In this paper, a semiclassical rate equation model has been established and the FM noise power spectrum calculated. We show that the spectral line width can be narrowed by the negative electrical feedback technique. Within the feedback loop bandwidth, the chirp-power-ratio (CPR) is reduced by a factor of 1+H, it shows that this kind of laser can be used as the light source in FSK coherent transmission system. Because of the direct modulation phase delay of the semiconductor laser, l/f-type FM noise is one of the important origin of the limitation, for linewidth reduction.