Study and mitigation of polarization fading in FBG-assisted direct-detection φ-OTDR

In this paper, the detrimental polarization effects in direct detection fiber Bragg grating (FBG)-assisted phase-OTDR sensing systems are investigated. The detrimental effects result from a mismatch between the states of polarization reflected by two successive FBGs. A numerical analysis is first performed to quantify the polarization dependence of the sensor response. It is shown that the use of standard single-mode and polarization-maintaining fibers could present a non-negligible polarization dependence. The mitigation of the polarization effect when using spun fibers is also quantified. Second, an experimental setup allowing the measurement of the polarization dependence of the sensor response is proposed, and results obtained for standard single-mode, polarization maintaining, and spun fibers are presented. The results showed a good agreement with the numerical data.