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Dear japon,

Good that you ask, as more people seem confused about this feature.

That offset between a bend and straight waveguide is intended in this demo interconnect. In various technologies (like InP) this offset is often needed for an optically smooth transition (looking “disjunct” geometrically). The light (mode) can be thought of as being swept to the outer side wall of the bend. Nazca takes care of this offset for you. Many designs made in other tools tend to be wrong by not having this offset automatically (you have to add it manually if you happen to know this) leading to underperforming photonic circuits (excess loss and higher order mode generation). As this is a demo interconnect, the offset is a bit exaggerated in size.

Note1: The arrows at the pins from straight to bend indicate this connection is correct (intended): they are aligned.

Note2: There are no arrows inside the sbend element (by choice) as this is a single interconnect.

The offset is a property ‘os’ of the xsection, here xsection “Shallow” in demofab. When creating an interconnect demo.shallow (in the demofab PDK) from xsection “Shallow” that property is automatically copied into the interconnect as property ‘offset’. To not have an offset, either define a xsection without ‘os’ or ‘os=0’ in the first place, or in this case when it is already added to an existing interconnect, simply switch of the offset in the interconnect via demo.shallow.offset = 0.