Mode overlap calculation in FDE


I am trying to simulate a nanotaper and a fiber (SM-400) for designing an edge coupler. As a part of the simulation, I am trying to make an overlap calculation between these two by the built-in overlap integral function in the mode solver. The mode profile of the nanotaper tip and fiber tip looks quite different yet I am getting an overlap of 81%. Could you please tell me, is it the correct way or not?

Also, the modal area of these two modes are different by a factor of 2, so having an overlap of 81% seems quite questionable. In the application library, I saw the edge coupler simulation was done in this way.
Please note that, the nanotaper has a dimension of 125 nm*30nm while the fiber has core of 1.5 µm and cladding of 20 µm.



Hi @arefin.2,

Thank you for your question. Yes, you are calculating the overlap in the right way. The modal area and the overlap are calculated in different ways:



I believe that the large difference in modal areas is due to the long tails of the taper’s mode profile. These tails significantly increase the modal area, but do not have as much of an effect on the overlap. In other words, the taper mode has a large area, but most of the mode’s energy is contained in the part of the mode that overlaps with the fibre mode. This is why there can be significant overlap between modes of different areas.

Let me know if you have any more questions.

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Thank you very much @kjohnson

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