Fiber-linked interferometric displacement sensor: analysis of residual sensitivity to fiber stress

In this paper an accurate analysis of the sensitivity to fiber-stress of a fiber-linked interferometric displacement sensor is presented. A numerical evaluation has been performed, aimed at evidencing the influence of (i) the nonideal behavior of the fiber, (ii) the nonideal polarization of the laser source, and (iii) the fiber-ends misalignment. An experimental verification of the model, based on measurements carried out on interferometric signals from a prototype of the system, is also presented. Both the incremental operating mode and the subfringe measurement mode of the interferometer have been considered. In the incremental mode, based on the interferometric fringe transition counting, the implemented system is insensitive to fiber stress for a few degrees of fiber-ends misalignment. However, for high-accuracy measurements based on subfringe measurements, in order to reduce the sensitivity to fiber stresses, the input fiber end has to be precisely aligned to the head containing the laser source and detectors. It is also shown that, due to the uncertain behavior of the real fiber and to the nonperfect polarization of the laser, also in the case of ideal fiber-end alignment, a residual sensitivity of the system to fiber stress is still present. This sensitivity is significant only for subfringe measurements.