Effect of injection-current fluctuations on the spectral linewidth of semiconductor lasers.

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-Markoffian 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/microA and can significantly affect the intrinsic linewidth at high power levels (>10mW). 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.