Directional couplers enable the transfer of light from one waveguide to another through evanescent coupling. In general, photonic directional couplers are implemented as two parallel waveguides.
coupler_length PCell property is the length of the straight part of the coupler. There are different flavors of directional couplers:
A directional coupler consisting of 2 parallel (horizontal) straight waveguides.
A directional coupler consisting of 2 parallel (horizontal) waveguides with bends at the start and end.
A directional coupler consisting of 2 parallel (horizontal) waveguides with S-shaped bends at the start and end.
In IPKISS, directional couplers are modeled using four waveguides and a logical coupler. The physical layout is built out of two waveguides, but the waveguides are split in a east and west part. This is done to match the layout and schematic, which is advantageous for circuit modeling and for verification purposes. The schematic is shown below:
Notice the logicalcoupler in the center. It is used for simulation purposes (and has no corresponding layout). The wave splitting occurs in this section. Two circuit models exist:
SimpleCircuitModel. Both use the physical length of the waveguides for the total phase propagation in the two arms. The model of the logicalcoupler differs:
CircuitModel: This model uses wavelength-independent fixed values for cross and bar coupling of the logicalcoupler.
SimpleCircuitModel: This model uses a wavelength-dependent model for the logicalcoupler, based on
coupler_lengthand the difference in effective indices between the two supermodes inside the coupling section.