Coherent transponder for optical PS-QPSK signals

The present document relates to optical transmission systems. In particular, the present document relates to a transmitter and/or a receiver for optical polarization switches (PS) QPSK (quadrature phase shift keying) signals. An apparatus (500) for an optical transmitter of an optical polarization switched M-PSK signal comprising a first and a second polarization, with M = 2m and m≥ 1 is described. The apparatus (500) comprises mapping means (200) configured to map, at a time instant k=1,...,K, m information bits to an M-PSK constellation point, thereby generating a first symbol at time instant k for the first polarization; assign, at the time instant k, one further information bit by generating a second symbol at time instant k for the second polarization, wherein the second symbol at time instant k corresponds to the first symbol at time instant k, or corresponds to the constellation point opposite the constellation point of the first symbol at time instant k; and repeat the mapping and assigning steps for all k=1,...,K to generate a sequence of first and second symbols for the first and second polarization, respectively. The apparatus (500) further comprises decorrelation means (501, 502) configured to decorrelate the first symbol at time instant k and the second symbol at time instant k, for all k=1,...,K.