Design theory of electrically frequency-controlled narrow-linewidth semiconductor lasers for coherent optical communication systems

We describe a design rule for the automatic frequency control of semiconductor lasers by electrical feedback for coherent optical communications. The linewidth achieved by the feedback control is not a good measure for evaluating the system performance, but the bandwidth of the feedback loop is the most important parameter. For example, a 500-MHz loop bandwidth is required for a heterodyne DPSK system when the free-running linewidth of the laser is 10 MHz.