Linearisation Method of DML-Based Transmitters for Optical Communications Part III: Pulse Amplitude Modulation

Abstract

A new linearization method for optical transmitters based on directly modulated lasers (DMLs), named the Stretched A method, was proposed in Parts I and II of this work. Parts I and II presented the theoretical framework of the method for non-return-to-zero (NRZ) modulation and related detailed simulation and experimental results. Here, we extend the method to pulse amplitude modulation schemes (PAM). Focussing on 4-level PAM (PAM-4), we present the theoretical background and discuss implementation options. A simplified variation of the method for the generation of PAM signals with a significantly lower number of sub-currents is proposed. Simulation studies for PAM-4 transmission at 50 GBaud (100 Gbs) and an experimental proof-ofprinciple demonstration at 16 GBaud (32 Gbs) are reported based on 850 nm vertical-cavity surface-emitting lasers (VCSELs). For PAM-4, products of effective eye diagram areas (${\boldsymbol{PS}}$) of 0.6 105 and 43 105 before and after applying the Stretched A method were measured, demonstrating an improvement ratio of 72. The sensitivity and tolerances of the method are analyzed using simulation and experiment.