I have had a problem with the simulation of a 3D structure using FDTD solutions.
My structure is composed of a silicon 10-um radius ring resonator (width=500 nm x height=220 nm) and at the center of the waveguide there are some fully etched holes (radius=50 nm).
The bottom and upper cladding are made of silica. The ring is coupled to a curved waveguide (width=300 nm x height=220 nm). Please, refer to this article in the following link (the only difference here is the radius of the ring): http://ieeexplore.ieee.org/document/7519087/
I ran some simulations to evaluate the coupling coefficent between the ring and the coupled waveguide (bus).
I varied the mesh accuracy (from 3 to 7) and I measured the transmission of the field which is coupled in the ring (the input mode is injected from the waveguide bus).
I have noticed that the values of the coupling coefficient as a function of the mesh accuracy are (for a wavelength=1548 nm) about:
Mesh accuracy Coupling coefficient [%]
The surprising fact is that the coupling coefficient at mesh=5 is comparable with the experimental results and its value is quite different from the coupling coefficient values obtained with higher mesh.
I cannot understand why. I tried to sweep the wavelength range and I tried to increase the simulation time (from 1500 fs to 3000 fs, they are both enough), but I am obtaining the same results.
Can you please help me to understand the problem?
Thank you in advance for your reply.