Diffraction on an isolated line

hello everyone,

I have a problem.

now I want to simulate the diffraction for an isolated line, and I want to observe the intencity of different diffraction orders, both transmitted and reflected. Presently I want to use scattering boundary condition, but the intencities of downward and upward waves are mixed, and I donot know how to seperate.

Whether only I used ports and S parameters could I get the intencity for a certain diffracted order? If I continued to used scattering boundary condition, is there any method to seperated the indicident wave and diffracted wave?


Thanks a lot