Guided-radiation mode interaction in off-axis propagation in anisotropic optical waveguides with application to direct-intensity modulators

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Abstract
A closed‐form solution of the coupled‐mode equations is derived for the guided–radiation mode interaction in optical waveguides by using a Lorentzian approximation to the coupling‐factor distribution. This solution does not neglect reconversion from radiation to guided modes as did the previous perturbation solution. It is thus more accurate for the case of higher‐order guided modes for which the power attenuation due to radiation conversion exhibits an oscillatory form and the ’’effective’’ perturbation assumption that the coupling factor is independent of the phase constant is not valid. Peaks in the mode conversion characteristics at certain eigenvalues of the relevant guided mode can be physically interpreted as the coupling to leaky waves. Illustrative examples and related discussions are given for the polarization‐rotated (TE→TM) guided‐to‐radiation mode conversion in off‐axis propagation in an anisotropic LiNbO3 waveguide. Also, an electro‐optic direct‐intensity modulator is described where the angle of propagation is used instead of a dc field to set a bias point for linear modulation.

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