Control modulation technique for client independent optical performance monitoring and transport of channel overhead

A control modulation technique for optical performance monitoring and transport of channel overhead independent of the client data and without optoelectronic conversion in the client data path was demonstrated. The technique involved an additional optical modulation of the WDM client signal by an RF subcarrier above the client spectrum in order to make the performance monitoring sensitive to degradation effects at high modulation frequencies. By modulating the RF subcarrier with a digital control signal, an over-all digital quality criterion BER was gained that took into acount the relevant degradation effects.