Hi Damian,
These are great answers 
They have helped alot. I have few more questions if you could help me understanding them:
1) How to know the fiber modes? I can select the number of fiber modes from the parameter LP(m,n) max but how to see the fundamental mode of the fiber itself? What I can see on the Spatial Visualizer is the result of mode mixing between the input modes and the fiber modes. Is there a way to see the fiber modes only?
2) As you say in the reply of point Nr. 3 that the spot size at the output is increased because of the larger spot size of the fundamental mode so how can I change the size of the fundamental mode of fiber? I do not see any parameter to do that. Does it depend on fiber core size?
3) you wrote in reply to the mode matching: “you could use a lens to focus the light into a smaller area or expand the beam into a larger area. OptiSystem has components to model this behavior”. Are you referring to Thin, Vortex lens and Spatial Aperture in OptiSystem?
Many thanks