Determination of SRI values using TFBG and PTFBG based on the wavelength shift of modes, mode groups, and differential methods, considering the influence of polarization.

A tilted fiber Bragg grating (TFBG) is sensitive to several physical quantities, which results in cross-sensitivity. The polarization of the incoming light influences the surrounding refractive index (SRI) measurement. In this paper, we analyze the effect of polarization on mode shift and the possibility of minimizing it using the differential shift method. For the second algorithmic method, the average shift of a mode group and the differences in the shifts between the two mode groups are determined. The calculations were based on experimental measurements. An effective reduction in the polarization effect can be achieved using a perpendicular TFBG (PTFBG) grating system. In the PTFBG-compensated system, the maximum and average polarization errors were reduced by more than 10-fold. An additional advantage of the proposed differential methods is the ability to achieve temperature-independent SRI measurements despite the lack of visibility of the core mode for systems with larger TFBG angles.