2D butt coupled non-identical fibers using EME solver


#1

Hi @bkhanaliloo

continuing discussion from:

I am starting out with a much simpler situation than an evanescent coupler. Its 2-D butt coupled non-identical fibers. And I want to extract the transmission / loss due to mode mismatch. I am aware that I can easily extract it through the mode overlap value. However, in order to understand the software better, I want to do the same by using EME. Please see the attached working model and a ppt with few queries on the same.

Would sincerely appreciate some pointers to the questions.

best regardslumerical query 20170515.pdf (423.9 KB)
butt_coupling-fibers.lms (739.6 KB)


Directional coupler simulation questions - varFDTD can not calculate coupling coefficient properly
#2

Dear @sxd

It looks like there is a big discontinuity in fiber sizes moving from fiber 1 to fiber 2 with core size of 4um to 3um, respectively. In such cases, fundamental mode in the first fiber will be coupled to higher order modes in the fiber 2. This should explain most of your questions. Here are my response to your queries:

1. How to interpret the increase in the value of |E|^2 after the fiber joint? and 3. How to visualize this increase in |E|^2 near the core of the fiber in a line plot? And what causes this? Is this the standing pattern due to the waves reflected at the output facet of the fiber? How to avoid it in simulations?

This can be due to three reasons: a) higher order modes might be scattering of the device b) you are visualizing total electric field which has higher order modes with complicated profiles c) modes are more confined in the smaller fiber.

Also, note that you are using only 6 mode in each EME section/fiber, which might not be sufficient enough to completely capture coupling to all the higher order modes. I recommend you to use a higher number of modes to propagate.

For example, fundamental mode in fiber 1 is coupled to fundamental mode and fifth mode in fiber 2 (you can check this by visualizing internal s matrix from EME result view):

and then clicking on view data in the visualizer:

You can learn more about the EME tool by checking the link below:
https://kb.lumerical.com/en/index.html?solvers_eigenmode_expansion.html

2.The value of transmission (S21)^2 is close to the expected analytical value although with some amount of discrepancy. Is there a way to improve accuracy of the result?

Yes, a) by increasing number of modes b) performing 3D simulations as in current 2D simulations the spherical shape of fibers are not considered. Software assumes that your geometry is identical in the z-direction (an infinitely wide waveguide).

4.Can the same configuration be modelled using EME 3D prop mode? How to extract the transmission in that case?

I am not quite sure if I understood this question, but you can use 3D simulations and then sum over S1j^2 (assuming mode 1 in fiber 1 and all the modes in fiber 2 denoted by j).

Also, I recommend you to use a larger EME simulation region and use metal for y-PML boundary conditions.

Please keep me update with your thoughts. It is also a good idea to discuss one question to make sure that it is answered properly.

Thanks


#3

Dear @bkhanaliloohttps://kx.lumerical.com/u/bkhanaliloo,

Thank you very much for the suggestions. I agree that the multimode nature of the fibers complicates matters significantly. So, in order to just understand the full functionalities of the software first, I have simplified the case as much as I could envisage.

So, now I would like to simulate two butt-coupled identical fibers and extract the propagation parameters (transmission/reflection/S21 etc.) using 3D simulations. (the 2D simulations work fine).
I have attached the modified model.
The results are attached in the pdf file.

Issue:

  1. The field profile of the 1st cell shows the correct fundamental mode (FM) of the fiber. However, when I try to view the same field profile using the EME profile monitor at the input (after PROPAGATE), I get an unexpected profile with a field zero at the center. I expected this to resemble the FM of the fiber, considering that I put “Port1” as “Source port” and “Mode 1” as the input in the EME Analysis settings. Where am I going wrong?

  2. The value of |S21|^2 (transmission) should be 1 since the fibers are identical but that is also not the case. Am I doing something wrong the setting up of the model? (I have executed the model in 2D XY plane, X prop mode, and it works fine. The field profile and the |S21|^2 values are 1, as expected.)

I eagerly look forward to some clarifications.

Best regards
Sonali

butt_coupling-fibers-3D EME.lms (1.1 MB)

lumerical query 2 - 20170522.pdf (379 KB)


#4

Dear @sxd

Thanks for the info.

Your simulation file looks fine. The only problem is that you are not injecting/detecting proper modes at port 1 and port 2. To select the modes on each port, click on select modes and choose the fundamental mode (for both ports):

and here is the mode profile:

Please note that the injected profile might not get updated automatically if you change the geometry. It is a good practice to check it before running simulations.

Here is the mode profile detected at input field monitor:

and finally abs(user s matrix)^2:

I think everything makes sense now.

I hope this was useful.

Thanks


#5

@bkhanaliloo: Yes absolutely! Thank you very much for the clarification.