Before a BPM simulation can begin, the optical field must be known for every point on the input plane. This usually requires a mode solver. OptiBPM will call one if set that way. The above message is sent when that modesolving fails, either because of incorrect settings in the modesolver or because the mode does not exist. Using Gaussian fields for the input plane will help, but you’ll have to supply a value for the reference index – OptiBPM will call the mode solver to find that parameter if you don’t specify it.
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