On Adiabatic Chirp and Compensation for Nonlinear Distortion in DML-Based OFDM Transmission

This paper provides detailed descriptions of the interplay between dispersion and chirp (specifically adiabatic chirp), in an optical orthogonal frequency-division multiplexing (OFDM) transmission system based on a directly modulated DFB laser (DML). We experimentally investigated various amounts of dispersion and adiabatic chirp by, respectively, varying the length of the fiber (0-150 km) and the bias current of the laser. By implementing DSP-based compensation for nonlinear distortion while taking adiabatic chirp into account, we determined that adiabatic chirp dominates nonlinear distortion after dispersive transmission. Adiabatic chirp was also shown to mediate dispersion-induced power fading and even provide a power gain. This is an indication that a specific amount of adiabatic chirp may be beneficial to transmission performance, particularly when the nonlinear distortion is mitigated by DSP-based compensation. The proposed adiabatic-chirp-related nonlinear compensation scheme enables an improvement of up to 11 Gbps in maximum capacity, and the presence of adiabatic-chirp-related gain makes it possible to achieve higher capacities following dispersive transmission.