An investigation of forward transmissive quasi-distributed fiber optic sensing for dual parameter measurement

The objective of this research is to experimentally and analytically demonstrate a forward transmissive quasi-distributed fiber sensor that simultaneously measures strain and temperature. An interferometer is constructed of polarization maintaining (PM) fiber that supports two polarization modes of the lightwave. These polarized states act as independent sensors so that two measurands can be detected. Frequency-modulated light is launched into the interferometer's sensing and reference fibers, and the time delay created by path length imbalance at their exit is measured by monitoring the beat frequency of the combined signal of each polarization state. A polarimetric sensing scheme that detects the power exchange between the two polarized states caused by changes in strain and temperature is expected to expand the measurement dynamic range. The measurands are localized along the length of the sensing fiber by tracking the phase of the beat signal. Simultaneous measurement of temperature and axial strain is both numerically and experimentally demonstrated. Individual control of input lightwave polarization and an applied lateral force enhances polarization modal coupling in the sensing arm. Coupled power analysis reveals how the orientation and the magnitude of the lateral applied force affect the sensor response at the coupling location. The results of this analysis are used to adjust the visibility and optimize the sensor response to reduce error.