A method for transmitting digital data includes splitting a coherent
optical carrier having a subcarrier into mutually coherent optical
carriers, producing corresponding sequences of phase shifts in each of
the mutually coherent optical carriers, and then, interfering the
mutually coherent optical carriers. The interfering produces an output
optical carrier whose subcarrier has modulated inphase and quadrature
components with a corresponding sequence of pairs of values. The pairs of
values of the modulated inphase and quadrature phase components produced
by the interfering correspond to a sequence of coordinate pairs for the
signal points the 4-PSK 2D, 16-QAM 2D, or 16-PSK 2D constellation.