We find that the onset of instability in many GaAs DH lasers that are capable of self‐induced modulation is always preceded by the strong emission of noise. The longitudinal optical‐mode width is fairly narrow, typically 0.1–0.3 A˚, when the optical power contains noise. It is only after the optical power exhibits coherent self‐induced modulation that the apparent longitudinal mode width increases to 1–2 A˚. This apparent increase in longitudinal mode width is attributed to a wavelength that changes periodically (sinusoidally, for small signals) with time. The wavelength oscillation is due to variations in refractive index brought about by oscillations in the carrier concentration. The implication is that in these lasers, the instantaneous longitudinal optical mode width is relatively narrow regardless of the temporal instability of the light output. In addition, it is shown analytically that there is a critical noise level above which a laser becomes unstable, i.e., exhibits coherent self‐induced modulation.