| 摘要 |
<p>A multilevel coherent optical system is disclosed in which a multilevel signal with a coherent optical carrier is provided by means of a modulation of the phase and the polarization of the electromagnetic field propagating through a single-mode optical fibre, a heterodyne transmitter and receiver for said signal being also disclosed. The transmitter comprises a coherent light source (1) providing the optical carrier, a phase modulator (2) modulating the phase of the carrier, a polarization modulator (3-8), and a modulation signal generator (10) providing control signals to the phase modulator (2) and the polarization modulator. The receiver comprises a first stage (12-23) carrying out the heterodyne detection of the phase component and the phase quadrature component of the polarization of the signal received through an optical fibre (11), a second stage demodulating the received signal to provide the multilevel signal, and a processing circuit comparing the received multilevel signal with predetermined reference signals. Such a system exploits the four degrees of freedom of the electromagnetic field propagating through the optical fibre so as to approach closer to the theoretical Shannon limit than conventional systems.</p> |
