Learning Forum

[Kevin Portner]

Hello, I want to know the optimal pitch for various grating coupler designs. I use the following file which already implements a grating coupler: https://optics.ansys.com/hc/en-us/articles/360042305334-Grating-coupler

 

I am quite confused by the TM and TE mode settings. I want to determine the optical pitch once for the fundamental TE mode and once for the fundamental TM mode in the waveguide. The simulating uses both TM modes for Port 1 and Port 2 in the standard settings. If I change both to TE (for Port 1 and Port 2) I get a very bad FOM. Is seems that Port1 needs to always be TM mode. Can anyone explain me why? On the other hand, if I set Port 1 to the Fundamental TM mode and Port 2 to the Fundamental TE mode, I again get the same results as I had in the case where both Port 1 and 2 were Fundamental TM modes. Can anyone tell me what I am doing wrong and how I can launch TM modes in the waveguides?

[Guilin Sun]

The waveguide is ridge-type, so it is very dispersive between TM and TE. The example is designed to work for one polarizatio, as it used "anti-symmetric" BC on ymin.  Please check the mode profile of the injection port before the simulation, and make sure it is the mode you want.

Once you change the polarization, you have to modify the BC. please refer to this article: https://optics.ansys.com/hc/en-us/articles/360034382694-Symmetric-and-anti-symmetric-BCs-in-FDTD-and-MODE

If you have questions for the symmetry BCs, please write a new post.

 

 

 

[Kevin Portner]

Thanks for the quick feedback! I assume that you are talking about the 3D simulation (where I see that the BC is anti-symmetric for ymin). However I was currently only using the 2D one. In that case I see that all the BC are PML and no symmetry is used.

Maybe I was not clear on what my goal is: I concretely want to find out what is the optimal pitch and etch depth for a fixed duty cycle of 0.5. I want to determine this for a Silicon WG with height 220nm (the one that is already implemented) and one with height 340nm. In both cases I want to know the pitch and etch depth for the fundamental TE but also for the fundamental TM mode. Here are the points that are not fully clear to me:

-It should be sufficient to use the 2D simulation only, right? Or is it necessary to go to the 3D simulation?

-The standard setting in the 2D simulation uses fundamental TM modes for both port 1 and port 2. However, for port 1 the injection axis is the y-axis while it is the x-axis for port 2. This confuses me. If the injection axis is the y-axis (as for port 1), TE would mean that the fields are polarized along y. On the other hand an injection axis along x (as for port 2) means that for TE = fields are along x. For port 2, this would however mean that I have a TE mode no? In the simulation the mode is however specified to be the fundamental TM mode. Could you explain me where I am wrong?

-What settings for Port 1 and Port 2 do I have to concretely select if I want to have the fundamental TM mode in the waveguide? And which settings do I have to select in the case of the fundamental TE mode?

 

Thanks already in advance for your answers.

[Guilin Sun]

A1: it will depend on the accuracy you desire. Usually we recommend at least to use 3D FDTD for verification.

A2: It is confusing for TE and TM. I usually recommend users to check the domiant component. in this case, TM means Ez dominant. Do not be limited by "definition".