Although experimental advances in the implementation and characterization of fiber speckle sensor have been reported, a suitable model to interpret the speckle-pattern variation under perturbation is desirable but very challenging to be developed due to the various factors influencing the speckle pattern. In this work, a new methodology based on the finite element method (FEM) for modeling and optimizing fiber specklegram sensors (FSSs) is proposed. The numerical method allows computational visualization and quantification, in near field, of changes of a step multi-mode fiber (SMMF) specklegram, due to the application of a uniformly distributed force line (UDFL). In turn, the local modifications of the fiber speckle produce changes in the optical power captured by a step single-mode fiber (SSMF) located just at the output end of the SMMF, causing a filtering effect that explains the operation of the FSSs. For each external force, the stress distribution and the propagations modes supported by the SMMF are calculated numerically by means of FEM. Then, those modes are vectorially superposed to reconstruct each perturbed fiber specklegram. Finally, the performance of the sensing mechanism is evaluated for different radius of the filtering SSMF and force-gauges, what evidences design criteria for these kinds of measuring systems. Results are in agreement with those theoretical and experimental ones previously reported.

A numerical approach based on the scalar finite element method is applied to analyse the modal properties of photonic crystal fibers having a solid core and a cladding region with either circular or non-circular microstructured holes. A correction which accounts for the polarization effects due to the large refractive index difference between silica materials and air holes is included in the analysis. Numerical results show that the proposed technique is an efficient and accurate alternative to vector ones.

Abstract:In this paper, we report some results about the effects of varying the wavelength in a structure of a non-holographic fiber specklegram sensor. In these arrangements, the speckle pattern produced by a multi-mode optical fiber is coupled to the asingle-mode optical fiber with lower numerical aperture, which produces a filtering effect that can be used as an optical transduction mechanism. The influence of the wavelength on the sensor performance is evaluated by changing the laser wavelength, and a strong effect on the linearity and reproducibility of its response is found. Lasers emitting at 1310 nm, 1550 nm, and 1625 nm are used.

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In this work, a technique for temperature measurement using non-holographic fiber optic specklegrams is demonstrated experimentally. In the experimental scheme, a 632 nm laser source is coupled to a mono-multi-mono mode fiber arrangement, which produces a filtering effect that is used as an optical transduction mechanism. Thermal perturbations between 25 0 C and 60 0 C were applied to the fiber array, finding a linear response of the system and a sensitivity superior to the previously reported for studies in similar systems. Due to the low cost and simplicity of the technique, these results are of great interest for the implementation of this type of optical fiber sensors in a large number of engineering applications.