An understanding of parasitic effects is essential to maximize the performance of a Photonic Integrated Circuit (PIC). Using
a circuit simulator, we are able to model mode conversion at the interface between straight and bent waveguides, parasitic
reflections in multi-mode interference couplers (MMIs), interference between multiple modes, residual facet reflections,
and reflections at junctions between components. Even though these effects are usually low in intensity, around -20 dB to
-30 dB from the main signal level, they can still have a strong influence on the circuit performance. This is because the
mentioned parasitic effects are coherent with the desired signal and interference between them is therefore a field effect.
By analyzing three different circuits, and comparing the results to measurements, we show that these effects need to be
carefully managed in order to ensure circuit performance. The circuits we investigate are a Fabry-Perot cavity, a Mach-
Zehnder interferometric structure, and a Michelson interferometer. Especially residual reflections coming from angled
facets and back-reflections in MMIs are shown to be the main parasitic effects in the investigated circuits.