| Summary: | 碩士 === 國立聯合大學 === 光電工程學系碩士班 === 105 === It is well known that optical fiber is made of fused silica, SiO2, which is a dielectric material and does not produce electric charges by itself. The fused silica is different from glass and crystalline silica in the periodicity length scale of [SiO4]4- tetrahedron and the ordering is in the preference of glassy network to form rings of 6 tetrahedra . A simple way to make optical fiber carry immobile positive or negative charges is to rubber it with silk or wool, respectively. However, the static charges in fiber can not last for a long time in the open air due to humidity. In addition, it is important to note that silica molecule has a tetrahedron unit with four oxygen atoms at the corners to surround the central silicon atom in a cubic symmetry
The angle of O-Si-O bond is about 109.5um and thus the Si-O bond is highly polar due to large electronegativity difference and the electrons are more strongly attracted by oxygen to result in uneven charge distribution. The central part and the four corners of tetrahedron is more positively and negatively charged, respectively, to generate electric dipole along each Si-O bond. However, for tetrahedron, the four dipoles point at different directions to cancel each other and thus no net dipole is created in this structure . Macroscopically, silica fiber is amorphous and electrically neutralized. This natural property changes a lot when the tetrahedron is deformed by stress including micro tensile and compressive forces along a certain direction so that the four dipoles are not be able to completely cancel. Consequently, the silica wire is electrically polarized to carry unbalanced electric charges. To make standard silica fiber with a 125um-diameter cladding polarized is difficult due to the thick thickness. However, when fiber is thinned down to a wavelength scale by flame tapering or chemical-etching, microscopically, the local unbalanced electric dipoles are automatically created. This is because the silica tetrahedron networks were originally omnidirectional symmetric in 3 dimensions but were further transformed into a structure more like a 1 dimensional wire. The created electric dipoles coming from hugely asymmetric fiber structure along transverse and axial directions can attract external small objects like paper and cotton pieces and the attracting force increases with decreasing fiber diameter. Interestingly, the fiber is cylindrically negatively charged on its surface because the Si4+ atom is always surrounded by oxygen atoms. This electrostatic attractive force between permanent dipoles in fiber and in external micro particles is one kind of van der Walls force. This interesting phenomenon has recently been proposed to make fiber gratings on micro fiber by attracting micro positively charged smoking particles to periodically cluster on micro fiber . Since the micro fiber can be electrically polarized, it can thus be controlled by an external electric source.
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