Low-chirp and enhanced-resonant frequency by direct push-pull modulation of DFB lasers

The first long haul experiment on the use of a new direct modulation scheme is reported. This scheme in principle permits simultaneous tailoring of the chirp and enhancement of the resonant frequency of a distributed feedback (DFB) laser. Numerical and analytical results are presented that demonstrate the properties of push-pull modulation along with supporting experiments. Measurement of the time resolved chirp shows that push-pull modulation results in a low-chirp and a unique-chirp shape, which improves pulse transmission along a dispersive fiber. Initial experiments, using a bulk active region unoptimized DFB device driven directly by push-pull modulation, demonstrate operation over 150 km transmission at a bit rate of 2.5 Gb/s with a practical system receiver giving 10^-9 bit-error rate at a dispersion penalty of -0.5 dB. Significant improvements are foreseen using quantum-well material. Simulations indicate that with appropriately optimized devices and drives, direct modulation at 10 Gb/s may give transmission over 100 km of standard fiber comparable to existing externally modulated systems