Optical signal transmission with polarization-insensitive coherent detection and frequency stabilization at the receiving end

摘要

An optical communication system comprises a transmitter (1), a receiver (3) and optical transmission path (2) between the transmitter and receiver. The transmitter (1) includes an electro-optical transducer (4) for converting a data input signal which is binary-coded according to a given bit period, into a polarized FSK-modulated optical signal, and a polarization switching device (5) for modifying the polarized FSK- modulated optical signal into a data-induced polarization switched signal (DIPS-signal), which is transmitted over the transmission path (2) to said receiver (3). The receiver (3) includes a local oscillator (8; 31) for generating a local optical signal; a mixer (9; 32) for mixing the received optical signal and the local optical signal to produce a mixed optical signal; and a detector (10; 33) for detecting and converting the mixed optical signal into a corresponding electrical signal. A device is provided at the receiver for deriving, from the electrical signal, a data signal in accordance with the binary data pattern, and a frequency control signal for controlling the local oscillator (8). A polarization variation device (6) is included in one of (a) the optical transmission path between the polarization switching device of the transmitter and the mixer of the receiver for periodically varying the polarization of the DIPS-signal, and (b) the receiver for periodically varying the polarization of the local optical signal before mixing of the received optical signal and the local optical signal by the mixer. The polarization variation device varies the polarization of the optical signal with a variation period longer than the bit period and shorter than a control time period needed for controlling the local oscillator.