A digitally operated optical quadrature-amplitude modulation (QAM) modulator based on a single multielectrode Mach-Zehnder modulator is presented. Generation and performance of 64QAM are studied in detail. Simulation results show that a single modulator with 13 electrodes, each of which is driven by either one of the voltages 0 or 1.68 vπ, provides close-to-ideal 64QAM constellation despite the inherent nonlinearity of the modulator. Moreover, employing a sufficient number of electrodes, close to ideal error performance can be obtained for any constellation order, or shape. Simulations results are demonstrated for several different square constellations: 16QAM and 256QAM, and nonsquare constellations: 32QAM and 128QAM. A brief discussion on the utilization of the proposed scheme as a predistorter is also given.