An Ultra Compactmulti Mode Interference Coupler with Parabolic Down Tapered Geometry

A 2x2 multimode interference structure with parabolic down tapering has been proposed and realized by using silica waveguides with silicon oxinitride (SiON) core for the reduction of coupling length. The coupling behavior and dependence of fabrication tolerances on power imbalance of the proposed tapered structure have been compared with those of other tapered MMI structures having both down and up tapering by using a mathematical model based on sinusoidal modes. It has been found both theoretically and experimentally that the beat length (Lπ) of the proposed tapered MMI coupler with Δn=5% is ∼ half that of conventional MMI coupler and the deviation of power imbalance from its minimum value with fabrication tolerances (± ∂w values) for the proposed tapered structure is more than that of other tapered structures.