Polarisation-independent coherent optical receiver

A polarisation-independent coherent optical receiver, comprising:

• an optical coupler (2) having three inputs and three outputs, which receives a modulated optical signal and an optical signal generated by a local oscillator (3) and creates the beat between such signals;
• an optical signal splitter (5;
• 5′), which is located on the path of one out of the modulated optical signal and the signal generated by the local oscillator, splits the signal applied thereto into two components having orthogonal polarisations and sends such components to two inputs of the coupler (2), which receives at a third input the other out of the modulated optical signal and the signal generated by the local oscillator (3);
• three photodetectors (6a, 6b, 6c) respectively connected to the coupler outputs and converting the optical signals outgoing from the coupler (2) into electrical signals;
• analogue processing circuitry (7, 8a, 8b, 8c, 9a, 9b, 9c, 10a, 10b, 10c, 11) receiving and processing the electrical signals outgoing from the photodetectors (6a, 6b, 6c) and generating a resultant electrical signal that represents the modulated optical signal and includes a baseband component;
• and
• a low-pass filter (12) filtering the resultant electrical signal;
• characterised in that:
• the optical coupler (2) is of a kind providing at the three outputs thereof optical signals having the same mutual phase shift;
• the analogue processing circuitry (7-11) includes at least:
• three squaring circuits (10a, 10b, 10c) squaring said electrical signals or combinations thereof;
• and
• an adder (11) summing up the signals outgoing from the squaring circuits (10a, 10b, 10c) and outputting said resultant electrical signal;
• wherein the resultant electrical signal includes, besides the baseband component, a spurious component that is centred on a frequency depending on a frequency difference between the modulated optical signal and the signal generated by the local oscillator (3) and that has an amplitude and a phase depending on the instant states of polarisation of the modulated optical signal and the signal generated by the local oscillator;
• said frequency difference is such that the receiver (100;
• 200) operates under intradyne conditions;
• and
• a passband of said low pass filter (12) and said frequency difference are such that said spurious component falls outside said passband or has a minimum spectral overlap with the modulated optical signal.