There are different optical amplifiers, Omni-directional and bidirectional. based on the V number (as function of core radius, the operating wavelength, refractive indexes) you can specify the number of modes to be excited inside the optical fiber.
One example is the “Er Doped Fiber” bidirectional component.
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.