Single-mode laser systems with modulated signals are studied by the method of numerical simulation. The signal-to-noise ratio, the output signal power, and the background noise power are analyzed near and far from the threshold. It is found that when the laser works near the threshold, the signal-to-noise ratio is present and the behavior of stochastic resonance takes place as the quantum noise increases, no matter whether the quantum noise is correlated with the pump noise. The correlation between the two noises, however, will increase the signal-to-noise ratio. And when the laser works far from the threshold, only when the two noises are correlated can the behavior of stochastic resonance appear. It is also found that the output signal power can be influenced by the noises. What is more, the simulation results are compared with those obtained by the method of linear approximation.