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Quadrature amplitude modulation (QAM) is a promising approach for increasing the spectral efficiency of wavelength-division-multiplexing transmission, but its optical implementation is challenging. We demonstrate a 64-level QAM (64QAM) modulator, which uses a signal synthesis in the optical domain. We achieved an integration of low-loss asymmetric couplers and three quadrature-phase-shift-keying modulation circuits using a hybrid configuration of silica-based planar lightwave circuits and an array of LiNbO3 phase modulators. The modulator successfully operated at 60 Gb/s (10 Gbaud).