An optical transmitter transmits a data signal. The optical transmitter has an encoder configured to encode the data signal by selecting based on a bit sequence a first symbol and a second symbol from a set of four symbols for each one of at least two transmission time slots. The optical transmitter further has a modulator configured to use in each transmission time slot the first symbol to modulate a first carrier wave and the second symbol to modulate a second carrier wave, and to transmit the two carrier waves over orthogonal polarizations of an optical carrier. Symbols in consecutive transmission time slots have non-identical polarization states.