Hi Damian,
Thank you for your reply. By the way, is it possible to use multi mode fiber in your PAM direct detection. I tried to replace the Laser and SMF in your diagram with Spatial VCSEL, Parabolic-index MMF and Spatial PIN photo-diode. The BER Test Set gives me results, but the Electrical Constellation Visualizer seems not work well. Could you help me check my setting in the attached files.
Thank you