does not converge because of overmeshing


I am designing a chirped Bragg grating. The problem is that I need very thin meshes to get the perfect results but when I do over meshing, the results become worst. I did some changes like changing the parameters of FDTD (stabilize) or decreasing the stability factor up to 0.5 to decrease the reflection, But it did not work at all. I have result with and without mesh. I was wondering if you can help me to get the perfect answer.



Hi @parisa.moslemi

Can you please upload your simulation file for a review?



To view test1.lms, sign in{7E5DE939-508B-40C3-ABA8-2B6FFFE76DB5}&listId=59e896db-b593-4d1e-8684-7893697810f7&itemId=d9b8cc84-ba81-4462-b2e8-a4862ffb87a3 or create an account.


Parisa Moslemi, Miss has shared a OneDrive for Business file with you. To view it, click the link below.


Hi Behzad,

Yes, Please consider the attached file. the mesh in the file is not active. if you active that and see the results, you will see the response is not correct.


Dear @parisa.moslemi

For the Bragg gratings, we use EME solver:

Since you are using varFDTD, simulations tend to take a long time thus making troubleshooting time consuming. If you are looking for an easier and faster simulation, please refer to the linked KB page and modify your simulation file.

Please let me know if this makes sense and I am here to help if needed.


EME is useful for a uniform Bragg grating which is periodic. This is a chirped one which is not periodic and EME is not useful for it.



Dear @parisa.moslemi

I just noticed the small variation between the periods. 8nm difference between adjacent teeth and an overall 8nm change over 1000 periods! which means 0.008nm between adjacent periods :cold_sweat::cold_sweat: and your device is ~320um long!

You are right, since the device is not periodic we can not use EME. But please note that the variations are less than the size of an atom (atom size is 0.1-0.5 nm). Can I ask what physical effect you are trying to see? Any references?

Simulating such a device will require a long time and huge amount of memory, and due to proximity effects will not be fabricable.