I want to plot a graph similar to above for our own designed structure. I want to study the effect of geometrical parameters of the ring on the coupling i.e. over-coupling, under-coupling and critically-coupling. On the x-axis, it says resonator round trip loss. This round trip loss is loss after the wave transfers the one full round of the circumference. The loss can also be tuned by applying the voltage to the graphene layer transferred onto the circumference of the ring. The y-axis plots the transmission into the bus waveguide. Please suggest how to approach to measure such losses using FDTD or MODE Solver. 

The picture is taken from the following article (Fig. 1c).  https://doi.org/10.1515/nanoph-2020-0363