rib Bragg grating


I have designed a rib uniform Bragg grating (for PN junction ) with Mode.
why I can not see the reflection of Bragg? file is attached.

PariPN_BG.lms (2.3 MB)


Dear @parisa.moslemi

It looks like all the material have a constant refractive index of 1.4 and your bragg grating basically behaves as waveguide. Please modify that and then check the source to make sure that you are injecting a proper mode into the left waveguide.

There are also a few more considerations:

  1. make sure that your varFDTD mesh is fine enough to resolve the slight change between adjacent gratings or you can use override mesh

  2. I don’t think that varFDTD is the best solver for these types of simulation. Since you have a periodic array, EME will be very fast and more convenient for you. Please refer to the link below for more information:

and maybe watch this webinar:



Thanks for your response. But the material is defined si for the core and sio2 for the cladding. which refractive index do you mean?


even after changing the material, I can not get the expected response


Dear @parisa.moslemi

Sorry for the confusion. If you check the simulation file, all the materials are set to have an index of 1.4. Can you check them again and make sure that they are set properly?

Is there a specific reason that you want to use varFDTD for these simulations?
Can you please attach your modified simulation file, and also plots/reference showing the results that you are looking for?



yes I want to add a asymmetric wave guide to it. So I can not use EME.
The file is attached.
ThanksUBG.lms (1.4 MB)


Dear @parisa.moslemi

EME should be able to solve the asymmetric cases. I still think that our goal should be to use EME which is optimized for this type of simulations.

I haven’t done Bragg grating simulations with varFDTD, and I think it will be a good practice for both of us to start with simpler cases and replicate the results of, for example. this KB page (which uses EME).

Anyhow, I did some modifications into your simulation file and I can see some grating effect. The main change was to use PML boundary conditions (BCs) along x so that light does not circulate inside the grating. Here are the results:

It looks like we have a bandgap around 1.54 um. Please take a look at it and let me know if the results make sense.

UBG_modified.lms (1.5 MB)



Thanks for your response. the place of the pulse is close to what I want. But the amplitude should be 1. Do you have any idea about that?


OK, good. At least the resonance makes sense.

The plot that I attached in the previous post shows both transmission and reflection (captured at two ends) of the grating. Since T+R will be one, I tend to think these results make sense and it demonstrates the actual behavior of the device. But we can try few things before coming into a conclusion: Can you use finer mesh and increase the simulation time and run them again? Also, try changing the y BC to PML, so that scattered light does not get back into simulation? Keep me updated with your results.


Thank you so much the result improved. I n the video at designing BG, it recoment using the lsf file called lito-simulation to see the desing after litography. I could not find it do you know where is the file?



That’s great, and it will be nice if you could upload some results.

For the lithography files, please refer to this link:

Make sure that you are logged into your account, and then download the related files.