Transient chirping in single-frequency lasers: lightwave systems consequences

The effect of modulation-induced laser wavelength shifts on optical transmission through dispersive fibres is shown to be a serious limiting factor at bit rates greater than 1 Gbit/s. Wavelength excursions with magnitudes as large as 6 ¿ are seen to occur in single-frequency lasers (C3 and DFB) during a transition from one power level to another. The wavelength shifts briefly toward shorter wavelengths and then back to the equilibrium value during turn-on and toward longer wavelengths and back during turn-off. These excursions, which are well explained by a model in which the carrier density is temporarily driven out of equilibrium by a change in injection current, last for hundreds of picoseconds or about one half of the relaxation resonance period. This time-dependent behaviour gives rise to a dramatic degradation of system performance with increasing bit rate.