A NOVEL MULTILEVEL COHERENT OPTICAL-SYSTEM - 4-QUADRATURE SIGNALING

A novel multilevel coherent optical system is proposed. It is based on the exploitation of the property that the electromagnetic field propagating in a single-mode optical fiber can be represented by a four-dimensional vector whose components are the phase and quadrature terms of the two polarization components of the electrical field. This allows a wider use of the resources of the electromagnetic field for information transmission so to obtain a spectrally efficient modulation format with a limited penalty. The different states of the transmitted field can be viewed as points in a four-dimension Euclidean space whose positions are optimized so to minimize the bit-error probability. The polarization state fluctuations due to conventional single-mode fiber propagation are compensated by means of a pure electronic algorithm allowing the state of polarization of the transmitted field to be recovered without additional optical processing. The net performance gain with respect to N-APK and N-PSK increases with increasing the number of levels N. For instance, for N = 32 the gain is of 1.6 and 7.7 dB with respect to N-APK and N-PSK system. Finally, the effect of lasers phase noise on the system performance is evaluated.