A Tapered Structure for Compact Multimode Interference Coupler

A new tapered structure based on general interference has been proposed and realized by using silica waveguides with a silicon oxinitride (SiON) core for the reduction of coupling length of the 2times2 multimode interference (MMI) coupler. The coupling characteristics of the proposed structure are compared with those of conventional MMI structures using a simple mathematical model based on sinusoidal modes. The beat length for the proposed tapered MMI coupler is reduced by 45% of that of a conventional MMI coupler. The effect of power imbalance on fabrication tolerance of the proposed tapered MMI coupler are also studied and compared with other tapered MMI couplers.