Patent ID: 8903253

Claim:
A 2 n -quadrature amplitude modulation optical modulator comprising: an optical input; a first optical modulation apparatus configured to apply a 2 n−2 amplitude modulation scheme having 2 n−2 constellation points arranged in a constellation in a first quadrant of its constellation diagram to a received optical signal; and a second optical modulation apparatus configured to selectively rotate the phase of a received optical signal, wherein the optical input is configured to deliver an optical signal to be modulated to one of the first and second optical modulation apparatus, the said apparatus generating an intermediate optical signal and further being configured to deliver the intermediate optical signal to the other of the first and second optical modulation apparatus, to thereby generate a modulated output optical signal having a square constellation diagram comprising 2 n constellation points distributed across the four quadrants of the constellation diagram, wherein n is an even number.