Torsion sensor based on a left- and right-handed helical sampled fiber Bragg grating pair inscribed by femtosecond...

In this Letter, we propose a torsion sensor based on a serially cascaded left- and right-handed helical sampled fiber Bragg grating pair (L&R-HSFBG-Pair) inscribed in a single-mode fiber (SMF) using point-by-point (PbP) femtosecond laser technique. Owing to their opposite response trends under torsion, the sensor can discriminate the twist direction and enable differential interrogation. The twist rate is quantified by monitoring the reflected peak power at the Bragg wavelength, which shows negligible dependence on temperature and axial strain within the tested ranges, thereby avoiding the need for additional temperature-compensation components. Differential interrogation of the two reflected peaks further improves the effective torsion sensitivity while suppressing common-mode fluctuations in light-source power. This sensor exhibits a torsion sensitivity of 0.25 dB/(rad·m^-1), which corresponds to 0.071/(rad·m^-1) after normalization. The proposed sensor therefore shows strong potential for torsion monitoring in harsh environments subject to temperature fluctuations and electromagnetic interference.