This paper describes optical quadrature amplitude modulation techniques using parallel Mach-Zehnder modulators (MZMs). Arbitrary constellations of optical signals can be generated by using a dual-parallel MZM which can independently control real and imaginary components of lightwaves, where each MZM should be driven by multi-level electric signals. Many parallel MZMs, such as quad-parallel MZMs, octet-parallel MZMs can synthesize optical multi-level signals from binary data stream generated by electronics designed for binary modulation formats. We also investigate characterization methods of parallel MZMs. Precise measurement of half-wave voltage, extinction ratio and chirp parameter of each MZM element can be achieved by using optical spectrum analysis.