The effect of the dispersive relation is investigated theoretically and a nominal linewidth broadening enhancement factor αe and an F factor related to the dispersive relation are introduced to modify the spectral linewidth formula of semiconductor lasers. It is found that the spectral linewidth not only can be suppressed by decreasing the linewidth broadening en-hancement factor, but also can be suppressed further by increasing the F factor. The mechanisms of the mode selection and linewidth compression in longitudinally coupled cavity (C3) are analyzed by means of the proposed effective reflectivity method, and the optimum condition for realizing single longitudinal mode with narrow linewidth is noted.