Mode propagation and overlap in Mode

noumanzianoumanzia Member Posts: 5
edited September 10 in Photonics


There is a feature in FDE to calculate the mode overlap between nearfields of two waveguide modes.

Is this possible to propagate the near-field some distance x and then take the overlap of this diverged beam with the near field of other waveguide mode using FDE or EME to calculate mode-mismatch.

I am well ware that it's doable via FDTD but it's computational expensive to propagate the mode in FDTD and calculate coupling efficiency. Also in that case I have to take into account the reflections and need to apply anti reflection coatings. FDTD is definitely more accurate but I am not sure what will be the level of accuracy if FDE/EME is used for this purpose.

Best regards,


Best Answer

  • gsungsun Posts: 953Ansys Employee
    Accepted Answer

    If it is so, the FDTD is the best method, whereas EME is second, since you will need many modes in the gap which can be very challenge to converge. FDE is not good for this. the farfieldexact is much better than the simple Fourier transform, as it is theoretically no approximation. However, all the Fourier transform will involve some kind of approximation.

    For such gap, and titled angle between two waveguides, FDTD is the best.


  • gsungsun Posts: 953Ansys Employee

    If you can provide more details about your device and simulation goal, we may provide better answers. For the limited information, yes, you can project mode in the farfield, 1 meter away in FDE; or using farffeildexact to project into finite distance. However the farfieldexact requires the mode profile in uniform material, which is physically incorrect in FDE/EME. If you want to project a waveguide mode into a uniform material such as air, it is better to use FDTD to simulate the end facet, using its monitor data to project to specified location, then do the coupling or overlap calculation using your own script, with farfieldexact:farfieldexact - Script command

    Facet reflection and transmission calculations

    when you say "anti reflection coatings", I guess you mean to coat the end surface of the waveguide. if not, please give more details.

  • noumanzianoumanzia Posts: 8Member

    Hi @gsun ,

    Thank you for your thoughts! I want to calculate fundamental TE mode coupling between two waveguides when there is a finite gap between them. In other words, I want to see how the divergence of mode along the propagation direction affects the mode overlap. I am already using FDTD and it works fine, but I assume there should be a much simpler way to find mode overlap by using some kind of a Fourier transform which will give the field after some propagation distance, which I can then overlap with the mode in another waveguide

    In the FDTD model, I do need antireflection coatings at the waveguide facets to match the effective indexes between each material, otherwise, there will be a significant reflection loss at each interface, which will make it difficult to calculate the mode mismatch loss due to propagation.

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