Study of optical fiber in-line sensors based on inner air cavity

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Study of optical fiber in-line sensors based on inner air cavity


Author: Hu, Tianyi
Title: Study of optical fiber in-line sensors based on inner air cavity
Degree: M.Phil.
Year: 2014
Subject: Optical fiber detectors
Fiber optics
Hong Kong Polytechnic University -- Dissertations
Department: Dept. of Electrical Engineering
Pages: x, 89 leaves : illustrations ; 30 cm
Language: English
InnoPac Record:
Abstract: Intrinsic fiber in-line sensors have attracted significant attention recently for their compact size, low cost and reliability. A number of methods to fabricate intrinsic fiber sensors have been reported. One of them is to fabricate fiber sensors based on inner air cavity. By using femtosecond (fs) laser micromachining and fusion splicing technique, we could fabricate fiber inner air cavity and control the morphology of the cavity. Parameters controlling on fabrication process of the fiber in-line cavity have been investigated theoretically and experimentally. The size of the air cavity could be controlled from 30 μm to 93 μm during the fabrication process. In this thesis, four fiber sensors based on air cavity by using fs laser micromachining and fusion splicing technique would be proposed. For a refractive index (RI) sensor use, we designed and fabricated an intrinsic Fabry-Perot interferometer (FPI) sensor based on inner air cavity. The interferometer cavity was formed by drilling a micro-hole at the cleaved fiber end facet, followed by fusion splicing. Two channels were drilled by fs to vertically cross the cavity to allow liquid to flow in and out. This device exhibits sensitivity of ~980 nm/RIU to ambient RI and low temperature cross sensitivity of ~4.8 × 10⁻⁶ RIU (refractive index unit) /°C.
We also developed a miniature fiber in-line Mach-Zehnder Interferometer (MZI) for high-temperature sensing at precise location. This MZI was based on an inner air cavity adjacent to the fiber core. After fabricating the air cavity adjacent to fiber core, a microstructure was fabricated on the surface of the cavity by fs laser beam scanning to eliminate an unwanted set of interference fringe. Such a device is robust and insensitive to ambient RI change, and has high temperature sensitivity of ~43.2 pm/°C up to 1000°C, and low cross sensitivity to strain. A platinum (Pt) -doped WO3 composite coated fiber device based on fiber inner air cavity with periodical microstructures was proposed for hydrogen sensing. The fiber core was structural modulated by the microstructures, making this optical device sensitive to ambient RI change. Coated with Pt-doped WO3 composite, whose RI is changed through the variation of hydrogen concentration, this optical device became sensitive to hydrogen. Such a device is compact and exhibits a high sensitivity as well as a low temperature cross-sensitivity. Based on dual internal mirrors formed by air spherical cavity surfaces, we also demonstrated another miniature fiber in-line cross-talk free MZI for simultaneous sensing for RI, temperature and curvature. We could monitor (1) RI by tracing output fringe visibility; (2) temperature by tracing fringe dip wavelength; (3) and curvature by tracing fringe dip intensity, respectively. No crosstalk between every two of these sensing results was found and no signal processing was required for this sensor. Such an inner structure-based fiber device is miniature, robust, suitable for high-accuracy measurement, sensitive to the surrounding environment, and free of temperature cross sensitivity, thus exhibiting high potential in versatile photonic applications.

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