Simulation of a laser modulator driven by NRZ pulses

In an earlier paper we presented a mathematical model of distributed feedback (DFB) and distributed Braggs reflector (DBR) semiconductor lasers that are integrated with an electroabsorption modulator and showed numerical solutions of this model for a sinusoidal modulator drive voltage. In this paper we use the model to demonstrate its behavior for a more realistic nonreturn-to-zero (NRZ) drive voltage of the modulator. In particular, we use the laser-modulator combination to launch the NRZ pulse train into a dispersive fiber and display the resulting pulse distortion by means of eye diagrams. As in the earlier paper, the objective of this study is to explore the interaction between the laser and the modulator in response to a residual reflection at the modulator output that provides unwanted optical feedback between the two devices. We find that frequency modulation of the laser due to optical feedback between the laser and the modulator is the principal cause of pulse distortion if the pulses are launched into dispersive fibers. However, the achievable transmission distance depends surprisingly strongly on the shape of the input pulses. Rectangular pulses with flat tops suffer far less distortion than pulses with more rounded edges.