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Unrepeatered optical transmission of 20 Gbit/s quadrature phase-shift keying (QPSK) signals over 210 km, using a homodyne phase-diversity receiver followed by carrier-phase estimation through digital signal processing is demonstrated. The advantage of the coherent detection with phase estimation leads to significant improvement of the receiver sensitivity compared with differential demodulation. It also enables the observation of the distortion of the complex amplitude due to fibre nonlinearity, offering new tools to monitor the transmission quality.
Date of Publication: 17 Feb. 2005