method for the formation of a multi-level signal on an optical carrier by modulating coherent phase and polarization of the carrier and apparatus for transmission and reception eterodinasignals formed with this method.

摘要

PCT No. PCT/IT91/00036 Sec. 371 Date Nov. 9, 1992 Sec. 102(e) Date Nov. 9, 1992 PCT Filed May 6, 1991 PCT Pub. No. WO91/18455 PCT Pub. Date Nov. 28, 1991.A multilevel coherent optical system, including a heterodyne transmitter and receiver, in which a multilevel signal with a coherent optical carrier is provided by modulating the phase and the polarization of the electromagnetic field propagating through a single-mode optical fiber. The transmitter comprises a coherent light source providing the optical carrier, a phase modulator modulating the phase of the carrier, a polarization modulator, and a modulation signal generator providing control signals to the phase modulator and the polarization modulator. The receiver comprises a first stage carrying out the heterodyne detection of the phase component and the phase quadrature component of the polarization of the signal received through an optical fiber, 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 fiber so as to more closely approach the theoretical Shannon limit compared with conventional systems.