Impact of Damping on 50 Gbps 4-PAM Modulation of 25G Class VCSELs

We present an investigation into the effects of photon lifetime and the associated damping of the modulation response on 50 Gbps 4-pulse-amplitude modulation (4-PAM) signal generation with directly modulated 25G class vertical-cavity surface-emitting lasers (VCSELs). While 4-PAM benefits from higher slope efficiency and output power, it is shown that the greater impact of intensity noise and dynamic nonlinearities implies that a VCSEL with a longer photon lifetime and more damped response is needed when transitioning from existing 25 Gbps OOK to 50 Gbps 4-PAM using the 25G VCSEL technology.