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.

Fiber-Optic Sensors for Measurements of Torsion, Twist and Rotation: A Review

“…Another MLPFBG PCF based fiber torsion-sensitivity was demonstrated in [58]. An alternative concept for improving sensitivity to twist/torsion in comparison to standard single mode fiber, is fabrication of MLPFBG with rear-earth doped fibers [40]. Reported peak wavelength shifts of the rejection bands were up to 25 nm in this work.…”

“…To design and manufacture an LPFBG torsion sensor, in principle, any known LPFBG inscription method can be used. These include methods of LPFBG fabrication with ultraviolet (UV)-light [33,34], electric arc induced perturbations [35], mechanical corrugation [36], mechanical load (microbending) [37,38,39,40] and application of CO 2 laser induced perturbation [41,42,43,44,45,46,47,48,49,50,51,52,53]. …”

“…Thus, coupling to the cladding modes in the MLPFBGs can become polarization dependent as, for example, demonstrated in [56] by Polarization Dependent Loss (PDL) and Differential Group Delay (DGD) measurements. Twist/rotation sensors using an MLPFBG sensor can also be built by using a variety of different fibers as, for example, step-index SM fibers [37], dispersion shifted fibers [57], photonic crystal fibers [58] and doped SM fibers [40]. The use of different fiber types requires proper tailoring of micro-bending deformer and mechanical setup of each fiber type.…”

A Review on Optical Fiber Long Period Grating, its applications in Optical Communication System

“…Thus as order of mode coupling m increases grating period Λ decreases [35]. decreases as modes increases [6].…”

“…The cladding modes are lossy and can be easily attenuated by introducing a bend. As a result the conventional transmission spectra of an LPG consists of a series of attenuation bands, each corresponding to coupling from the propagating core mode to a particular cladding mode [6]. Defining   as the difference between the two propagation constants co .…”