problem about the simulation of grating coupler
Hi，I have a problem on the simulation of grating coupler. When I optimized 2D grating coupler, I got a smooth transmission.
But, when I use the same parameter to run 3D grating coupler, the transmission have great fluctuations.
the transmission of port1(input)->T:
port1->mode expansion monitor->T_out:
the transmission of port2(output)->T:
port2->mode expansion monitor->T_in:
I increased the simulation time and PML layer but the transmission still have the fluctuations. So, I think the problem is the back reflection.
I do not know why the back reflection is less in 2D simulation but great in 3D simulation?
Where the back reflection is from?
the back reflection seems to introduce from port1(input)?