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.

中文翻译：

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斜角半导波介电管渐逝激发的混合耦合模式建模

考虑了具有管状轮廓的介电阶跃折射率光纤，其放置在介电平板波导顶部的小间隙中。我们提出了一种 2.5-D 混合分析/数值耦合模式模型，用于通过以斜角在板坯中传播的半导波对管进行渐逝激发。该模型将平板支持的定向偏振模式与管状光纤的 TE-、TM- 和轨道角动量 (OAM) 模式的解析解相结合。讨论了该方案的实施细节，并辅以有限元模拟以进行验证。我们的结果包括具有大轨道角动量和强场增强的 OAM 模式的谐振光纤内激发的配置。