Grating Coupler with normal (theta = 0) incident light

fdtd

#1

Hello everyone, I’m trying to get some decent transmissions through a 2D grating coupler.

Starting with the basic introductory file found at https://kb.lumerical.com/en/pic_passive_grating_coupler_2d.html , I am trying to couple some light in the visible range (400-700nm) that is perfectly normal to the grating coupler. However when the light is normal to the surface, the back reflections are huge.

After doing some research I’ve found that some authors have modified the grating coupler pitch and duty cycle to have additional in-between structures. I’m trying to modify my GC to mimic those structures, but I’m not sure how to add my own custom patterns in the Edit Object menu. I’ve tried doing the Add option but I can’t seem to get anything physical to show up. Any tips on customizing our grating?

Also, I’m desperately in need of more tutorial vids like the one found in the above link. Does anyone have any recommendations?

All the best,
Josh



#2

Hi @jossue.montes,

If you want to add some additional structures inside the grating coupler structure group, you will need some procedures. First, you need to define the geometrical parameters describing physical feature; second, according to your parameters you can add your parameters in the User properties in the Properties tab as you already did; third, this would be not familiar to you, based on the parameters, you have to edit the script command lines in “Script” tab.
For examples, as you can see the following images, the parameters such as “input length”, “h total”, and “etch depth” shown in the User properties also appear in the Script tab. In the Script tab, physical structures are building up with regard to the pre-defined parameters; i.e., the “addrect” is a command for generating a rectangular geometry.

I’m guessing you just typed the physical parameters in the Properties tab for creating in-between geometries and did not modify the script command lines in Script tab according to your parameters. You must modify the script lines to construct the in-between structures. There is a webinar video for the script language, it could be helpful for you; here is the link: https://www.lumerical.com/support/courses/fdtd_scripting_webinar/video.html.


#3

Can you upload the paper or a snapshot of the new grating structure you are try to simulate? To tell you if there is a shortcut for this.


#4

Hello guys, my apologies for the late reply. Thanks for answering! I (un)fortunately no longer need to use asymmetric gratings. The type of structures are called Sub Wavelength Gratings (SWG), and one of my motivations can be found in this PDF.

Thanks again![8] Focusing Sub-Wavelength Grating Couplers with Low Back Reflections for Rapid Prototyping of Silicon Photonic Circuits.pdf (3.0 MB)