Abstract
In this paper, we present a comparison analysis between directional couplers (DC) and multi-mode interferometers (MMI) based on high-index contrast ridge waveguides. It is found that the two devices are intimately related as the MMI is structurally derived from the DC. For the first time, the continuous evolution from the two-mode coupling characteristic of DC to the two-mode interference and multi-mode interference of MMI is demonstrated. The resulting MMIs are compared with the DC in terms of coupling length, polarization dependence, excess loss, and fabrication tolerances. We show that practical directional couplers with reasonable gap size can also be quite compact and have the same coupling length for both TE and TM polarizations. Consequently, the DC can be just as polarization insensitive as the MMI. These features, however, require careful design control involving a large set of design parameters. By comparison, the MMI design is more robust and involves fewer design variables.
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