Influence of polarization on demodulation methods for determining the SRI value for a single TFBG and an orthogonal grating system

• Published in: Optics express 2024-11, Vol.32 (24), p.43264
• Main Authors: Cięszczyk, Sławomir, Harasim, Damian, Skorupski, Krzysztof, Panas, Patryk, Klimek, Jacek
• Format: Article
• Language: English
• ISSN: 1094-4087; 1094-4087
• DOI: 10.1364/OE.540761

This paper presents a comparison of the spectra of single and cascade of two tilted fiber Bragg grating (TFBG) sensors. These sensors are characterized by high sensitivity to the refractive index. The disadvantage of a single TFBG is cross-sensitivity to changes in the polarization of the light introduced to the sensor, which is visible both when analyzing individual modes and when analyzing a wide spectral range with global demodulation algorithms. In response, a cascade structure was created consisting of a set of two TFBGs connected in series with 90 degrees of mutual rotation between them. This work presents a comparison of the polarization cross-sensitivity of sensor spectral demodulation algorithms for measuring the surrounding refractive index (SRI). While maintaining the high accuracy of the SRI measurement typical of TFBG gratings, a cascade of two such gratings properly oriented toward each other allows for a significant reduction in the sensitivity of the SRI measurement to polarization changes. Importantly, the reduction in the polarization cross-sensitivity is significantly greater for the demodulation methods that determine the cutoff wavelength shift; owing to this, a perpendicular TFBG(P-TFBG) measurement system without polarization control remains highly reliable. The application of the proposed P-TFBG system also allows greater SRI sensitivity to be obtained for a single tilted grating with the same tilt angle.