My simulation file and results are attached to this link:
Basically it is a simulation of the coupling between a 5um Si3N4 ring and waveguide, with 1um cladding layer on top and on bottom of it. The simulation time is not converged for this case, but that shouldn’t matter to the discussion below.
However, the results, as can be shown in the “after” monitor, has a wired free spectral range.
In general, we expect FSR=c/(n_eff*L), where L=2piR
Plugging in the above equation I got n_eff for this system is in the range of (2.40,2.43).
However, all the materials I used in the simulation has refractive index smaller than 2.40. (SiN is 2.34, SiO2 is 1.444, background air is set to 1) And all of them should have no or little wavelength dependence as can be seen in material explorer window.
So, how does it predict n_eff larger than any of the materials? If that is true phenomenon, please give a brief explanation. If not, please tell me how to get rid of this error.