Application of Cascaded TFBG for Wavelength-Shift-Based SRI Measurement with Reduced Polarization Cross-Sensitivity

The wavelength shift in TFBG cladding resonance is a practical parameter due to its independence from power fluctuations coming from the light source or fiber bends. It is possible to select the cladding modes that are characterized by the center wavelength shifts while changing the refractive index of the surrounding medium. In the case of a single TFBG, this parameter is strongly dependent on the input light polarization angle. In this paper, we present the possibility of reducing interference of polarization changes to measure the refractive index by using the wavelength shift in Bragg grating cladding modes with the cascade structure called the perpendicular TFBG (P-TFBG). The wavelength shift in the selected cladding mode was analyzed and compared in the case of a single grating and proposed cascade. In the case of P-TFBG, the dependence on the corresponding cladding mode of a single TFBG with the same inscription parameters is reduced to 16.15%. The analysis of mode wavelength instead of the previously reported amplitude provides a wider measurement range of possible SRI and protects the results from power fluctuations.