Abstract
We study analytically and numerically the possibility of vortex modes propagation over planar dielectric rectangular waveguides, and consider the problem of waveguide geometry optimization for the support of vortex modes. The results show, that theoretically rectangular waveguides can provide transmission of quasi-TE and quasi-TM modes with high purity states of orbital angular momentum (OAM) in the dominant field component. However, only for the quasi-generate mode of azimuthal order ±1 the constituent eigenmodes can propagate in a phase-matched regime, and the vortex modes of higher azimuthal orders can propagate only with a certain beat length. Moreover, as the target azimuthal order increases, the normalized power of the corresponding OAM state in the modal superposition decreases. The analytical predictions have been verified by numerical electromagnetic simulations of silicon nitride waveguides providing field distributions and OAM spectra of the corresponding modal superpositions.
Highlights
IntroductionSince the time when it was recognized by Allen et al [1] that a helically-phased light beam (commonly referred to as an optical vortex) inherently possesses an orbital angular momentum (OAM) of per photon (where integer is the topological charge of vortex beam and is the Planck constant), voluminous applications of optical vortices have been developed [2], [3]
We study analytically and numerically the possibility of vortex modes propagation over planar dielectric rectangular waveguides, and consider the problem of waveguide geometry optimization for the support of vortex modes
Since the time when it was recognized by Allen et al [1] that a helically-phased light beam inherently possesses an orbital angular momentum (OAM) of per photon, voluminous applications of optical vortices have been developed [2], [3]
Summary
Since the time when it was recognized by Allen et al [1] that a helically-phased light beam (commonly referred to as an optical vortex) inherently possesses an orbital angular momentum (OAM) of per photon (where integer is the topological charge of vortex beam and is the Planck constant), voluminous applications of optical vortices have been developed [2], [3]. We will discuss the advantages and disadvantages of the first two approaches, which can be implemented based on planar PICs. Out-of-plane generation of optical vortices implies their transmission via free space or optical fiber (using some lens in front of vortex beam emitter in order to have paraxial or focused beam, respectively). The weak points of the in-plane approach are its questionable scalability and the need for custom chip fabrication or the use of shorter wavelengths The latter is caused by the waveguide height of the existing photonic integration platforms, which is usually defined for single-mode operation at telecom wavelengths. Transmission of an ideal vortex mode is known to require a circular cross-section of the waveguide, which inherently provides a propagation with the term e±i φ, defining the phase front helicity (here φ denotes the azimuthal angle) Whereas this condition is met automatically in optical fibers, it is not.
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