System and method for coherent detection of optical signals

An optical communication system comprising:

• at least a transmitter apt to emit a differential phase shift keying (DPSK) optical signal having a bit-rate equal to R;
• at least a receiving system receiving the differential phase shift keying optical signal the receiving system comprising:
• a coherent optical receiving device apt to coherently receive the propagated DPSK optical signal and emitting at least one electrical signal related to the received DPSK optical signal, the coherent optical receiving device comprises a local oscillator emitting an optical field having a phase, and an optical hybrid apt to mix the optical field and the received DPSK optical signal;
• at least one electrical filter for filtering the at least one electrical signal and having −3 dB double-side bandwidth greater than or equal to 0.44 R and lower than or equal to 0.68 R;
• and
• at least one squarer for squaring the at least one filtered electrical signal;
• and
• an optical link optically connecting the transmitter and the receiving system for transmitting the DPSK optical signal from the transmitter to the receiving system;
• wherein the system keeps a difference in absolute value between a central frequency of the received DPSK optical signal and a central frequency of the optical field of the local oscillator at a predetermined value comprised between R and 3 R.

An optical communication system comprising:

• at least a transmitter apt to emit a differential phase shift keying (DPSK) optical signal having a bit-rate equal to R;
• at least a receiving system receiving the differential phase shift keying optical signal, wherein the receiving system comprises:
• a coherent optical receiving device to coherently receive the propagated DPSK optical signal and to emit at least one electrical signal related to the received DPSK optical signal, the coherent optical receiving device comprising:
• a local oscillator apt to emit an optical field having a phase, and
• an optical hybrid apt to mix the optical field and the received DPSK optical signal, the optical hybrid has at least a pair of output ports for respectively outputting either two in phase components or quadrature components of the received DPSK optical signal with respect to the phase of the optical field of the local oscillator, and a further pair of output ports and wherein the optical hybrid is configured for outputting in-phase and quadrature components of the received DPSK optical signal with respect to the phase of the optical field of the local oscillator respectively at the at least a pair of output ports and the further pair of output ports;
• a further photodetector connected to at least one output port of the further pair of output ports for detecting at least one quadrature component and for generating a second electrical signal (Q) directly proportional to the at least one quadrature component;
• and
• a pair of multipliers connected for receiving the first (I) and second (Q) electrical signal and for multiplying them respectively by an electrical local oscillator signal and by the electrical local oscillator signal phase shifted by 90°, and an adder connected for receiving the multiplied first and second electrical signal and for outputting a sum of the multiplied first and second electrical signal, the sum being the at least one electrical signal;
• at least one electrical filter for filtering the at least one electrical signal and having −3 dB double-side bandwidth greater than or equal to 0.44 R and lower than or equal to 0.68 R;
• and
• at least one squarer for squaring the at least one filtered electrical signal;
• and
• an optical link optically connecting the transmitter and the receiving system for transmitting the DPSK optical signal from the transmitter to the receiving system;
• wherein the system is further configured for keeping a difference in absolute value between a central frequency of the received DPSK optical signal and a central frequency of the optical field of the local oscillator at a value below or equal to 0.02 R.

An optical communication system comprising:

• at least a transmitter apt to emit a differential phase shift keying (DPSK) optical signal having a bit-rate equal to R;
• a receiving system for receiving the differential phase shift keying optical signal, the receiving system comprising:
• a coherent optical receiving device apt to coherently receive the propagated DPSK optical signal and emitting at least one electrical signal related to the received DPSK optical signal, and wherein the receiving system further comprises at least one electrical filter for filtering the at least one electrical signal and having −3 dB double-side bandwidth greater than or equal to 0.44 R and lower than or equal to 0.68 R;
• at least one squarer for squaring the at least one filtered electrical signal;
• and
• an electrical dispersion compensator placed upstream the squarer with respect to the direction of propagation of the at least one electrical signal, for compensating the dispersion of the at least one electrical signal;
• and
• an optical link optically connecting the transmitter and the receiving system for transmitting the DPSK optical signal from the transmitter to the receiving system;
• wherein the electrical dispersion compensator comprises delay-lines having delays equal to an integer multiple of a given amount comprised between 0.7/R and 1.4/R.

An optical communication system comprising:

• at least a transmitter apt to emit a differential phase shift keying (DPSK) optical signal having a bit-rate equal to R;
• at least a receiving system for receiving the differential phase shift keying optical signal;
• an optical link optically connecting the transmitter and the receiving system for transmitting the DPSK optical signal from the transmitter to the receiving system, wherein the receiving system comprises a coherent optical receiving device to coherently receive the propagated DPSK optical signal and to emit at least one electrical signal related to the received DPSK optical signal, and wherein the receiving system further comprises at least one electrical filter for filtering the at least one electrical signal and having −3 dB double-side bandwidth greater than or equal to 0.44 R and lower than or equal to 0.68 R, and at least one squarer for squaring the at least one filtered electrical signal;
• and
• a polarization matching system for matching the polarization of the received DPSK optical signal with the polarization of the optical field of the local oscillator.

An optical communication system comprising:

• at least a transmitter apt to emit a differential phase shift keying (DPSK) optical signal having a bit-rate equal to R;
• at least a receiving system for receiving the differential phase shift keying optical signal, the receiving system comprising:
• a coherent optical receiving device to coherently receive the propagated DPSK optical signal and to emit at least one electrical signal related to the received DPSK optical signal;
• at least one electrical filter for filtering the at least one electrical signal and having −3 dB double-side bandwidth greater than or equal to 0.44 R and lower than or equal to 0.68 R;
• and
• at least one squarer for squaring the at least one filtered electrical signal;
• and
• an optical link optically connecting the transmitter and the receiving system for transmitting the DPSK optical signal from the transmitter to the receiving system;
• a further transmitter;
• a polarization multiplexer optically connected to the transmitter and the further transmitter and having an output port optically connected to the optical link,
• a polarization demultiplexer placed at the end of the optical link and having a first output port optically connected to the receiving system;
• a polarization matching system for matching the polarization of the received DPSK optical signal with the polarization of the optical field of the local oscillator;
• and
• a second output port optically connected to a further receiving system configured according to the receiving system.

A method of optical transmission comprising the steps of:

• generating, using a transmitter, a differential phase shift keying (DPSK) optical signal having a bit-rate R, the transmitter comprising a laser source, an optical modulator for phase modulating an output of the laser source so as to generate the DPSK optical signal phase-modulated among two phase levels differing by 180° and a differential precoder electrically connected to the optical modulator for driving said optical modulator;
• propagating the DPSK optical signal along a transmission line;
• coherently receiving the propagated DPSK optical signal in order to obtain at least one electrical signal related to the propagated DPSK optical signal;
• filtering the at least one electrical signal by way of an electrical filter having −3 dB double-side bandwidth greater than or equal to 0.44 R and lower than or equal to 0.68 R;
• and
• squaring the at least one filtered electrical signal, and;
• keeping a difference in absolute value between a central frequency of the received DPSK optical signal and a central frequency of the optical field of a local oscillator at a value below or equal to 0.02 R.