The effect of current fluctuations on the linewidth of semiconductor lasers is analyzed using the single-mode rate equations. Since the time scale of such fluctuations can generally be longer than the intrinsic time scale of relaxation oscillations, current fluctuations are modeled using a non-Markovian random force in the rate equation governing the carrier dynamics. In the absence of nonlinear-gain effects, the contribution of current fluctuations to the linewidth is negligible at all power levels. However, when the gain saturation resulting from spectral hole burning is included, current fluctuations are found to give rise to a power-independent contribution to the linewidth. At low operating powers, this contribution is small compared with the spontaneous-emission contribution. For InGaAsP lasers, the power-independent contribution is estimated to be ∼1 MHz/μA and can significantly affect the intrinsic linewidth at high power levels (>10 mW). Furthermore, the line shape is not strictly Lorentzian and tends towards Gaussian with increasing power. We have discussed the dependence of the line shape and linewidth on various device parameters.

• Received 21 August 1987

DOI:https://doi.org/10.1103/PhysRevA.37.2495