Multifrequency modulation for high-sensitivity coherent optical systems

A new instantaneous multilevel modulation format is proposed that allows the optical bandwidth to be fully exploited without requiring an electrical bandwidth of the same order of magnitude. The format is more efficient than multilevel wide-deviation frequency-shift-keying (FSK), suffering only a limited penalty with respect to coherent pulse position modulation. The modulated signal is generated by the same criterion adopted for multilevel wide-deviation FSK, but for each optical carrier a modulation format more efficient than binary amplitude-shift-keying (ASK) is chosen. Multilevel four-quadrature modulation is used. In order to obtain power-efficient signaling, the transmitted symbols are associated with the field configurations in such a way that just one frequency carrier at a time is modulated