Noise in nearly-single-mode semiconductor lasers

We report the results of a numerical study of nearly-single-mode semiconductor lasers. The stochastic nonlinear dynamical equations of the field and population inversion are integrated and the intensity fluctuations and line shapes of the laser are investigated. The second mode affects the coherence of the light output appreciably, particularly near the laser threshold regime. The accuracy of the linearized theory for a single-mode laser is tested from far below to far above threshold. It is found to be very accurate both far below and far above threshold, but significant discrepancies are seen in the threshold region. We automatically include the coupling of the intensity and phase fluctuations in the line-shape calculations, resulting in the experimentally observed asymmetry of the relaxation oscillation sidebands. The numerical technique developed here is easily applicable to multimode lasers and is shown to be a convenient and powerful probe of the coherence properties of semiconductor lasers.