Torsion Sensor Using a High-Birefringence Nine-Hole Optical Fiber

This paper presents a novel high birefringence fiber-based torsion sensor based on a microstructured optical fiber with nine holes and seven cores (MHCF) embedded into a Sagnac interferometer. A segment of this fiber is inserted into a symmetric SMF-MMF-MHCF-MMF-SMF arrangement, which provides efficient coupling to the multiple cores of the birefringent fiber and, consequently, multimode interference. Fast Fourier Transform (FFT) analysis of the spectral data is employed to enhance measurement stability and reduce dependence on optical source variations. The sensor demonstrates a linear response to torsion angles between -50° and +50°, with a sensitivity of 16 mrad/°. The high sensitivity and good linearity of the sensor are enhanced through the application of machine learning techniques.