Learning Forum

[lastskybender]

Doubt regarding LNOI waveguide: I am trying to simulate LNOI waveguide ( lithium niobate waveguide on SiO2). During simulation I found that as I increase the core size (0.5um to 0.9um) Neff increases. In commercial available LNOI, LN thickness vary from 500nm to 900nm. Now anyone would choose core size 0.9um because its giving higher Neff, then what is the point of having LNOIs with LN thickness vary from 500nm to 900nm?

Doubts regarding Mode: How do I find TE mode(for which I want to design waveguide)?Is it mode 2 in simulation, with TE polarization fraction 99?

Regarding Boundary condition: In attached screenshot I have chosen metal boundary. Should I choose PML instead?

Thanks for reading. An screenshot of simulation is attached for reference.

[Guilin Sun]

"then what is the point of having LNOIs with LN thickness vary from 500nm to 900nm?"
This depends on customer's goal. Some may want to have different result, eg, to couple with other waveguide/fiber etc.
"Doubts regarding Mode:"
If the waveguide is symmetric, mode degeneracy can happen. If you know exactly what does TE mode mean, eg, its dominant component is Ey or others, you can choose that one. To make sure it is a pure mode without degeneracy, you can use symmetry BCs: Power coupling and TE polarization returning different results for same simulation (FDE) ÔÇö Ansys Learning Forum
"Regarding Boundary condition:"
Metal BC is recommended to get quickly the result, and it works in most cases.
PML BC is required when the modes have leakage/radiation, eg, bent waveguide, etc. The PML may also introduce a very small loss, eg, or the order of 1e-5 or so to imag(neff). Sometimes you see a "negative loss", which is numerical error. You may need to increase the distance between PML and the waveguide core.

[lastskybender]

"This depends on customer's goal. Some may want to have different result, eg, to couple with other waveguide/fiber etc." 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?
I have two more doubts:
We design MZM waveguides for single mode(preferable TE), right? Now considering the direction of propagation is in Z direction, for TE Electric field will be either in X or Y direction. In simulation which TE I am looking for, is it TE polarized in X or Y? Because I think TE polarization fraction is close to 100% only for TE in X direction in Lumerical. So wouldn't it be wrong to say that TE polarization fraction should be 100% for TE polarized light, I mean it could be Y polarized also.
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?

[Guilin Sun]

"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.