Tunable semiconductor laser system

A multiplexer for a wavelength division multiplexed optical communication system includes an optical circulator with at least first, second, third and fourth circulator ports. An optical fiber with a first optical transmission path is coupled to the first circulator port and carries a wavelength division multiplexed optical signal that includes signals 1-n. A second optical transmission path is in optical communication with the second circulator port. A first laser is coupled to the second optical transmission path. The first laser reflects the 1-n signals and adds a signal n+1. A control loop is coupled to the first laser. In response to a detected change in temperature the control loop sends a signal to adjust a voltage or current supplied to the first laser and provide a controlled frequency and power of an output beam. A third optical transmission path is in optical communication with the third circulator port and transmits the signals 1-n and the signals n+1 that are received from the optical circulator. A fourth optical transmission path is in optical communication with the fourth optical circulator port. The fourth optical transmission path is positioned after the second optical transmission path and before the third optical transmission path. A first optoelectronic device is coupled to the fourth optical transmission path.