variety of sensors based on the effect of external stress on the SOP of the output light. The phase difference between the two polarized modes in these fibers can be significantly changed by stretching the fiber. This phenomenon has formed the basis of strain gauge. Here we have evaluated the performance of three different HiBi fibers for sensing axial strain by mounting them on the surface of specimens. In most strain sensing applications, the protective coating over the fibers plays a crucial role in transfer of strain to the fiber core. The effect of the protective coating in each of these fibers is observed. Three fibers were used in our experiment.