Abstract
The mode-matching/finite-element (MM/FE) method based on complex power matching is applied for the design of strongly guiding coupled ring-core fibers (RCFs) to modulate orbital angular momentum (OAM) beams. The evolutions of polarization and OAM states of light in the coupler are investigated. Due to the effects of linear birefringence and spin-orbit interaction (SOI), the OAM beam with opposite polarization and topological charge (TC) is observed at a certain distance to the end of the coupler after the coupling region in the form of complex supermodes. The impacts of separation distance and wavelength on the maximum coupling efficiency and inter-modal crosstalk are investigated to achieve the optimum trade-off between the crosstalk and the coupling length. Two types of OAM couplers with separated OAM modes of TC <inline-formula><tex-math notation="LaTeX">$l = \pm1$</tex-math></inline-formula> are proposed, both of which can achieve high coupling efficiency (<inline-formula><tex-math notation="LaTeX">$\gt 99\%$</tex-math></inline-formula>) and low crosstalk (<inline-formula><tex-math notation="LaTeX">$\rm{\lt - 21\,dB}$</tex-math></inline-formula>) at 1550 nm wavelength without mode conversion. The proposed design approach has great potential in the (de)multiplexing of OAMs since it leads to a novel mechanism for sorting OAM modes in waveguides without destroying the integrity of OAM modes.
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