Feature Highlight: Phase Aware Routing

Tools Used: OptoDesigner

OptoDesigner PDKs include information on optical properties of the various waveguide types that are available. These properties describe the effective index and group index as a function of wavelength, radius, width and polarization. The program's script can use this information to automatically calculate the optical length or group length of a waveguide of any shape; the tool automatically takes into account the local radius of curvature and width to integrate the effective, or group index, along the length of a waveguide. Moreover, a set of connectors is available that uses these features to draw a waveguide shape that has a desired geometrical, optical or group length. We call using these connectors "Phase aware Routing," as shown in the figure below.

For each of these connectors, there are three available options for the length type: Geometrical, Optical and Group.

  • Geometrical optimizes the geometrical, physical length of the center path of the waveguide
  • Optical optimizes for the integrated effective index along the waveguide
  • Group optimizes for the integrated group index along the waveguide

These connectors are very useful in designing phase sensitive devices, such as an asymmetrical Mach-Zehnder interferometer and 90 degree hybrid.

For example, the asymmetric Mach-Zehnder model, shown below, combines two 50-50 MMIs with a small asymmetric delay to achieve any desired splitting ratio.

The length of the lower arm is set first. The optical path for the upper arm is then calculated to achieve a given phase difference compared to the lower one.

Another example is this 90 degree hybrid:

There are four paths in between the two sets of MMI2x2. All of these paths except one should have the same optical length (having the same phase difference from the input MMIs to the output MMIs) while the fourth path needs to be 90 degrees of phase longer. The phase aware connectors allow one to define this structure in a very simple fashion.