Bragg reflector


#1

Hello everyone,

I followed this tutorial in order to make my own bragg reflectors. I get surprising result for a specific grating depth. As you can see on the following pictures, the maximum reflection drop for a grating depth around 75nm. I was expecting a maximum value between the maximum of the 50nm and 100nm gratings.



I check the mesh parameters, and a finer one does not change my results. All the calculated parameters in my script seem correct.

Am I missing something ?

Here are my .lsf and .lms files :
wavelengthsweep.lsf (4.3 KB)
BraggReflector.lms (120.8 KB)

Thanks for your help :slight_smile:


#2

I checked the file and did some testing by changing the “tolerance 1” and “tolerance 2” settings of the EME advanced options tab based on the recommendations from the following page under the “advanced settings and how to use them” heading:
https://kb.lumerical.com/en/index.html?layout_analysis_eme_error_diagnostics.html

I used tolerance 1 = 0.5 and tolerance 2 = 1e-5

I also tested whether the method of sweeping the group spans to get the broadband result was valid by setting up a parameter sweep to sweep over wavelengths from 1.54 to 1.56 um over 50 points.

I found that the results stayed very similar where the peak in reflection has a much smaller amplitude for the etch depth of 75 nm compared to 50 nm and 100 nm etch depths.

Since the results don’t change with a finer mesh or increased number of modes, and the results are consistent between using the method of sweeping the wavelength and sweeping the group span, it suggests that the results could be physical. If you have access to FDTD Solutions, it might be worth doing a check between a 3D FDTD simulation and the EME results (for a fewer number of periods so that the simulation time is reasonable).


#3

Thanks for your answer, I check with FDTD and a I had some results going in the same way for the 75nm etched depth.

I also changed the tolerances in the EME simulation, do you have any advice to have good value ? I get similar result for an etch depth below 100nm for tolerance1=0.5 and 0.75, above the results are different. Does it depends on the expected losses ?


#4

Yes, the setting should be set based on the expected loss, and since for the Bragg waveguides the loss should be 0, I would expect that setting the tolerance 1 value to a higher value would give better results.


#5

Hello again,

I am wondering how I could implement the propagation loss on this model is it possible ? I want to fit my results with some experiments.

thanks in advance


#6

Typically for the EME simulation of a Bragg waveguide we enforce energy conservation since small errors in loss can lead to large discrepancies over many periods. You could change the energy conservation method to “make passive” to allow losses, however, the result may not be trustworthy. I believe that for this type of device the loss is usually obtained from experiment rather than simulation for this reason.