Simulation of Surface Plasmon Polaritons on Kretschmann Configuration

Hi,

When simulating SPPs on Comsol so that I can calculate the magnetic reflected field on the plasmon ressonance to determine the Reflectance x Incident angle at the surface (prism/material/air), which type of excitation port is the best to select: numeric, rectangular or periodic? What are the diference between them? How can I determine the magnetic incident field from the optical incident power of 1Watt?

Thanks,