Chirp Characteristics of Directly Modulated Lasers During Multilevel Modulation

The chirp characteristics of directly modulated lasers (DMLs) are the major constraint on the performance of directly modulated optical communication systems. However, studies on the chirp characteristics of DMLs are typically focused on binary-level modulation. An analysis of the chirp characteristics is presented for DMLs during multilevel pulse amplitude modulation (PAM) to explore the similarities and differences in chirp behavior between DML applied to multilevel modulation and two-level modulation. Simulation results of transient chirp and adiabatic chirp of four-level, eight-level, and sixteen-level PAM signals agree with the analytical findings. Increasing the number of levels in the PAM signal reduces the probability of larger amplitude chirps and improves interference immunity. Moreover, the adiabatic chirp is more influential at lower bit rates, while transient chirp dominates at higher bit rates. Finally, this paper analyzes the impact of chirp on fiber transmission dispersion and the compensation methods. The results show that the chirp exacerbates the dispersion, especially the adiabatic chirp, which leads to severe eye diagram tilt. Analysis of chirp characteristics facilitates further compensation and utilization to improve the performance of DMLs.