In-line tapered fiber Mach-Zehnder interferometer for biosensing applications

Alharbi, Maryam (2018) In-line tapered fiber Mach-Zehnder interferometer for biosensing applications. Masters thesis, Memorial University of Newfoundland.

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Abstract

Due to the advantages of compact size, light weight, immunity to electromagnetic interference and remote sensing, optical fiber sensors have been studied extensively since the 60s. In particular, interferometric Mach-Zehnder fiber sensors are favourable for their flexibility and high sensitivity. The principle of operation for these sensors bases upon the interference between the light propagation in the core mode and the excited cladding modes. The work presented in this thesis focuses on the fabrication of tapered Mach- Zehnder interferometer in a single-mode fiber and its applications in biosensing. In this study, two kinds of tapered fiber Mach-Zehnder interferometer, either symmetric or asymmetric, have been fabricated using fusion splicing technique. In both cases, the effects of varying the waist diameter, separation distance between tapers, and propagation direction are studied. The symmetrical tapered fiber Mach-Zehnder interferometer has been explored for biosensing applications. Using the dipping layer-bylayer method, multiple thin films have been deposited on the fiber structure for the detection of streptavidin which is a common target material used to test the effectiveness of a biosensor. The sensing mechanism here relies on the electrostatic attraction between cationic and anionic materials, in which the cationic material used in this study is poly (allylamin hidrocloride) (PAH). While, the anionic material adopted here is either SiO₂ core/Au shell nanoparticles or poly (sodium 4-styrenesulfonate) (PSS). For the best of our knowledge, this is the first time the tapered fiber Mach-Zehnder interferometer is used for streptavidin sensing by applying the layer-by-layer technique. Two types of multilayered structure are fabricated on the tapered region of the fiber interferometer. The first multilayered structure consists of SiO₂:Au nanoparticles, in which the deposition of a monolayer of the PAH polymer and a monolayer of the negatively charged SiO₂:Au NPs represents a single bilayer of (PAH/SiO₂:Au NPs). The second multilayered structure bases on the deposition of PAH and PSS polymer. Following the fabrication of the multilayered structures is the functionalization with biotin which is a vitamin that possesses a strong binding ability with streptavidin so it functions as an adhesive material to streptavidin. The sensing response of the sensors on detecting aqueous solutions of streptavidin has been observed by measuring the shift in the wavelength of the transmission spectrum of the tapered fiber interferometer. The effects of depositing (PAH/SiO₂:Au NPs) and (PAH/PSS) films in multilayered structures, as well as the influences of different device specifications are investigated.

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