Optical Transmitters and Receivers Using Polarization Multiplexing

An optical transmitter, comprising:

• a laser having an output;
• an optical beam splitter having an input coupled to the output of the laser and two outputs configured so as to produce first and second optical signals;
• first and second optical intensity modulators having respective inputs coupled to the outputs of the optical beam splitter and configured to impress multi-level intensity modulation on the first and second optical signals, respectively, based on respective first and second information streams;
• a phase modulator coupled in line with the second optical intensity modulator and configured to impress a multi-level phase modulation on the second optical signal, using a multi-level phase modulation signal derived from a third information stream;
• and
• an optical beam combiner having first and second inputs coupled to receive the intensity-modulated first optical signal and the intensity-and-phase-modulated second optical signal, respectively, and configured to combine the intensity-modulated first optical signal and the intensity-and-phase-modulated second optical signal to produce a polarization-multiplexed optical signal for transmission;
• wherein the optical transmitter is configured so that the intensity-modulated first optical signal and the intensity-and-phase-modulated second optical signal in the polarization-multiplexed optical signal have first and second optical states of polarization, respectively.

An optical receiver, comprising:

• an optical receiver front-end configured to receive a polarization-multiplexed optical signal and to split the polarization-multiplexed optical signal into sub-components comprising first and second signals, proportional to a time-varying intensity for each of first and second orthogonal states of polarization of the received signal, respectively, and third and fourth signals proportional to S2r=2Re{ExrEyr*} and S3r=2Im{ExrEyr*}, respectively, where Exr and Eyr are complex time-varying values for the first and second orthogonal states of polarization of the received signal, respectively;
• a digital signal processing circuit operatively coupled to the optical receiver front-end and configured to receive digitized versions of the first, second, third, and fourth signals, said digital signal processing circuit being further configured to implement a matrix arrangement of Finite Impulse Response, FIR, filters receiving the digitized versions of the first, second, third, and fourth signals and to thereby substantially remove polarization rotation induced by transmission of the polarization-multiplexed optical signal from a polarization-division-multiplexing optical transmitter to the optical receiver and estimate first and second information streams used by the polarization-division-multiplexing optical transmitter to intensity-modulate first and second optical states of polarization in the transmitted polarization-multiplexed optical signal.

A method of generating a modulated optical signal, comprising:

• impressing a first intensity modulation on a first optical signal, to obtain a first intensity-modulated signal, wherein the first intensity modulation is based on a first modulation signal derived from a first stream of information bits;
• impressing a second intensity modulation on a second optical signal, to obtain a second intensity-modulated signal, wherein the second intensity modulation is based on a second modulation signal derived from a second stream of information bits;
• impressing a multi-level phase modulation on the first intensity-modulated signal, to obtain an intensity-modulated and inter-polarization-phase-modulated signal;
• and
• rotating the polarization of the first optical signal or the second optical signal, or both;
• combining the intensity-modulated and inter-polarization-phase-modulated signal and the second intensity-modulated signal and transmitting the combined signal via an optical fiber;
• wherein rotating the polarization of the first optical signal or the second optical signal, or both, is performed in such a way that the intensity-modulated and inter-polarization-phase-modulated signal and the second intensity-modulated signal, as combined and transmitted, have first and second states of optical polarization.

A method, comprising:

• receiving an intensity-modulated, polarization-division multiplexed optical signal with inter-polarization phase modulation;
• detecting a time-varying intensity for each of first and second orthogonal polarizations of the received optical signal;
• detecting signal components S2r=2Re{ExrEyr*} and S3r=2Im{ExrEyr*}, where Exr and Eyr are the first and second orthogonal polarizations of the received signal;
• de-rotating the detected intensities for the first and second orthogonal polarizations of the received optical signal and the detected signal components S2r and S3r, so as to compensate for polarization rotation imposed on the signal between a transmitter of the received optical signal and said detecting operations;
• demodulating first and second information bit streams from the de-rotated first and second orthogonal polarizations of the received optical signal, the first and second information bit streams corresponding to first and second polarization-division-multiplexed intensity modulations on the optical signal as received;
• and
• demodulating a third information bit stream from de-rotated signal components S2 and S3 corresponding to the detected signal components S2r and S3r, the third information bit stream corresponding to the inter-polarization phase modulation on the optical signal as received.