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Optical Waveguide simulation with Lumerical: propagation loss, how is it calculated?

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LandLack

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Dear All,
I am simulating with Lumerical: Mode solutions (2013) a single mode Si waveguide on SiO2; the numerical method used by this simulator is EigenModeExpansion. The geometry I am using as a reference is similar to a standard 500x220nm, which corresponds to very low simulated losses (it has already been manufactured, and probably simulated by experts).

I have simulated the structure with my tool and, while at first the losses were around 2dB/cm, it started increasing as I improved the meshing conditions; it became 100dB/cm with 3nm as minimum meshing step, which is very refined. The mode profile, as well as the dispersion curves are normal and similar to litarature...

To study how the code calculates the losses, I have modified the materials in order to have no losses in any wavelength, but there was no change in the calculation of the losses (so I think that the material is not related); at first I thought that Lumerical added just the material's losses to that of the waveguide (a mode is theoretically lossless), but now I am confused:

Do you know what are the parameters that lumerical is using to calculate the losses? I don't understand why it is not working; I have tried with Comsol, and it goes efficiently, following my expectations, so I am really curious.

P.S. My workstation is powerful, so it cannot be an issue of the machine. I would like to continue working with Lumerical as it has been efficient with other calculations.

Thank you in advance for your suggestions,

Best Regards,
Lack Land

- - - Updated - - -

In summary, the question is:

1) since a mode is lossless, and the materials are lossless, how does it still have propagation loss?

This propagation loss is related apparently to the geometry (since it increases with the mesh density), but it's just a simple rectangle. Also, EigenModeExpansion, calculates the modes by using just the geometry (I didn't have to give any signal as input).
 

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