This paper presents a miniature, all-fiber fiber optic twist/rotation sensor. The proposed sensor consists of polarization maintaining lead-in fiber (PMF), a short section of standard (fully circularly symmetric) single-mode fiber, which is exposed to twist/axial rotation, an all-fiber quarter-wave wave plate, and a reflective layer. This sensor exploits the inability of a short-section circular symmetric single-mode fiber to change the polarization state or spatial E-field vector orientation of an optical wave when exposed to mechanical twist/rotation. Thus, the twisting of a single-mode fiber in the proposed configuration causes displacement of the lead-in PMF and wave-plate axis. This enables encoding of the twist angle into a power ratio of the leadin PMF LP modes that is propagated in a backward direction by the application of a wave plate, which can be straightforwardly measured by a simple interrogation system. The proposed design allows for compact sensor design with sensors active lengths below 5 mm. The unambiguous angular measurement range of the proposed sensor is 45°. An angular resolution better than 0.0355° was demonstrated, while the temperature sensitivity proved to be less than 0.011 °/C. In order to achieve low temperature sensitivity of the sensing system, a combination of different PMFs was used to eliminate the temperature sensitivity of the PMF based wave plate, which is the main source of temperatureinduced errors in the presented setup.
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