Torsion sensing characteristics of fibre ring laser based on nonlinear polarisation rotation

A fibre active sensor based on fibre laser using nonlinear polarisation rotation is proposed and experimentally demonstrated. The laser wavelength strongly depended on polarisations, and the torsion sensing characteristics were investigated by twisting a section of polarisation-maintaining fibre which was incorporated into the laser ring cavity. It was observed that the change of laser wavelength with twist rate contained two stages. The laser wavelength had quasi-linear and periodic relationship with the twist rate in the first and second stage, respectively. There was a linear region in each period of the second stage. Utilising the active sensing method can realise high sensitivity torsion sensing measurement in the linear regions.