Guilin Sun
Ansys Employee
"What would be design constraints if I am designing to couple to fiber? I understand that smaller waveguide leads to confined mode but then again wouldn't E field of mode be leaking outside the core to clad region, decreasing Neff? So my design goal is to design waveguide with biggest core size which can propagate only one mode TE?"
This is a more theoretical question than simulation: I guess you want to optimize the coupling efficiency, right? other than the waveguide size, what else parameters you can optimize?
For a given wavelength, the waveguide has a limit size to maintain single mode operation. You can find this size using FDE by sweeping.
You can quantify the confinement factor. You can investigate the possible loss.
"TE polarization fraction is close to 100% only for TE in X direction in Lumerical" You may need to confirm your own definition of "TE", as it is not uniquely defined. I would use the dominant field component as the indication of polarization.
"What is the purpose of TE/TM fraction? suppose its value is x/y so x should be close to 100(most of the power in electric field should be in transverse direction) but what about y field? What is the significance of this value?"
Please refer this link FDE solver analysis - Mode List and Deck you can check the y-field. TE/TM is to quantify how much the total intensity is in the transverse (the rest is the longitudinal).
I believe you can define your own quantity to measure what you expect.
I agree with you that, no reason to prefer one polarization to the other when both are transversal and the waveguide is symmetric.