Temperature- and strain-independent torsion sensor utilising polarisation-dependent loss of Hi-Bi FBGs

A temperature- and strain-independent torsion sensor based on analysis of polarisation-dependent loss (PDL) characteristics of high-birefringent (Hi-Bi) fibre Bragg gratings (FBGs), is proposed and demonstrated. Unlike the amplitude spectra, the PDL response has higher sensitivity to torsion. Experimetal results demonstrate that the difference between the two peaks in the PDL spectra has a linear relationship with the twist rate, but remains invariant with changes in temperature and strain. In addition, it was observed that the PDL responses to the clockwise twist are just opposite of the responses to the couterclockwise twist.