Abstract
The coupling of guided modes obliquely incident to periodic and aperiodic waveguide gratings has been modeled by a variety of analytical formalisms including coupled-mode and boundary perturbation techniques. While the various methods used to model the grating response result in the same analytical expression for the near-Bragg-matched reflectivity, there are striking differences in the predicted coupling coefficients needed to evaluate this formula. This is especially evident for the case of TM-TM coupling. The TM-TM coupling coefficients predicted using the local normal and ideal mode expansions of coupled-mode theory demonstrate significantly different angular dependences. In particular, the zeros obtained by these methods occur for different angles of incidence. Thus, in certain circumstances, the ideal mode theory predicts a peak grating reflectivity of 100% while that predicted using the local normal mode expansion is virtually 0%.
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