Abstract
We demonstrate high sensitivity fiber refractive index (RI) sensor based on asymmetric supermode interferences in tapered four core fiber (TFCF). To make TFCF-based RI sensors, the whitelight was launched into any one of the cores to define the excitation orientation and is called a vertex-core excitation scheme. When the four-core fiber (FCF) was gradually tapered, the four cores gathered closer and closer. Originally, the power coupling occurred between its two neighboring cores first and these three cores are grouped to produce supermodes. Subsequently, the fourth diagonal core enters the evanescent field overlapping region to excite asymmetric supermodes interferences. The output spectral responses of the two cores next to the excitation core are mutually in phase whereas the spectral responses of the diagonal core are in phase and out of phase to that of the excitation core at the shorter and longer wavelengths, respectively. Due to the lowest limitation of the available refractive index of liquids, the best sensitivity can be achieved when the tapered diameter is 10 μm and the best RI sensitivity S is 3249 nm/RIU over the indices ranging from 1.41–1.42. This is several times higher than that at other RI ranges due to the asymmetric supermodes.
Highlights
For those refractive index (RI) sensors using four-core fiber (FCF), the laser lights were launched into the center of FCF, namely the central silica area surrounded by the four cores, and tapered [32]
The asymmetric supermode interferences based on the tapered four core fiber (TFCF) in the vertexcore excitation scheme were demonstrated to achieve high sensitivity RI sensors
This can be verified to explain that the spectral responses of the diagonal core c#4 are in-phase and out-of-phase relative to that of the excitation core c#1 at the short- and long-wavelength side of 1450 nm, respectively
Summary
Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations. If the lights are launched into one of four cores in the FCF, the optical characteristics of the fiber sensors are very different from the above-mentioned excitation method from the silica center These sensors using FCFs were not tapered to excite the supermodes to investigate the interferences. It is superior to most of the RI sensors using other kinds of special fibers [14,37,38] This asymmetric supermode interference based on the vertex-core excitation scheme was found to be helpful in improving the S and exploring the underlying physics for developing TFCF interferometric sensors. It is highly promising for fiber sensors with multi-parameters monitoring simultaneously. These formulas can help explain the working principles for RI sensors
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