Abstract
Understanding optical propagation in lossy planar waveguides is important for photonic integration. "Steady-state eigenanalysis", ie: an analysis using only the eigenvalues (propagation constants (βi+jαi for modes i) of the complex-index slab waveguide problem, is often used to model such structures, [1-5] and was recently used to optimize vertically-coupled detectors (fig. 1) by maximizing αi It predicts that αi can be enhanced by adjusting the absorber thickness [1,3] or by inserting an additional, transparent "matching layer",[2] as indicated by a sharp resonance in the dependence of αi on layer thickness (fig. 2, solid curve). However, recent experiments and BPM simulations indicate that steady-state eigenanalysis fails to adequately describe devices optimized with matching layers: [6,7] while enhanced absorption is observed, the predicted resonance is absent. Here we show that the resonance predicted by steady-state eigenanalysis for the conventional structure of fig. 1 is quenched by multimode interference associated with modal power non-orthogonality, and can be accurately simulated using an eigenmode-based propagation method.
© 1992 Optical Society of America
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