Spectral domain demodulation of fibre optics position and displacement sensors by Fourier-transform spectral interferogram

A fibre optics position and displacement sensor demodulated in spectral domain by Fourier-transform spectral interferogram (FTSI) is described. The approach is based on the application of a transverse point-like force on the polarization maintaining fibres (PMF) that causes coupling between the two polarization eigenmodes and utilizes their spectral interferogram. The sensor employs a Michelson white-light interferometer (WLI) and an optical spectrum analysis (OSA). Changing the position of the coupling point or moving the movable mirror will change the phase function of the interferogram. A FTSI is applied to obtain the group time delay of the two modes, and then the position variation of the coupling point or the displacement of the movable mirror can be calculated from the group time delay. Experiment results are presented for position measurement over a 7.2 millimetre range with a measurement precision of 2.87μm.