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Wavelength excursions with magnitudes as large as 6 Å are seen to occur in single frequency lasers (both C3and 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 behavior gives rise to a dramatic degradation of lightwave system performance with increasing bit rate. Laser structures which heavily damp the relaxation resonance peak are seen to exhibit the least chirp and to perform best in high speed transmission systems.