Learning Forum

[Po-Yu Hsiao]

Hi,

I am trying to design a Y-branch combiner that connects three single-mode waveguides(only fundamental TE and TM modes allowed).

First, I used MODE to check whether a waveguide is single-mode. As shown in the attachment, the waveguide only showed two fundamental guided modes (TE & TM) when the number of mesh cells is set to 800 in x and y.

Thus, I used this waveguide to design a Y branch and set a fundamental TM mode source injecting from the right-upper port(Please refer to the attachment) in FDTD. However, I found the output port showed a mode that is neither fundamental TE nor TM mode. I think only the fundamental TE and TM modes are allowed and will be guided in this waveguide. Why does this happen? Is this because the mesh of FDTD is too coarse?

[Guilin Sun]

This post has been replied in Zendesk support. For the benefit of other users, I give a short reply here:

Most likely there are other higher order mdoes that are excited due to bending and waveguide geometry change. To check if the waveguide supports higher order modes, please use more numer of trial modes.

[Then Elango]

Hi, I think changing the mesh settings will not affect the number of modes in the waveguide. Because Meshing posint would be required to determine the accuracy of the system. 

Is this assumption right?

[Guilin Sun]

Not necessarily right. you can try to use 2*2 mesh and check the result. FDE solves eighgen values so it needs some details of the structure.

The most important thing is: when Metal BC (the default BC for mode expansion monitor) is used it has box modes, which are not necessarily the propagation modes, whereas using PML BCs can have a lot of PML modes that need to e removed. Since you set a certain number of tiral modes, when the solver reaches this number it will stop, so some higher order modes may not be found.

 

[Then Elango]

Thank you for the detailed reply.