Carrier density oscillations and frequency chirping of pulse code modulated single frequency semiconductor lasers are shown to be greatly reduced by use of a small current step in the leading edge of the drive pulse. For a 100 km single mode fiber link the dispersion power penalty at 1. 55 mu m resulting from dynamic line broadening is predicted to be reduced from 3. 7 to 1. 0 db by use of the modified current pulse.