optical Waveguide

Hi
I am defining 2D waveguide from Er:Ta2o5 waveguide in top and bottom of sio2 substrate, now i want to define electromagnetic wave to this.and then i want to calculate the gain.
1-how can i excite the waveguide?
2-i am using of ports?
3-from where of this waveguide i define the wave excitation?from the bottom or top?

Thanks in advance
Micheal