A method for conveying signaling information through an optical fiber link between a transmitter and a receiver of an optical communication system, the method comprising: generating, by the transmitter , a predetermined spectral change in an optical signal, and transmitting the optical signal including the predetermined spectral change through an optical fiber link to the receiver , the predetermined spectral change being indicative of the signaling information; and detecting, by a detector associated with the receiver , the predetermined spectral change in the optical signal received through the optical fiber link, and generating a detection signal in accordance with the detection result; wherein the detecting is performed independently of demodulation of the optical signal by the receiver ; wherein generating the predetermined spectral change in the optical signal comprises: generating a tone signa using an electrical oscillator signal at a predetermined frequency; and inserting the tone signal into the optical fiber link; and wherein detecting the tone signal comprises: tapping a photodetector signal generated by a photodetector of the receiver in response to the optical signal received through the optical fiber link; mixing the tapped photodetector signal with an electrical oscillator signal having a second predetermined frequency to generate a combined signal; filtering the combined signal using a filter characteristic having a pass band corresponding to the tone signal, to generate a filtered signal; and comparing the filtered signal to a predetermined threshold, and generating the detection signal based on the comparison result. generating, by the transmitter , a predetermined spectral change in an optical signal, and transmitting the optical signal including the predetermined spectral change through an optical fiber link to the receiver , the predetermined spectral change being indicative of the signaling information; and detecting, by a detector associated with the receiver , the predetermined spectral change in the optical signal received through the optical fiber link, and generating a detection signal in accordance with the detection result; wherein the detecting is performed independently of demodulation of the optical signal by the receiver ; wherein generating the predetermined spectral change in the optical signal comprises: generating a tone signa using an electrical oscillator signal at a predetermined frequency; and inserting the tone signal into the optical fiber link; and wherein detecting the tone signal comprises: tapping a photodetector signal generated by a photodetector of the receiver in response to the optical signal received through the optical fiber link; mixing the tapped photodetector signal with an electrical oscillator signal having a second predetermined frequency to generate a combined signal; filtering the combined signal using a filter characteristic having a pass band corresponding to the tone signal, to generate a filtered signal; and comparing the filtered signal to a predetermined threshold, and generating the detection signal based on the comparison result. tapping a photodetector signal generated by a photodetector of the receiver in response to the optical signal received through the optical fiber link; mixing the tapped photodetector signal with an electrical oscillator signal having a second predetermined frequency to generate a combined signal; filtering the combined signal using a filter characteristic having a pass band corresponding to the tone signal, to generate a filtered signal; and comparing the filtered signal to a predetermined threshold, and generating the detection signal based on the comparison result. <EOS>