High index contrast structures often result in propagation that is not paraxial, especially gratings. On the other hand, not always. Sometimes long period gratings have paraxial propagation. If you know in advance that the propagation will be paraxial, it is possible the Beam Propagation Method will accurately simulate the propagation, and therefore you can use OptiBPM. If you know it will not be paraxial, don’t try it.
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