Linewidth reduction of 1.5-μm grating-loaded external cavity and semiconductor lasers

To meet the stringent linewidth requirements tor coherent optical fiber communication, a dispersive passive cavity has been added to the multimode laser to impose a strong spectral selectivity so that a narrow linewidth single longitudinal mode is allowed to oscillate.1 Optical feedback level, power, and facet reflectivity are the important parameters affecting the laser linewidth. We report here the study of the strong influence of laser geometry on the laser linewidth of such external cavity lasers.