Method of automatic calibration and measurement of the light polarisation plane rotation with tilted fibre Bragg grating and discrete wavelet transform usage

Fibre optic sensors are used to measure various physical quantities, including polarisation plane rotation. Existing solutions for measuring the rotation of the plane of polarisation in optical fibres are based on sensors using tilted fibre Bragg gratings (TFBGs). Articles describing the possibilities of measuring the rotation of the plane of polarisation are generally concepts that show the effect of the rotation of the plane of polarisation on quantities such as the change in optical power of the light transmitted through the TFBG, or the change in the position of the selected minimum of the light spectrum. The only method that allows the measurement of the rotation of the plane of polarisation bases on optical spectrum analysis and requires manual calibration by an experienced operator. The paper proposes a fully automatic method of sensor calibration and processing the signal from a TFBG to measure the light polarisation plane rotation. The method uses the discrete wavelet transform (DWT) to process the light spectrum. An automatic algorithm to choose optimal DWT coefficients to use has been developed. The presented method offers calibration of light polarisation plane rotation angle sensors avoiding the influence of manufacturing imperfections of the measurement system components. In addition, it allows the calibration process to be fully automated without operator involvement. The developed measurement method is also fully automated. It allows measurement of angles of rotation in the range of 0-180, making it possible to distinguish between 0-90 and 90-180 rotation angle ranges without any problems. The mean square error of measurement over the entire range is 0.37 degrees, which is better than that of competing methods. In addition, an independent measurement method operating in the 82-98 rotation angle range is proposed to increase measurement precision in this range.