2d fdtd solver

I've read SUllivan implementation of plane wave, and I compare it with Taflove's theory of TF\SF. I found some incompatibilities.
Now I'm trying to simulate plane wave, by implemetation of taflove's thery. In free space I have plane wave which distributes through the cell. But when I put in the main grid (in Total field zone) double-slit, then I get simulation Errors. I get difracted wave and waves generated from right and left border :( .
So who can help me to solve this problem???

So? Nobody can help me?