Low-cost measurement of light polarisation plane rotation using dual-channel tilted fibre bragg grating–fibre bragg grating power-based interrogation

This article presents a new method for measuring the angle of rotation of the light polarisation plane (LPPR) using a dual-channel power-based approach with tilted Bragg gratings (TFBG) and Bragg gratings (FBG), and optical power measurement.Unlike previous solutions based on the analysis of the full transmission spectrum of the TFBG, the proposed method does not require the use of an optical spectrum analyser.The measurement system consists of two optical paths containing TFBGs positioned at a 45° angle to each other, working in conjunction with FBGs acting as spectral filters.The optical power values recorded in both channels are used to determine the angle of rotation of the polarisation plane based on previously determined calibration characteristics approximated by a Fourier series.An analysis was carried out of the effect of the relative positions of the TFBG and FBG spectra on the amplitude of power variations and the distortion of measurement characteristics.It was demonstrated that the appropriate selection of the FBG reflection band position relative to the selected TFBG cladding mode allows the amplitude of the measurement signal to be increased whilst maintaining low distortion of the characteristics.The effectiveness of the developed method was experimentally verified in the range of angles from 0° to 180°.For an independent test set, a mean absolute error of 0.27°, an RMSE of 0.33°, and a maximum error not exceeding 0.76° were obtained.The coefficient of determination reached a value of R² = 0.999.The results confirm that the proposed method enables precise measurement of the angle of rotation of the plane of light polarisation whilst significantly simplifying the measurement setup, reducing measurement time and lowering costs compared to solutions requiring the recording of the full optical spectrum.