Patent ID: 7920787

Claim:
A method for detecting a check-back signal in an optical transmission system for optical signals, comprising: concentrating a constant proportion of an output in a defined frequency range of the check-back signal in a narrow-band spectral range; feeding the check-back signal into the transmission system at a sending end; decoupling the check-back signal after a section of the transmission system; modulating, amplifying and filtering the decoupled check-back signal to isolate the narrow-band spectral range of the check-back signal; and determining the output of the isolated narrow-band spectral range for the detection of the check-back signal, wherein the amplification of the check-back signal decoupled from the transmission system is linear and an amplitude limiting process is not performed on the check-back signal so that if there is a high proportion of noise, the check-back signal is still detected in the narrow-band spectral range; wherein a concentration of a constant proportion of the output of the check-back signal is created in the narrow-band spectral range by evenly distributing ones and zeros from data of the check-back signal, followed by encoding; and wherein scrambling is used to evenly distribute ones and zeros from the data of the check-back signal and then a CMI or RZ encoding is used to create a spectral line.