How to introduce the source in fdtd or direct the source

HI,

I try to simulate the propagation of light in one waveguide. So I need to give out the transverse profile of source. I do it by hard source. The transverse profile is really tailored. However, the source introduce some diffracting in all directions because it is confined in transverse direction.

I know the popular bootstrapping method. It is used to find the transverse profile but not to eliminate the diffraction.

Could some body tell me how to introduce some transverse mode of a waveguide without this problem: the source diffract in any direction?

Thanks.

use the Total field /Scattered field for directional sourse

Thanks for your reply.

However, TL/SL is only used for plane wave source.

I am not sure if I understand your problem correctly, but if your talking about transients generated by
switching on the source suddenly or by not introducing it over the whole wave guide then I guess the
simplest way to deal with your problem is to use some Gaussian envelope.

time dimension => weaker transients
Spatially => weaker diffraction