Learning Forum

[Hrishikesh Iyer]

Hello, I need help figuring out how to estimate the number of orders scattered by a dielectic grating coupler with a waveguide mode input. When I use the grating function on a field monitor above a uniform sub-wavelength grating coupler I see a very large number of orders (>800) which doesn't seem correct to me. 

Any idea what could be going wrong? 

Here is the cross-section view:

Thanks! 

[Afroditi Petropoulou]

Dear Hrishikesh,

 

Thank you for reaching out to us.

 

Could you please give me some more information about your structure? When you use the grating - Script command – Ansys Optics and gratingn - Script command – Ansys Optics commands, do you have most of the power to the expected order?

I guess for your simulation you use PML boundary conditions. In this case, your grating is finite and probably the extra orders that are calculated are due to the scattering from the edge elements.

The grating - Script command – Ansys Optics is mostly used for periodic structures, for which a single unit cell is simulated. This assumes an infinite number of periods and the grating orders that are supported are discrete and depend on the period of the grating (which is the same as the width of the simulation region). In your case though, where the waveguide mode is used for the excitation, you don’t assume an infinite structure. When the mode hits the first grating, the light can be scattered in many angles (practically every possible angle) and this might be translated into grating orders by the monitor.

 

Best regards,

Afroditi