I am interested in determining the reflection from a straight waveguide to bent waveguide interface. e.g., 500x220 nm straight strip waveguide connected to a 90º bend with a radius of 1 µm. I know such an interface has a large mode-mismatch loss (0.5 dB for a 90º bend with two interfaces). Most of the power scatters away (See Fig 3.26 in my book)
How much is reflected back into the fundamental mode of the source waveguide?
I looked through your manual, https://kb.lumerical.com/en/ref_scripts_expand.html, which calculates the S11. Can this be interpreted as S11 = reflection coefficient when going from mode 1 to mode 2?
I wrote a script that does this calculation, and it gives numbers of about -30 to -40 dB for ranges of 1-3 µm bends. Seems small, but not unreasonable.
I also did this using 3D-FDTD, using mode expansion monitors. I get values between -40 and -65 dB for a 1 µm bend, depending on the mesh accuracy (between 1 and 5). I suspect that in FDTD it will be challenging to eliminate all the reflections from the PML boundaries, and I haven’t been able to get a converged result where I can be sure I have the actual reflection coefficient.
So I am wondering what is the best method to determine the reflection for this case?