A new design for ultracompact multimode interference-based 2 x 2 couplers

We introduce here a new multimode interference-based 3-dB coupler that has a substantially smaller device geometry than conventional multimode interference structures. This new structure is based on a special multimode region shape and an appropriate launching technique which aids in circumventing the proximity limitations. Simulations show this structure can provide a decrease of a factor of three or more in the device size of the smallest 3-dB MMI coupler made to date.