Analytical and Experimental Study of Perturbed Abrupt Fiber Tapers and Their Sensing Applications

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Ghasemi, Pourya
abrupt taper , bending , coupling , fabrication imperfection , fiber-optics , single-mode fiber , sensing , strain , single-wavelength , torsion , coupled-mode theory
Optical-fiber sensors are vastly employed in sensing applications due to their immunity to electromagnetic interference, high sensitivity, environmental ruggedness, and compactness. The development of fiber-optics technology introduced many matured sensor technologies, such as fiber Bragg grating (FBG) and long-period grating (LPG). However, the fabrication and interrogation of these sensors are costly and complex. Instead, fiber-taper (FT) can be quickly fabricated with the standard fiber-optics components, namely the single-mode fiber and fusion-splicer. So, FT has been considered as a sensing component in several sensing applications. Despite FT fabrication's convenience, it is prone to some imperfections inducing a bend in its structure, not theoretically addressed in the literature. Thus, in our research, the bent-taper is analytically studied through a new mode-coupling formalism. Moreover, we have established a relation between the bending curvature and the emergence of new modes. Afterward, we optimized our model with the simulated annealing method to match the spectra of several FTs, and a new modal decomposition technique for FTs was developed. We have then statistically examined the modal content of a series of fabricated FTs, showing the excitation of distinct modes due to perturbations. On the application side, the FTs' can be employed to form in-line interferometric fiber-sensors. In contrast, their sensing interrogation needs spectral measurements with such bulky and pricy instruments as broadband source (BBS) and optical-spectrum analyzer (OSA). Alternately, this thesis has proposed a new interrogation setup for these sensors comprising a laser source and a photodetector, significantly reducing future integration cost and complexity. In our work, the sensor's design parameters are conditioned to satisfy a single-wavelength measurement. Then, supporting our theory, several sensors were fabricated and examined for the strain sensing application. The FTs have been reviewed primarily for the refractive index and strain sensing. Nevertheless, in this research, torsion measurement is introduced as a new application of these sensors. So, a setup is proposed combining epoxy with the conventional in-line FT sensors. The epoxy's spectral filtering effect upon the sensor's response is experimentally characterized. The sensor's response under strain and torsion is modelled and experimentally confirmed, exhibiting high sensitivity upon applied tension and torsion.
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