At the moment, OptiFDTD cannot find modes of bent waveguides.
I can suggest the following:
1) Design a linear waveguide portion in front of your bent waveguide. Launch light from this linear waveguide. This way the mode solver will pick-up the waveguide geometry. The linear waveguide doesn’t have to be long, you just need to ensure that it uses the same profile as your bent waveguide.
OR
2) Design your profile cross-section from OptiMode, solve the modes and export the mode from OptiMode solver. Then go back to OptiFDTD and use the “user-defined” input plane option to load your mode file.
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