Optical Fiber Torsion Sensor with Mechanically Induced Long Period Fiber Gratings in Rare-Earth Doped Fibers

Abstract

In this work wavelength sensitivity in mechanically induced long period fiber gratings (MLPFG) is analyzed. This analysis is first carried out both in standard single-mode fiber SMF-28 and in Er-doped fibers. The mechanical analysis for both types of fibers under different torsion conditions is presented. In order to apply the torsion one of the fiber ends is fixed while torsion is applied on the other end. A MLPFG whose period is 503 µm is used to press the fiber after torsion is applied. This allows for micro curvatures to be formed on the fiber, which in turn generates a periodical index perturbation on it. Here, it was noted that the sensitive wavelength shift of the rejection bands is bigger for Er-doped fibers. For a torsion of 6 turns applied to 10 cm of doped fiber the wavelength peaks can be moved up to 25 nm, which is longer to what was detected on standard fibers. Therefore, by using Er-doped fibers to monitor torsion on structures will give more sensitive and accurate results than using standard fibers. These results can be employed for sensing applications, especially for small to medium size structures, which can be mechanical, civil or aeronautics.