Fiber optic transmission with fiber amplifiers and supervisory signals

A method of providing supervisory signals to amplifiers of a fiber optic system, the fiber optic system carrying traffic signals at one or more traffic signal carrier wavelengths from a first terminal station of the fiber optic system, wherein the traffic signals are amplified by the amplifiers so that the traffic signals are maintained in strength along the fiber optic cable system for detection at a second terminal station, characterized in that a carrier wavelength different from the one or more carrier wavelengths used for the traffic signals is employed to carry a supervisory signal but no traffic signals, wherein the fiber optic system is adapted such that the supervisory signal is amplified such that the supervisory signal is maintained in strength along the fiber optic system by the same the amplifiers amplifying the traffic signals, and wherein supervisory information is provided on its carrier wavelength by modulation at a frequency substantially lower than that of a traffic bit rate for detection by detectors located at the amplifiers.