Changing cross-section

A taper usually goes from one width to another. Given two cross-sections instead, it changes one cross-section into the other:

shape taper with 20um, rib, strip as t.start at self.origin

The sections are paired by layer: the core on si at the start tapers into the core on si at the end. A section that exists at only one end narrows into the main section of the other end, so a slab that a strip waveguide does not have disappears under the core. That is exactly a rib-to-strip converter.

The start of such a taper carries the first cross-section and its end the second, which matters on the next page.