# Polarization Influence on Algorithms of TFBG Sensors Data Analysis for Bending Application
## 2 Materials and Methods  
The measurement setup was built in a way that allowed the bending radius of the optical fiber to be changed in the range from 15 to 50 mm. At the same time, the use of a half-wave plate allowed us to change the angle of the polarization plane of the introduced light in the full range from 0 to 360 degrees. The system allows for recording the spectrum of light generated by a broadband source, transmitted through an optical fiber with a recorded tilted fiber Bragg grating. A fragment of the fiber with inscribed periodic structure was placed in the central part of the arc created by the optical fiber placed between sliding tables with a variable distance.  
Measurements were conducted by using laboratory-class equipment. Tilted fiber Bragg gratings used for experiments were inechived in photosensitive Ge-doped fiber by using the phase mask method. The fiber was illuminated by 248 nm excimer BraggStar laser. The experimental setup was placed in the NEXUS vibroisolating table to provide the isolation from additional interference from the ground vibrations and consisted of (according to Figure 1a): 1--Thorlabs SLED broadband light source; 2, 3--optical lenses; 4--polarizer; 5--half-wave plate; 6--single mode optical fiber; 7--optical fiber with a TFBG inscribed in its core; 8--motorized translation stage; 9--fixed mount; 10--optical spectrum analyzer; 11--PC computer.  
Figure 1b presents a detailed view of the optical fiber with a TFBG inscribed in its core, which was formed in the arc and placed between two mounts with adjustable distance marked with the letter L. Decreasing the distance created an arc with a smaller radius so the curvature was stronger. This setup presents one of the possible applications of TFBG bending sensors--measurement of distance between objects.  
The light from source was transmitted by optical fiber through the lens to create the parallel beam, which was directed to polarize, thereby resulting in strong linear polarization of light. The beam was then transmitted through a half-wave plate mounted in motorized rotation stage, allowing for precise adjustment of the angle of the light polarization plane. Modified light was focused by a second lens to be introduced into second fiber, which had a TFBG inscribed in its core and was formed to the arc with an adjustable radius. The curvature of the fiber with an inscribed TFBG was modified by changing the distance between the two plates, as shown in Figure 1b. One of the plates was fixed and the second one was mounted at a motorized translation stage which provided precise control of the distance, and as a result the radius of the fiber curvature. Figure 2 shows two transmission spectra of a TFBG with a \(2^{\circ}\) internal tilt angle.  
Figure 1: Schematic view of experimental setup: (**a**) main view; (**b**) detailed presentation of the translation stage for bending radius adjustment.  
The spectra presented in Figure 2 show that the different cladding modes of TFBG reacts with different change of their spectral parameters. It can be seen that the strongest change was observed for the so-called low order cladding modes, which have the longest wavelengths, next to the Bragg wavelength. Figure 2b presents the spectrum measured in the range in which the amplitude and the wavelengths of resonances was the most changed. This is a reason that the algorithms used for the demodulation of bending radius should be based on the analysis of the part of the spectrum. Appropriate selection of the wavelength range should result in the improvement of the sensitivity and accuracy of the developed methods.