Starting from the analytical solutions to the multimode rate equations, the influence of facet reflectivities on the output characteristics of laser diodes have been studied. For laser diodes biased above the threshold, the output spectra and the mode suppression ratios under different facet reflectivities have been simulated numerically. For laser diodes biased below threshold, with the emphasis on the superluminescent diodes (SLDs), the dependences of such important operation parameters as output spectral width (FWHM) and output power on the facet reflectivities have been investigated.