50 Gbps With 4-PAM

Good morning!
I need a helping hand.
In fact, i’m simulating an optical link with 50gb/s speed. This link uses the 4-PAM modulation format in direct intensity modulation on a VCSEL laser which sends the signal over a multimode type fiber. The signal is directly detected by a photodiode.
For reasons of laser configuration, I had modeled my own VCSEL laser by making a matlab-optisystem co-simulation.
My simulation does not work. I believe that my problem lies in the setting of the various components of the link such as the VCSEL laser, the MMF fiber, and the PIN photodiode.
Could someone help me get the right parameters for these components for a 50Gb / s (single channel) link?
The help of someone who has already simulated such a speed on such a link would be very precious to me.
NB: I do a cosimulation using matlab 2018a and optisystem7
thank you!