Design of polarization-insensitive 2×2 multimode interference coupler based on double strip silicon nitride waveguides

A polarization-insensitive 2×2 multimode interference coupler based on double strip silicon nitride waveguides is proposed and optimized by using the three-dimensional finite difference time domain method. By optimizing the device's structure parameters in detail, polarization independent excess loss of −0.32dB is obtained, and negligible output uniformities of −0.02dB and −0.03dB could be achieved for the TE and TM mode, respectively. The optimized polarization-insensitive 2×2 multimode interference coupler could be served as a building block on the double strip silicon nitride waveguides platform.