The linewidth reduction of a external cavity semiconductor laser with strong optical feedback is analysed. The effects of residual reflectivity r 2 of the AR coated laser facet, external cavity reflectivity r 3, and external cavity length L ex on the linewidth are studied theoretically. A fiber grating is coupled to a conventional multi longitudinal mode semiconductor laser with on facet AR coated. Single longitudinal mode operation with side mode suppression ration over 30 dB and linewidth below 120 kHz is achieved.