Learning Forum

[Then Elango]

Hi!

I'm trying to find the supported modes in the waveguide of different material. I use varFDTD to find the effective index of the mode in waveguide(Si on SiO2) . The simulation setup is 

Now I add another material (new = refractive index1.62) to the materials tab and change the waveguide material to new  and when I save the ocnfiguration the source automatically changes the direction to this: 

Why does this happen? Should it mean that the direction change will alter TE and TM mode to be vice versa?

 

[Guilin Sun]

By default, the solver uses the first mode it finds. When the material is changed, the modes may change. For example, in one waveguide the first mode is TE, but in another material, the first mode can be TM. In addition, when the modes are degenerated it can be either of them. If you want a specific mode, please choose "fundamental TE" or "fundamental TM", or you use "use select" and calculate the modes, then you can choose whatever the mode you want.

[Then Elango]

 

Thank you sir.  I just tried this and I find that that using user select to view the mode I see,

So, first two are my guided modes. Since my cladding refractive index is 1.595 and I have low index contrast. 

When I select the first mode: 

 

 

and my second mode is similar to the first.

2nd: 

 

are these both TE polarised. In the mode index table, 2nd one is 0% TE polarization, which means it should be TM polarisation. I have this confusion, because in my opinion, TE and TM mode profiles, should be inverted like this:

 

 

 

 

I tried to check the Ey, and Hy and they are similar too. WHat could I have done wrong?

 

 

[Guilin Sun]

The definition of TE TM is actually not universal and can be confusing in some times.

As you can see, the TE polarization fraction is zero for the 2nd mode, so it is NOT a TE mode. Please check the dominant field component. Please check this link for more information.  https://optics.ansys.com/hc/en-us/articles/360034396734-FDE-solver-analysis-Mode-List-and-Deck

 

[Then Elango]

Thank you very much