High-speed optical transmission system using 1.55-μm directly modulated lasers

Abstract

We present the small-signal frequency responses of single-mode fiber used in directly modulated laser/direct detection (DML/DD) and externally modulated transmitter/direct detection (EXT/DD) systems, and compare the dispersion tolerance of these two systems. We find out that DML/DD system could be more tolerant to fiber chromatic dispersion than EXT/DD system when an electrical equalizer is employed at the receiver. We also present the transmission of 56Gb/s 4-level pulse amplitude modulation signals generated from a 1.55-mu m DML over 20-km standard single-mode fiber with the aid of a linear electrical equalizer. The performance behavior of this system with respect to the transmission distance is explained by using the frequency response.