This paper presents a fiber-optic quasi-distributed in-line twist/rotation sensor. The presented sensor consists of a polarization maintaining lead-in fiber and series of in-line fiber polarizers with integrated semi-reflective mirrors that are interconnected by a standard single mode fiber. Sections of interconnecting single mode define multiple twist/rotation sensitive sensor segments, which can be interrogated individually by a simple optical time domain reflectometer. The presented system exploits the inability of the standard single mode fiber to change the polarization state, i.e. E-field vector orientation of a wave propagating down twisted/rotated fiber. Thus, twisting of the fiber segments between consecutively located polarizers causes modulation of the transmitted optical power along each sensor segment, which is further determined by observation of back-reflected signals from semi-reflective mirrors integrated together with in-line polarizers. A quasi-distributed sensing system with nine sensing segments is demonstrated with twist/rotational resolution better than 0.3 degrees and low cross-talk. The entire system is built out of a small number of simple and efficient optoelectronic components.
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