Shaping Current Accurately to Suppress the Transient Chirp for Multi-Level PAM Signals of Directly Modulated DFB Lasers

Low-cost direct modulation direct detection (DM-DD) systems in optical communications are severely limited by transient chirps when adopting higher-order modulation formats to meet the high-capacity transmission. To break through this constraint, a shaping current waveform technique combined with multi-level pulse amplitude modulation (PAM) is proposed to suppress the transient chirp of directly modulated distributed feedback (DFB) lasers. This pre-compensation technique only requires simply changing the injected current waveform, which in combination with phase space trajectories visualizes the effect of the technique in obtaining the optimum output from the laser. Experiments show that the pre-compensation technique can minimize the transient chirp of higher-order modulated signals and reduce the overshoot of the eye diagram. Furthermore, to further validate the effectiveness of the pre-compensation technique, the eye diagrams are compared and analyzed at the receiver side. The simulation results show that the current shaping technique achieves the desired laser output, which could improve the higher-order modulation performance of the DM-DD system.