Coherent phase-shift-keying

A receiver for coherent detection of a PSK modulated optical carrier, comprising:

• an optical detector being configured to mix the modulated optical carrier with different phase components of a reference optical carrier and to produce analog output signals representative of optical signals produced by said mixing;
• analog-to-digital converters connected to receive the analog output signals and to produce digital signals from the received analog output signals;
• a digital module being connected to the receive the digital signals and to perform one of compensating the received digital signals for a conjugate phase misalignment between the mixed components, extracting phases of the received digital signals, and estimating a frequency offset between the two carriers from the received digital signals;
• a laser source connected to transmit the reference optical carrier to the optical detector;
• and
• an electronic feedback loop being connected to the laser source and being configured to maintain a frequency of light from the laser source in a selected range;
• and
• wherein the digital module is configured to decode the PSK modulated optical carrier to obtain signal points in a PSK constellation having at least four signal points and the digital module comprises a module configured to use the received digital signals to estimate a frequency offset between the two carriers;
• and
• wherein the electronic feedback loop is configured to maintain a below-threshold frequency difference between the reference and the modulated optical carriers.

A method, comprising:

• mixing a PSK modulated optical carrier with first and second phase components of a reference optical carrier;
• producing electrical signals by measuring intensities of optical signals formed by the mixing;
• digitalizing the electrical signals to produce a series of digital signals;
• evaluating derivatives of phases of the digital signals;
• demodulating the PSK modulated optical cater based on the evaluated derivatives;
• digitally transforming the digital signals to compensate for a conjugate phase misalignment between the first and second components;
• and
• wherein the evaluating step is based on the transformed digital signals.