SIMULATION OF A LASER MODULATOR DRIVEN BY NRZ PULSES

In an earlier paper we presented a mathematical model of distributed f eedback (DFB) and distributed Braggs reflector (DBR) semiconductor las ers that are integrated with an electroabsorption modulator and showed numerical solutions of this model for a sinusoidal modulator drive vo ltage. In this paper we use the model to demonstrate its behavior for a more realistic nonreturn-to-zero (NRZ) drive voltage of the modulato r. In particular, we use the laser-modulator combination to launch the NRZ pulse train into a dispersive fiber and display the resulting pul se distortion by means of eye diagrams. As in the earlier paper, the o bjective of this study is to explore the interaction between the laser and the modulator in response to a residual reflection at the modulat or output that provides unwanted optical feedback between the two devi ces. We find that frequency modulation of the laser due to optical fee dback between the laser and the modulator is the principal cause of pu lse distortion if the pulses are launched into dispersive fibers. Howe ver, the achievable transmission distance depends surprisingly strongl y on the shape of the input pulses. Rectangular pulses with flat tops suffer far less distortion than pulses with more rounded edges.