An apparatus comprising an optical data transmitter and an electronic controller , the optical data transmitter including a laser configured to generate a carrier wavelength for an optical output signal sent to a remote receiver through a span of optical fibers; wherein the apparatus is configured to read a first value of a measure of transmission quality, the measure of transmission quality representing an error rate at which an FEC code used at the remote receiver encounters errors in data transmitted using the optical output signal; and wherein the electronic controller is configured to change an optical output power of the laser in response to the first value of the measure of transmission quality, characterized in that the apparatus further comprises an optical data receiver , the optical data receiver including a photodetector configured to detect an optical input signal carrying a first data frame, wherein the optical data receiver comprises a frame decoder configured to read the first value of the measure of transmission quality from the first data frame, wherein the first value of the measure of transmission quality is provided to the electronic controller by the frame decoder , wherein the controller is connected between the optical data receiver and the optical data transmitter , wherein the frame decoder is further configured to count a number of errors corrected in the first data frame using an FEC data block thereof; wherein the electronic controller is configured to compute a second value of the measure of transmission quality using the number of errors counted by the frame decoder; wherein the optical data transmitter comprises a frame generator configured to write the second value of the measure of transmission quality into a second data frame; and wherein the optical data transmitter is configured to cause the optical output signal to carry the second data frame to the remote receiver through the span of optical fibers. wherein the apparatus is configured to read a first value of a measure of transmission quality, the measure of transmission quality representing an error rate at which an FEC code used at the remote receiver encounters errors in data transmitted using the optical output signal; and wherein the electronic controller is configured to change an optical output power of the laser in response to the first value of the measure of transmission quality, characterized in that the apparatus further comprises an optical data receiver , the optical data receiver including a photodetector configured to detect an optical input signal carrying a first data frame, wherein the optical data receiver comprises a frame decoder configured to read the first value of the measure of transmission quality from the first data frame, wherein the first value of the measure of transmission quality is provided to the electronic controller by the frame decoder , wherein the controller is connected between the optical data receiver and the optical data transmitter , wherein the frame decoder is further configured to count a number of errors corrected in the first data frame using an FEC data block thereof; wherein the electronic controller is configured to compute a second value of the measure of transmission quality using the number of errors counted by the frame decoder; wherein the optical data transmitter comprises a frame generator configured to write the second value of the measure of transmission quality into a second data frame; and wherein the optical data transmitter is configured to cause the optical output signal to carry the second data frame to the remote receiver through the span of optical fibers. <EOS>