Abstract
This paper presents the influence of a thin metal layer deposition on the surface of a tapered optical fiber surrounded by a low liquid crystal, on light propagation inside the taper structure. In this research, three types of liquid crystal cells were under investigation: orthogonal, parallel, and twist. They differed by the rubbing direction of the electrodes in relation to the fiber axis determining the initial molecule arrangement inside the cell. Gold films with thickness d = 30 nm were deposited on the tapered fiber surface in the tapered waist area. Cells including a tapered optical fiber with no metallic layer were also examined and presented as a reference. All measurements were performed at room temperature for a different steering voltage U from 0 to 200 V, with and without any amplitude modulation with a frequency f = 5 Hz, and the wavelength λ range from 550 to 1200 nm. As a result, the resonant peaks were obtained, which depends on a liquid crystal cell type and steering voltage, as well. This paper shows the possibility of sensing the change of applied voltage by the constructed system. During measurements, additional effects as signal overlapping and intermodal interference were observed reducing measured voltage value. In the future, the improved, similar systems that will have a better response could be used as a sensor of factors to which liquid crystal (LC) will be sensitive, especially temperature and electric field.
Highlights
Phenomenon of a surface plasmon resonance (SPR) is commonly used in the sensing techniques because of its simplicity and very high accuracy [1]
Dimensions of the frame were chosen for the initial protection of the tapered fiber and enabled a uniform deposition of metallic layer, and the frame, allowing further assembling of the liquid crystal cell
In the first photos obtained for crossed polarizers, it can be seen the plain dark field in the case of the orthogonal and parallel cell and the right one in the case of the twisted cell. It indicates the same arrangement on liquid crystal (LC) molecules in a whole volume of the cell
Summary
Phenomenon of a surface plasmon resonance (SPR) is commonly used in the sensing techniques because of its simplicity and very high accuracy [1]. Configuration, sensors based on Bragg’s grating, optical fiber, and microfiber [4]. The first one is a section of an untapered optical fiber, the region with a diameter gradual change and, subsequently, the region that shows a quite uniform diameter and the same on a reverse order. The geometry of these parts has a significant influence on the light propagation in the tapered fiber. The rate of taper’s diameter change is one of the most important criteria in the manufacturing of good-quality tapered optical fibers. In the case of some tapered fibers, Materials 2020, 13, 4942; doi:10.3390/ma13214942 www.mdpi.com/journal/materials
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