Multilevel coherent optical system

An optical receiver for receiving a multilevel optical signal, comprising:

• a first stage including an optical local oscillator to generate a coherent optical signal, a 90.degree. optical hybrid to receive said multilevel optical signal and said coherent optical signal and to output a first signal, corresponding to a sum of the multilevel optical signal and said coherent optical signal, and to output a second signal, corresponding to a sum of the multilevel optical signal and said coherent optical signal with a phase of one of said multilevel optical signal and said coherent optical signal shifted by 90.degree., two beam splitters, each to separate said first and second signals, respectively, into orthogonal polarization component signals, and four photodiodes each to detect said separated component signals, said first stage coupled to an optical fiber to carry out a heterodyne detection of the phase terms and the phase quadrature terms of a beat signal generated from a polarized signal received by the optical fiber and the coherent optical signal, said first stage further including four bandpass filters centered about the intermediate frequency of the respective component signals detected by said four photodiodes;
• a second stage coupled to said first stage to demodulate the received signals and to provide a multilevel signal, including means for converting the intermediate frequency signals of said four bandpass filters to respective four base band signals;
• and
• a processing circuit coupled to said second stage to compare said multilevel signal with predetermined reference signals.

An optical transmitter for transmitting a multilevel optical signal modulated in phase and polarization, to transmit information through a single-mode optical fiber, comprising:

• a coder to code a first, second and third control signal by coding a binary succession representing information to be transmitted and formed of a plurality of symbols each representing a predetermined state of the multilevel signal to be transmitted;
• a coherent light source to generate an optical carrier;
• a first phase modulator, connected to said coherent light source and said coder, to modulate a phase of the carrier with the coded first control signal;
• a polarization selection beam splitter, connected to the first phase modulator, to split the modulated carrier into two components of the polarization state of the carrier;
• a first polarization rotator, connected to the polarization selection beam splitter, to rotate the polarization of a first of the two components by 90.degree.;
• a second phase modulator, connected to the polarization selection beam splitter and said coder, to modulate a phase of a second of said two components with the coded second control signal;
• a directional first coupler, connected to both the first polarization rotator and the second phase modulator, to superimpose the rotated first component with the modulated second component and to output a first and a second output signal, respectively;
• a second polarization rotator, connected to the directional first coupler, to rotate the polarization of the first output signal by 90.degree. so that the rotated first output signal is orthogonal to the second output signal;
• a third phase modulator, connected to the directional first coupler and said coupler, to modulate a phase of the second output signal with the coded third control signal;
• and
• a second coupler, connected to the second polarization rotator and the third phase modulator, to couple the modulated second output signal with the orthogonal first output signal to produce an optical signal modulated in phase and polarization.

An optical transmitter for transmitting a multilevel optical signal modulated in phase and polarization, to transmit information through a signal-mode optical fiber, comprising:

• a light source to generate a carrier;
• a coder to code a plurality of control signals based on a binary succession of information to be transmitted and a predetermined state of the multilevel signal to be transmitted;
• polarization and modulation means for modulating a phase of the carrier with one of said plurality of coded control signals, and for generating a first and a second signal from the modulated carrier, the first signal being orthogonal to the second signal and the second signal being further modulated with another one of said plurality of coded control signals;
• and
• a coupler to couple the first and second signals to generate an optical signal modulated in phase and polarization.

A method for transmitting a multilevel optical signal modulated in phase and polarization, to transmit information through a single-mode optical fiber, comprising the steps of:

• generating a carrier;
• coding a plurality of control signals based on a binary succession of information to be transmitted and a predetermined state of the multilevel signal to be transmitted;
• modulating a phase of the carrier with one of said plurality of coded control signals;
• generating a first and a second signal from the modulated carrier, the first signal being orthogonal to the second signal and the second signal being further modulated with another one of said plurality of coded control signals;
• and coupling the first and second signals to generate an optical signal modulated line in phase and polarization.