Multimode Interference Waveguides, also termed MMI Couplers, are used to split light from one waveguide into two or more paths. MMI couplers are designed to match the power at each output port. The length, width and positioning of the output ports are critcal to the design of the MMI coupler. The MMI coupler is also difficult to build in device fabrication due to the sensitivity of the width of the multimode waveguide to the performance.
Below are two MMI couplers, designed in Rsoft. The 3dB Coupler has two output ports of half the input power. The approach for both couplers is to monitor the optical power at each output port in the simulation. Initially, we design the length of the multimode section to be longer than estimated. The length of the multimode waveguide section is reduced to the length at which the optical power in each of the output paths is equal.