A fiber optic transmission system including a fiber optic link. Transmitting apparatus transmits an optical signal having a concentration of energy at a particular wavelength. Routing apparatus then routes optical signals in response to the concentration of energy at the particular wavelength of the transmitted optical signal. The routing apparatus is disposed to receive the transmitted optical signal and also reflects a small portion of the transmitted optical signal. A first apparatus for receiving the reflected portion of the optical signal is disposed to cooperate with the routing apparatus and further has apparatus for converting the reflected portion into a control signal. Temperature sensing apparatus is disposed to sense temperature variations in the first receiving apparatus and apparatus for controlling the transmitting apparatus is adapted to receive the control signal and further is disposed so as to regulate the transmitting appratus responsively to the control signal. The controlling apparatus further operates to regulate the power of the transmitting apparatus as a function of temperature sensed by the temperature sensing apparatus so as to maintain a preselected power margin in the fiber optic link.