The structure of hybrid higher order modes of two coupled weakly guiding identical
optical fibres is studied. On the basis of perturbation theory with degeneracy for
the vector wave equation expressions for modes with azimuthal angular number
l ≥ 1
are obtained that allow for the spin–orbit interaction. The spectra of polarization
corrections to the scalar propagation constants are calculated in a wide range of distances
between the fibres. The limiting cases of widely and closely spaced fibres are
studied. The obtained results can be used for studying the tunnelling of optical
vortices in directional couplers and in matters concerned with information security.
We study the evolution of circularly polarized optical vortices (OVs) in the system of two
coupled few-mode optical fibres. We demonstrate that upon propagation OVs tunnel into
the adjacent fibre as a complex superposition of OVs that comprise also OVs of opposite
polarization and topological charge. The initial OV may tunnel into the other fibre as the
same vortex state of lesser energy. The evolution of the orbital angular momentum in
coupled fibres is studied.
We study the structure of supermodes of a double-ring array of identical evanescently coupled single-mode fibres with the same coupling constant, which describes interaction between the fibres. We obtain the expressions for normal modes of such an array and the spectrum of their propagation constants. We show that these supermodes are represented by symmetric and antisymmetric combinations of supermodes of a single-ring circular array.
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