Novel Stepped-Width Design Concept for Compact Multimode-Interference Couplers With Low Cross-Coupling Ratio

We show through detailed simulations that the length of conventionally designed multimode-interference couplers for low (<50%) cross-coupling ratios can be shortened significantly by replacing a segment that is 3/2-times the coupling length by a minimum-width design. For the 15% cross-coupling device, this stepped-width design results in 32% length reduction. With additional incorporation of adiabatic tapering, the length reduction can increase to over 44%. Shortening of the device makes it possible to fabricate ring resonators with larger free spectral range, significantly increases the 1-dB bandwidth for the total transmittance, and also helps to reduce the radiative loss