Learning Forum

[Hrishikesh Iyer]

Hello, 

I have a dielectric grating coupler tooth with a plane wave input with dimensions matching the waveguide. If I set Bloch boundary conditions, will I be able to simulate an infinite (in the Bloch-boundary direction) grating coupler? 

The simulation does seem to work, however, I'm not sure whether it's physically correct to do this. If it is correct, can anyone help me extrapolate the field intensity along the grating length? 

Thanks! 

[Dimitris Polyzos]

Hello Hrishikesh.

Thank you for contacting us.

Could you please share a screenshot of your model to have a better perception of your issue? Correct me if I am wrong, but you mentioned that the plane wave matches the dimensions of the waveguide, while I would recommend to extend the plane source edges outside of the simulation region. I will have a clearer image if I could check your model though.

In general, Bloch-boundary-conditions-in-FDTD-and-MODE is mostly used for periodic structures illuminated with a plane wave source propagating at an angle for a single wavelength. For broadband source you should use BFAST BCs. 

Also, cuold you explain a little bit more what do you mean by this: If it is correct, can anyone help me extrapolate the field intensity along the grating length? 

Do you mean the far filed when you refer to extrapolation? 

Thanks a lot, in advance.

Kind regards

Dimitris

 

[Hrishikesh Iyer]

Thank you for the reply Dimitris. I have a monochromatic plane wave source behaving like a waveguide mode. 

I would like to simulate the angle of scattering from the grating tooth, and also break down the several grating orders. 

[Dimitris Polyzos]

Hello Hrishikesh, Thanks for your reply and sharing your model. As I suggested in a previous post of mine, I would recommend to extend the span of the source through the FDTD simulation boundaries. Please refer to the following articles: https://optics.ansys.com/hc/en-us/articles/360034382874-Understanding-field-truncation-issues-with-finite-sized-plane-wave-sources. https://optics.ansys.com/hc/en-us/articles/360034382854-Plane-wave-and-beam-source-Simulation-object I hope I helped. Please let me know if you need further assistance. Kind regards Dimitris

[Dimitris Polyzos]

Hello  Hrishikesh,

I hope you got my answer as something was woring with my PC. I will post it again just to be sure that you got it completed.

Thanks for your reply and sharing your model. As I suggested in a previous post of mine, I would recommend to extend the span of the source through the FDTD simulation boundaries as I am afraid that you didn not set it up correctly. More information why you should proceed with this extension of source spans can be found at this article:

Understanding-field-truncation-issues-with-finite-sized-plane-wave-sources

Plane-wave-and-beam-source-Simulation-object

Also, if you want to analyz the grating orders you should check the Gratings-List-of-Examples that inlcudes some good articles as well :

Nanobeam-grating

Diffraction-grating-DGTD

Grating coupler

I hope I helped. Please let me know if you need further assistance.

Kind regards

Dimitris