Strategy of optical negative feedback for narrow linewidth semiconductor lasers

The coherent optical negative feedback scheme is systematically investigated by calculating rate equations that model a noise-added semiconductor laser coupled to a Fabry-Perot optical filter for the FM noise reduction. The calculated results indicate that the FM noise is minimized when a lasing frequency of the free-running laser matches a valley frequency of the filter (the point where power reflectivity becomes zero) under a specific feedback phase, where the slope of the electric field reflectivity for the lasing light and frequency discrimination efficiency to electric field amplitude of the feedback light becomes maximum. And the linewidth is also minimized at a lasing frequency corresponding to the valley frequency of the Fabry-Perot optical filter. It is also made clear that the laser frequency becomes less sensitive to the fluctuation of the injection current of the laser under optical negative feedback.