Digital Signal Processing of an Optical Communications Signal in a Coherent Optical Receiver

A digital signal processing unit for use in a coherent optical receiver for an optical communications network, the digital signal processing unit comprising:

• an adaptive equalizer comprising;
• a number of input ports for receiving electrical signals, each electrical signal corresponding to a different state of polarization of an optical signal received by the coherent optical receiver;
• and
• a number of output ports;
• wherein each of the output ports is connected to a processing branch, wherein a processing branch comprises:
• at least one symbol sequence estimator;
• and
• a carrier phase estimator comprising an input for receiving signal tapped from an output of the processing branch;
• wherein an output of the carrier phase estimator is connected to a phase adjuster interconnecting the respective output port of the adaptive equalizer and the symbol sequence estimator;
• and
• a processing block connected to an output of the carrier phase estimator, an output of the processing branch, and at least one of the output of the phase adjuster and the outputs of the adaptive equalizer.

A coherent optical receiver for use in an optical communications network, the coherent optical receiver comprising:

• an optical input arranged to receive an optical signal;
• a splitter arranged to split the received optical signal into two orthogonal states of polarization;
• a laser arranged to operate as a local oscillator to generate a second optical signal;
• two 2×2 90° optical hybrids arranged to combine the optical fields of the two orthogonal states of polarization received from the splitter with the optical field of the second optical signal;
• four pairs of balanced photodetectors, a first two pairs of the photodetectors connected to a first optical hybrid and a second two pairs of the photodetectors connected to a second optical hybrid;
• the four pairs of photodetectors comprising four outputs arranged to output electrical signals representing in-phase and quadrature components of the two orthogonal states of polarization of the received optical signal, the four outputs being connected to four analog-to-digital converters and outputs of the four analog-to-digital converters being connected to a digital signal processing unit
• wherein the digital signal processing unit comprising:
• an adaptive equalizer comprising;
• a number of input ports for receiving electrical signals, each electrical signal corresponding to a different state of polarization of an optical signal received by the coherent optical receiver;
• and
• a number of output ports;
• wherein each of the output ports is connected to a processing branch,
• wherein a processing branch comprises:
• at least one symbol sequence estimator;
• and
• a carrier phase estimator comprising an input for receiving signal tapped from an output of the processing branch;
• wherein an output of the carrier phase estimator is connected to a phase adjuster interconnecting the respective output port of the adaptive equalizer and the symbol sequence estimator;
• and
• a processing block connected to an output of the carrier phase estimator, an output of the processing branch, and at least one of the output of the phase adjuster and the outputs of the adaptive equalizer.

A method for digital signal processing of an optical communications signal in a coherent optical receiver, the method comprising:

• a) receiving electrical signals, each electrical signal corresponding to a different state of polarization of an optical signal received by the optical receiver;
• b) performing adaptive equalization on the samples to obtain equalized samples, the adaptive equalization using equalizer coefficients;
• c) performing symbol sequence estimation on the equalized samples to obtain decided symbols;
• d) performing carrier phase estimation on the decided symbols to obtain a phase compensation, and applying the phase compensation to the equalized samples;
• and
• e) performing simultaneous channel response coefficients estimation and equalizer coefficients estimation,
• wherein step e) is repeated and steps b) to d) are performed iteratively using channel response coefficients and equalizer coefficients resulting from step e) until an error between the equalized samples after phase compensation and estimated channel samples is minimized.

A non-transitory computer readable recording medium storing a computer program product for digital signal processing of an optical communications signal in a coherent optical receiver, the computer program product comprising software instructions which, when run on processing circuitry of a device, causes the device to:

• a) receive electrical signals, each electrical signal corresponding to a different state of polarization of an optical signal received by the optical receiver;
• b) perform adaptive equalization on the samples to obtain equalized samples, the adaptive equalization using equalizer coefficients;
• c) perform symbol sequence estimation on the equalized samples to obtain decided symbols;
• d) perform carrier phase estimation on the decided symbols to obtain a phase compensation, and applying the phase compensation to the equalized samples;
• and
• e) perform simultaneous channel response coefficients estimation and equalizer coefficients estimation,
• repeat step e) and iteratively perform steps b) to d) using channel response coefficients and equalizer coefficients resulting from step e) until an error between the equalized samples after phase compensation and estimated channel samples is minimized.