Abstract
A dielectric step-index optical fiber with tube-like profile is considered, being positioned with a small gap on top of a dielectric slab waveguide. We propose a 2.5-D hybrid analytical/numerical coupled mode model for the evanescent excitation of the tube through semi-guided waves propagating in the slab at oblique angles. The model combines the directional polarized modes supported by the slab with analytic solutions for the TE-, TM-, and orbital-angular-momentum (OAM) modes of the tube-shaped fiber. Implementational details of the scheme are discussed, complemented by finite-element simulations for verification purposes. Our results include configurations with resonant in-fiber excitation of OAM modes with large orbital angular momentum and strong field enhancement.
Highlights
Slab-coupled microstrips (Hammer et al 2019), when considered in a context of photonic microresonators (Chremmos et al 2010), can exhibit rather remarkable filtering properties
In the most basic configuration, a simple dielectric strip, positioned at some distance above a slab waveguide, is evanescently excited, at oblique angles of incidence, by the semiguided waves supported by the slab
The present work is to be seen in a context of high-contrast 2.5-D integrated optics (Hammer et al 2010), where the specific features related to the oblique excitation of infinite structures, with two-dimensional cross sections, are exploited
Summary
Slab-coupled microstrips (Hammer et al 2019), when considered in a context of photonic microresonators (Chremmos et al 2010), can exhibit rather remarkable filtering properties. The present work is to be seen in a context of high-contrast 2.5-D integrated optics (Hammer et al 2010), where the specific features related to the oblique excitation of infinite structures, with two-dimensional cross sections, are exploited Apart from the former microstrip-resonators, the list of proposals includes step- and corner-folds in slab waveguides (Hammer et al 2015, 2016; Ebers et al 2018), an anti-reflection-coating of a slab waveguide transition (Hammer et al 2019), and configurations that realize filter functions related to bound states in the continuum (BIC states) in a waveguide photonics setting (Bezus et al 2018a, b; Bykov et al 2020). One needs to distinguish two cases: For a sufficiently large effective mode index Nout with k2No2ut > ky and real k , the outgoing field propagates at an angle out with wavenumber k = kNout cos out. Separate contributions are specified for the slab waveguide and for the dielectric tube, respectively
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