Hello,
Mohamed is no longer with Optiwave.
As he said to you, you need to choose the proper parameters of the erbium-doped fiber such as core radius, refractive index of core and cladding and index profile to give you 4 modes for the “Erbium Doped MM Fiber” component in OptiSystem. You can also use the “Spatial Visualizer” to view the produced mode in OptiSystem.
regards,
Ahmad