Evanescent and plasmonic sensing using linear and radial polarization modes in tapered microfibers

Date of Award


Degree Name

M.S. in Electro-Optics


Department of Electro-Optics and Photonics


Advisor: Joseph W. Haus


We present the optical transmission results for biconic tapered optical fibers using coupled mode theory (CMT) and perturbative analysis. In this area of research, much of the theoretical and empirical studies on biconic fiber tapers for sensor applications have focused primarily on the behavior of the quasi-linear polarized hybrid modes (HE1ℓ ,EH1ℓ ). In this thesis, we begin by examining the properties of the quasi-linear polarized modes. That is a prelude to our study of radial (TM0ℓ ) and azimuthal (TE0ℓ ) polarized modes in biconic tapered fibers. We performed a detailed comparison of hybrid, radial, and azimuthal mode behavior for tapered fibers down to various fiber waist radii. The comparison study was divided into two parts. First we looked at the differences in design and optimization of the taper regions to retain the launched mode as the primary mode in the fiber. Our main goal in designing an adiabatic taper was to minimize the power exchange to higher order modes. To quickly obtain results for the fiber output, we modeled the amplitude of the modes propagating in a tapered fiber as a set of coupled differential equations, and numerically solved the system. We confirmed the efficacy of the model by comparing our simulations with experimental data. Coupled mode analysis of these types of tapers revealed that the radial/azimuthal modes require taper lengths on the order of tens of centimeters to suppress coupling energy to higher order modes. The adiabatic taper lengths found for the radial/azimuthal modes were in stark contrast to those of the hybrid modes, which required only tens of millimeters. Our CMT approach was then successfully used to design non-adiabatic tapers, a necessity for standard compact biconic sensors. The additional losses were quantified for cases of non-adiabatic tapers, where short taper sections were fabricated in the lab. Initial experimental results comparing transmission for EH-HE modes are shown and compared with our simulations. The second portion of our study was to discover whether the tapered fibers and the waist region between the two tapers would be useful for sensing analytes near the fiber's surface. Our investigation of both hybrid and radial/azimuthal polarized modes revealed that sensors benefited significantly from the latter modes higher energy stored outside the fiber in the evanescent regime for the same fiber radius. This was especially true for small fiber radii. We found that fiber sensors designed using the TM-TE modes would generally have higher sensitivity than those using EH-HE modes. At the cost of introducing propagation losses for the modes, we added plasmonic nano-films on the surface of the waist region of the tapered fibers to draw the fields from the glass into the evanescent regime on the surface. The coupling losses incurred from the transition to and from the waist region, were addressed within a CMT framework. The losses due to absorption were analyzed within a perturbative framework. At longer wavelengths, the field amplitude in the metal for radial polarization is suppressed due to a boundary condition effect, and the mode can propagate further along the fiber waist. At the same time the plasmonic nano-film creates more field energy extending into the evanescent region outside the fiber that improves the sensitivity of the biconic taper sensor. Again, we found that in the TM modes generally have more energy displaced to the outside than the EH-HE modes, which makes them interesting candidates for future fiber sensor designs.


Optical fibers Testing, Detectors Design and construction, Mode-coupling theory, Optics; biconic tapered fibers; TM; TE; radial modes; azimuthal modes; plasmonic fibers; metallic thin film; evanescent sensing

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Copyright © 2013, author