I’m an exchange student from France in China, currently working on my final research project, dealing about optimizing an optical transmission using PAM4 modulation.
The simulation I built consists in a chain using two PRBS converted via NRZ to be used as the two entries of a MZM modulator, which generated optical signal is received and converted by a photodetector, to be finally displayed on an eye diagram.
Here, you can see the result I obtain by doing so.
As you can see, a weird curve appears and crosses the diagram in the middle. Since an eye diagram represents the signal periodically superimposed on itself, I don’t see any reason for the signal to stop being periodical at a certain time (hence creating this curve)…
Is this a common issue or bug of Interconnect ? Am I doing something wrong ? Is there a way to correct it ?
 I forgot to say that the greater the datarate (here, 25Gbit/s), the more weird curves there are.
Thank you all for your support.