Edit: The contents of this post have been moved to this page: https://optics.ansys.com/hc/en-us/articles/12614264530323
Occasionally the results of an FDTD simulation will show normalized transmission values greater than one, which is, in general, incorrect for linear simulations. In this post I will discuss a few of the more common causes of this issue and how they can be fixed.
Note that while this post specifically addresses transmission values greater than one, the issues discussed here can also be sources of inaccurate transmission results in general. If you suspect that your transmission results are inaccurate, you should consider if these issues are relevant for your simulation.
Simulation Ends Before Autoshutoff Minimum Reached
One of the more common causes of this issue is the simulation ending before the light leaves the simulation region. This is generally the most common cause of transmission results greater than one, and is particularly relevant for resonator structures.
In most cases, we would like the simulation to end when it reaches the “autoshutoff minimum”, which is a measure of the energy left in the simulation region. However, the simulation can also end when the “simulation time” is reached, while there is still a significant amount of energy left in the simulation. This causes artifacts in the transmission spectra which appear as “oscillations” or “ripples” that can increase the transmission to values greater than one. For example, here are the transmission results for a varFDTD ring resonator simulation with different simulation times:
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