Abstract
We study the coupling between the fundamental guided modes of two identical parallel nanofibers analytically and numerically. We calculate the coefficients of directional coupling, butt coupling, and self coupling as functions of the fiber radius, the light wavelength, and the fiber separation distance. We show that, due to the symmetry of the system, a mode of a nanofiber with the principal quasilinear polarization aligned along the axis joining the nanofibers or the perpendicular axis is coupled only to the mode with the same corresponding principal polarization of the other nanofiber. We find that the effects of the butt coupling and the self coupling on the power transfer are significant when the fiber radius is small, the light wavelength is large, or the fiber separation distance is small. We show that the power transfer coefficient may achieve a local maximum or become zero as the fiber radius, the light wavelength, or the fiber separation distance varies.
Highlights
Optical nanofibers are vacuum-clad, ultrathin optical fibers [1] that allow tightly radially confined light to propagate over a long distance and to interact efficiently with nearby atoms, molecules, quantum dots, or nanoparticles [2, 3, 4]
We show that the effects of the butt coupling and the mode energy changes on the power transfer are significant when the fiber radius is small, the light wavelength is large, or the fiber separation distance is small
We have calculated the coefficients of directional coupling, butt coupling, and mode energy changes as functions of the fiber radius, the light wavelength, and the fiber separation distance
Summary
Optical nanofibers are vacuum-clad, ultrathin optical fibers [1] that allow tightly radially confined light to propagate over a long distance (the range of several millimeters is typical) and to interact efficiently with nearby atoms, molecules, quantum dots, or nanoparticles [2, 3, 4]. Photonic structures composed of coupled micro- and nanofibers have been proposed, fabricated, and investigated [30, 31, 32, 33, 34, 35] Such structures have been used to build miniaturized ring interferometers and knot resonators in the visible light wavelength range in recent work by Ding et al [35]. They used the linear coupling theory [26, 27, 28, 29, 36, 37, 38, 39, 40, 41, 42, 43, 44] to calculate the directional coupling coefficient for different fiber radii and different polarization angles.
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