Fourier transform usage to analyse data of polarisation plane rotation measurement with a TFBG sensor

The paper presents a method of measuring the angle of light polarisation plane rotation. Measurement is done with a tilted fibre Bragg grating (TFBG), with a tilt angle of 7°, and an optical spectrum analyser. Data obtained with the analyser are processed with a Fast Fourier Transform (FFT) to obtain frequency representation (FFT coefficients). The rotation angle is calculated by comparing these coefficients obtained from the measurement with the ones collected during measurement set calibration. It has been shown that FFT coefficients change in the function of polarisation plane rotation and, in the case of some of them, these changes have a regular character and can be used to determine rotation. The method shown works in the range of 0 – 180° of rotation with an average error of 0.076° and a median error of 0.033°. The highest values of errors appear at about 0, 45, 90, 135 and 180°, which is caused by flat characteristics of many frequencies for these angles of rotation. The method discussed could find applications in many fields of structure monitoring and maintenance, where rotation or twist could be used as a quality parameter.