Answer:
I have attached an example of a single mode VCSEL exciting multiple modes in a Measured-Index Multimode Fiber. The Gaussian beam at the output of the VCSEL first propagates through 300 μm of free space and is then coupled to a multimode fiber. The spreading of the beam causes multiple modes to be excited, which can be observed at the Spatial Visualizer_7 by changing the Mode number. After more free space propagation the signal is recovered and the BER is calculated.