Frequency estimation in an intradyne optical receiver

An apparatus, comprising:

• an optical receiver for receiving a PSK modulated optical carrier from an optical communications channel, comprising:
• an optical hybrid configured to output first mixtures of the modulated optical carrier and a reference optical carrier at first outputs and to output second mixtures of the carriers having different relative phases at second outputs;
• a first light detector configured to generate first digital values by sampling light intensities at the first outputs;
• a second light detector configured to generate second digital values by sampling light intensities at the second outputs, the first and second digital values of a sampling period forming a first complex sampled value thereof;
• and
• a digital signal processor configured to receive the complex sampled values and to correct a phase of each complex sampled value to compensate for a frequency offset between the modulated and reference optical carriers, the digital signal processor being configured to average M-th integer powers of a sequence of signal values to evaluate the corrected phase, each of the signal values of the sequence being a product of one of the complex sampled values with a complex conjugate of another of the complex sampled values, the one and another of the complex sampled values being successive ones of the complex sampled values, the digital signal processor being configured to estimate symbols on the modulated optical carrier from the corrected phases, M-th powers of the complex sampled values having phases independent of the PSK modulation of the modulated optical carrier.