An efficient multilevel coherent optical system: M-4Q-QAM

A coherent multilevel transmission system based on the modulation of all the optical field degrees of freedom (the four quadratures of the optical field) is presented. The transmitted symbols are represented in the space of the field quadratures by a square lattice of points, so that both the transmitter and the decision devices can be easily implemented. The proposed modulation formats allow high spectral efficiency systems to be designed without paying a great sensitivity penalty with respect to binary systems. For this reason the proposed modulation format seems promising for application in densely spaced very-high-capacity FISM networks and in high-speed data transmission in parallel computers