Transformation of directional couplers to multi-mode interferometers based on ridge waveguides and its applications

We present a comparison of directional couplers and multi-mode interferometers based on the unique properties of high-index contrast ridge waveguides. 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 multi-mode mixing and interference characteristic of MMI is shown. 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. On top of that, we also show a novel design of polarization splitter based on both DC and MMI. By comparison, the MMI design is more robust and involves fewer design variables.