Abstract
A fiber reshaping-based refractive index (RI) sensor is proposed relying on both optical intensity variation and wavelength shift. The objective of this study is to completely reshape the core and to ultimately mimic a coreless fiber, thereby creating a highly efficient multimode interference (MMI) coupler. Thus, propagation modes are permitted to leak out into the cladding and eventually escape out of the fiber, depending on the surrounding environment. Two interrogation mechanisms based on both the intensity variation and wavelength shift are employed to investigate the performance of the RI sensor, with the assistance of leaky-mode and MMI theories. By monitoring the output intensity difference and the wavelength shift, the proposed RI sensor exhibits high average sensitivities of 185 dB/RIU and 3912 nm/RIU in a broad range from 1.339 to 1.443, respectively. The operating range and sensitivity can be adjusted by controlling the interaction length, which is appealing for a wide range of applications in industry and bioscience research.
Highlights
Refractive index (RI) sensors have been the subject of a significant number of investigations with the intention of measuring the RI and concentration of liquids or gases for diverse applications in industry and bioscience research
D-shaped fibers [12,13,14,15]; waist-deformed fiber tapers fabricated by heat or chemical tapering [16,17,18]; heterostructures formed by the splicing of hetero-core fibers, including multimode fiber (MMF)-single-mode fibers (SMFs)-MMF [19], SMF-MMF-SMF [20], SMF-tapered claddingless fiber-SMF [21], SMF-hole-assisted dual-core fiber-SMF [22], and cascaded single-mode-no-core-hollow-core-no-core-single-mode structures [23]
Admitting that the current work presented in this paper is preferentially focused on the theoretical design of a RI sensor, its fabrication might be realized by translating the fiber under the focused femtosecond laser beam exhibiting a visible central wavelength, as in the case of ref. 26 in which the central wavelength, pulse energy, repetition rate, and translating speed are 520 nm, 3 μJ, 200 kHz, and 0.2 mm/s, respectively
Summary
Refractive index (RI) sensors have been the subject of a significant number of investigations with the intention of measuring the RI and concentration of liquids or gases for diverse applications in industry and bioscience research. A substantial number of fiber-optic designs for RI sensing have been presented These include tapered multimode fiber (MMF) tips [3,4]; bent fibers incorporating a plastic. For the heterostructure-based designs, in order to facilitate the interaction between the core waveguide and surrounding medium, the cladding of the sensor portion may be fully or partly removed through additional processes, such as chemical etching [24] or femtosecond laser ablation [25]. Sensors 2019, 19, 2477 was recently reported by introducing a line of negative RI modification, which partly overlaps with the fiber core over an interaction length of mm scale. A fiber reshaping-based RI sensor featuring a high sensitivity is proposed and designed, allowing for both direct detection of intensity variation and wavelength shift simultaneously. To verify the feasibility and flexibility of the RI sensor, several SMFs with different core diameters and NAs are considered, with the help of a simulation tool, RSoft BeamPROP (Synopsys)
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