Transmission results greater than one in a Single Bus ring resonator — varFDTD

[Daniel Pimbi]

Dear Ansys Forum,

I am using the example of a double bus ring resonator (ring_resonator2.lms) provided by Ansys https://optics.ansys.com/hc/en-us/articles/360042800233  as an initial step to simulate a single bus ring resonator.

Here is the transmission from the double bus ring resonator:

 

However, using the same simulation file Ansys provided, I removed the drop waveguide to make a single bus ring resonator, as shown below. The transmission result at the through port consistently exceeds one.

 

I implemented all the solutions given in the post “Transmission Results Greater Than One” https://optics.ansys.com/hc/en-us/articles/12614264530323 , but I could not resolve this issue.

Additionally, all the suggestions given in previous posts /forum/forums/topic/ring-resonatorabout-my-transmission-more-than-1/ , such as increasing the simulation time and reducing the auto shutoff, could not still solve the problem. I noted that varFDTD only gave me reliable transmission for the double bus ring resonator.

Is there any reason for the transmission to exceed one in a single bus ring resonator? And why more ripples in the transmission spectrum?

 

Thanks for your help!

[z.l.li-1]

 

 

 

 

 

[Guilin Sun]

This is common when the simulation time is not long enough, and probably the autoshutoff min is a little large. Single bus ring needs much longer time to decay the signal. So please significnatly (say 100 times more ) increase the simulation time, until you can get smooth result. If it diverges, please use more number of PML.

[Guilin Sun]

I did some tests, and found that it can reach convergence by modifying the following:
1: use narrowband instead of broadband (the refractive index does not change much)

2:increase xy simulation span

3: use 36 or 64 PML layers

4: delete the current varFDTD object, add a new one and modify the settings

5: increase simulation time.

Mostly the divergence is caused by PML, as you can extend the Profile monitor to cover PDF and record the data inside the PML. There is strong field intensity/power density in front of PML.

[tao wang]