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Unexpected S-Parameters in Waveguide Simulation
Posted May 4, 2023, 7:57 a.m. EDT Wave Optics, Results & Visualization Version 5.6 4 Replies
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Hi all,
I'm trying to simulate a 3D nanophotonic waveguide. The structure is very simple, a slab of material in perfect vacuum. I applied a PML all around, and now I want to investigated the propagation of supported modes. For that, I follow the post in Copying and Resuing Boundary Mode Analysis Results, copying faces and using withsol() to "listen" to the correct mode.
I created an excitation port (1) and an observer port (2) at the same face, as well as some additional ports. Mainly, I would expect S11 and S21 to be the same value, and I would expect that the excited mode propagates very well as the effective refractive index has a small imaginary part.
The Multislice seems to show a nice propagation, yet the S-parameters are way worse than I would expect. Especially the difference between S11 and S21 is surprising to me.
Am I missing something in the way the S-parameters are calculated or how the ports act? Or are my expections wrong? I'd be glad about any input!
Thanks!
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I can't run your model because I don't have wave optics. But- I don't understand why you have so many ports.
Seems to me you have one input port and one output port. You only need more ports if there are multiple propagating modes.
When set up properly with linear materials and a single mode type you should find S11=S22 and S12=S21 (or is there a minus sign, I forget).
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Hi Dave,
thanks for your answer. Indeed, I added ports to check for the cross-talk between the two allowed propagating modes. One port (port 6) was only added after I realized that the other S-parameters do not behave in the way I would expect. With this one, I wanted to monitor if the wave propagates as I would imagine.
Do ports influence the solution? Or are they only evaluated in post-processing?
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Do ports influence the solution? Or are they only evaluated in post-processing?
You can see that the ports are part of the calculation because for every port there is a corresponding S parameter that appears under "dependent variables."
I think it is fair to say it this way- Comsol calculates a solution to Maxwell's equations including outgoing waves at the ports with magnitude and phase that are variables.
What this means- if physically there should be outgoing waves and the port does not allow them the result will be incorrect. This could happen, for example, if you specified an output port with an electric field perpendicular to the electric field for the input port.
I think what you should be doing in your calculation is doing a boundary mode analysis for an input port consisting of the diamond and the surrounding vacuum. Same thing on the output port. If there was something that would result in mode conversion to another mode (say a disconinuity or obstacle of some sort) then there could be additional modes that propagate. In that case you would need to explicitly add additional ports.
Note that when you do the boundary mode analysis, you specify the effective mode index or out of plane wavenumber. These specifications allow you to select particular modes.
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Hi Dave,
thanks for the reply. I am indeed doing a boundary mode analysis and propagating the solution to the various numeric ports. I now realized that having multiple "observer" ports listening to the same mode is probably not a good idea. They seem to influence each other. If I disable most of the off-ports in my model, the results make sense. Mainly port 2 in my model seemed to have caused the issues.
Thanks for your input!