When creating a PIC layout people mostly use simple components like pre-configured PDK components, simple straights or polar bends. However the Synopsys PIC Design Suit layout tool OptoDesigner (OD) is capable of creating any desired shape, while discretizing it to mask files accurately, which gives a broad range of design freedom.
Waveguide shapes – even simple ones like straights – in OptoDesigner consist of two curves; an ‘upper’ curve and a ‘lower’ curve, when looking at it running left-to-right. Each layer associated to the active mask cross-section has its own ‘upper’ and ‘lower’ curve, which are related to the original curves – usually either directly or through widening. These are all analytic curves. When exporting to a mask file, these curves are turned into polygons. The rules in the OD PDK (Process Design Kit) determine how accurately this happens; the curve is analyzed, and an algorithm makes sure that all vertices of the mask polygons lie within a given distances from the analytical curve (typically 1 nm). Note that this discretization into polygons is still done with full precision; a snap to the mask file grid will introduce extra errors. Since this algorithm happens for any waveguide shape, it also enables accurate discretization of fully arbitrary shapes.
The OptoDesigner Element category ‘Curves’ contains a number of different ways of creating arbitrary curves. This Feature Highlight will dive into two of those: CurveUpDown and CenterPath.