In recent years, with the potential advantages of compact size, high resolution, low cost and light weight, fiber optic devices such as fiber optical gratings have attracted increasing attention for twist sensing applications. In principle, fiber-based torsion sensors are divisible into two classes: the sensors based on long-period gratings (LPGs), and the sensors based on fiber Bragg gratings (FBGs). However, the former are fragile and suffer from large cross sensitivity of the axial strain and temperature, the latter are expensive and have relatively low torsion sensitivity.

To solve the above problems, PhD student, Bo Huang and Prof Xuewen Shu from Wuhan National Laboratory for Optoelectronics developed a novel temperature- and strain-independent optical fiber torsion sensor based on a phase-shifted FBG fabricated with the femtosecond-laser line-by-line inscription technique. By simply monitoring the variation of the amplitude difference between the two polarization-peaks, the fiber torsion angle and the fiber torsion direction can be simultaneously deduced without temperature and strain confusion. The torsion sensor exhibits a high torsion sensitivity of up to -1032.71 dB/(rad/mm), with the distinct advantages of low manufacture cost, small dimension (just ~1.72mm), and extremely robust and simple structure, which make it very attractive for practical applications. To the best of our knowledge, this is the smallest torsion sensor ever reported.

Fig. 1. (a) Transmission spectra evolution of π-PSFBG under torsion in clockwise direction from 0° to 80° in an elevation step of 20°. (b)Measured P1-P2 under torsion in clockwise and anticlockwise directions from 0° to 360°.

Fig. 2. For 0°, 30° and 60° twisted PSFBG, the measured P1-P2 under different (a) temperature and (b) strain levels.

On August 8, 2016,this work “Ultra-compact strain- and temperature-insensitive torsion sensor based on a line-by-line inscribed phase-shifted FBG” was published on Optics Express(vol. 24, no. 12, pp. 17670-17679, 2016).

Prof. Shu’s group recently also developed a novel passively mode-locked fiber laser based on a chiral fiber grating. The work “All-fiber passively mode-locked laser based on a chiral fiber grating” was published on Optics Letters (Vol. 41, No. 2, pp.