Take a look at Paul Falstad's amazing applets:

http://www.falstad.com/mathphysics.html

There is a wave guide applet that may do what you want to simulate.

I am also working on an FDTD solver for photonics, including optic fibers. I will keep you posted when I have something. It may take me a long time to get there because I do not have much time for doing it.