A torsion sensor made of a corrugated long period fibre grating

Based on the photoelastic effect, the strain distribution within a corrugated long period fibre grating (LPFG) subjected to tensile stress and twisting will cause a periodic index variation, which may result in resonant couplings between fibre core and cladding modes. The resonance wavelength corresponding to the peak loss is directly sensitive to the torsional angle per unit length; thus a novel torsion sensor can be made from the corrugated long period fibre grating. We also present a phenomenological theory based on the scattering matrix formalism to qualitatively explain the mode coupling behaviour in the corrugated grating.