A system for stabilizing a frequency separation between two coherent light sources includes first and second current sources driving first and second semiconductor laser devices with first and second frequencies, respectively. Light from the semiconductor laser devices is combined in a multiplexer, and a portion the combined light is diverted to an optical-to-electrical transducer. Output current from the transducer is combined with an output of a local oscillator driven at a selected frequency. A combined signal is then filtered, and used to form a control signal of one of the semiconductor laser devices.