US Patent 7184672 Analogue maintenance detection

Methods and apparatus are described for processing an optical signal. The optical signal comprises at least a first channel signal at a first wavelength modulated by a first frequency, a second channel signal at a second wavelength modulated by a second frequency, and a third channel signal at a third wavelength modulated by a third frequency. The method includes receiving the optical signal, and splitting the received optical signal into at least two optical bands, a first optical band comprising the first and second channel signals, and a second optical band comprising the third channel signal. A first electrical signal is formed, proportional to the total optical power of the first optical band.