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Expand periodic models

Posted Jan 6, 2022, 5:46 a.m. EST Wave Optics, Results & Visualization Version 5.6 2 Replies

Jesco Schönfelder
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Hello everyone,

I am trying to simulate a diffraction pattern in COMSOL. As an optical element I would like to use the official example of a diffraction grating from COMSOL.

In the example they used periodic ports and periodic boundary conditions so they only have to calculate the solution for a single unit cell.

Is it possible to get a diffraction pattern from this example? With this model one can calculate the transmission and reflection of the individual diffraction orders. I would like to expand this model in a useful way so I can apply Fraunhofer diffraction on the bottom line of the model. From my understanding, even behind an infinitely expanded grating the electric field can differ from one unit cell to another so it would not make sense to just duplicate the solution of this model.

Thanks for your help

Jesco

2 Replies Last Post Jan 7, 2022, 5:24 a.m. EST
Robert Koslover Certified Consultant
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Posted: 3 years ago Jan 6, 2022, 11:33 a.m. EST

You say: "From my understanding, even behind an infinitely expanded grating the electric field can differ from one unit cell to another..." Are you referring merely to the phase?

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Scientific Applications & Research Associates (SARA) Inc.
www.comsol.com/partners-consultants/certified-consultants/sara
You say: "From my understanding, even behind an infinitely expanded grating the electric field can differ from one unit cell to another..." Are you referring merely to the phase?
Jesco Schönfelder
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Posted: 3 years ago Jan 7, 2022, 5:24 a.m. EST
Updated: 3 years ago Jan 7, 2022, 5:26 a.m. EST

I think the intensity of the electric field behind the grating can differ from one unit cell to another as well.

Let's assume the electric field propagates towards the grating with the angle alpha = 15°. I will attach a picture of the duplicated solution for ewfd.Ez for this case. The problem with this solution is that in each unit cell the electric field does not reach the grating in a different phase and the respective electric fields of the unit cells do not interfere with each other. Thus, there is not the proper near or far field.

I am looking for a solution with a continuous wave that is propagating with the angle alpha towards the grating. There is the option to duplicate the geometry in the first place before computing but in my case I would have to compute the solution for 5000 unit cells which would be too much work for my PC.

Can you think of a way to get the correct near field for a width of about 2 mm without building a geometry this big? Is it possible to use the periodic boundary condition for this?

I think the intensity of the electric field behind the grating can differ from one unit cell to another as well. Let's assume the electric field propagates towards the grating with the angle alpha = 15°. I will attach a picture of the duplicated solution for ewfd.Ez for this case. The problem with this solution is that in each unit cell the electric field does not reach the grating in a different phase and the respective electric fields of the unit cells do not interfere with each other. Thus, there is not the proper near or far field. I am looking for a solution with a continuous wave that is propagating with the angle alpha towards the grating. There is the option to duplicate the geometry in the first place before computing but in my case I would have to compute the solution for 5000 unit cells which would be too much work for my PC. Can you think of a way to get the correct near field for a width of about 2 mm without building a geometry this big? Is it possible to use the periodic boundary condition for this?

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