LPFGSLong Period Fiber Grating Sensor
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The capability to discriminate twist direction is an important advantage of LPFGs in comparison to interferometric and standard polarization maintaining fibers (PANDA, Bow Tie).
LPFGs are obtained by periodically sweeping the bare fiber with a C[O.sub.2] laser beam (Synrad, 25 W) focused with a cylindrical lens (working distance of 50 mm) into a vertical line of 3 mm length and 125 fm wide.
A set of LPFGs were fabricated with a 600 fm period and lengths varying between 2.4 mm and 7.2 mm.
In Figure 5, we compare results for LPFGs with different lengths, in the interval from 2.4 mm to 7.2 mm, and sensitivities for the resonance shift are of the same order of magnitude.
An efficient method for fabrication of short-length and highly birefringent LPFGs is presented, demonstrating its effectiveness for torsion sensing applications.
Rebordao, "Repeatability analysis on LPFGs written by a C[O.sub.2] laser," in Proceedings Volume 9286, Second International Conference on Applications of Optics and Photonics; 92863W, Aveiro, Portugal, August 2014.
LPFGs are characterized, among their fabrication parameters (period, length), by the coupling coefficient, [kappa], which describes the optimal coupling, according to
LPFGs created by C[O.sub.2] laser radiation have some particularities regarding the three main physical mechanisms that modulate refractive index and mode coupling: residual stress relaxation, changes in the glass structure, and physical deformation.
Regarding refraction index sensing, most authors report LPFGs' response to the refractive index in terms of resolution, publishing values one or two orders of magnitude higher than those measured by Abbe refractometers.
The LPFGs fabrication technique consists in a focused C[O.sub.2] laser beam (Synrad, 25 W) periodically sweeping the fiber using an automatic system controlled by LabVIEW[C], as described in detail in [8, 45].
A set of LPFGs were fabricated with a 600 [micro]m period and lengths varying between 2.4 mm and 4.8 mm.
According to the coupled-mode theory (Section 2.1), and using a Matlab[C] simulation [46-48], under the conditions previously described for the LPFGs, the coupling was found to occur between the [LP.sub.01] core mode and the [LP.sub.13] azimuthally asymmetric cladding mode (m = 3).