Flexible-Format Modulator With a Lattice Configuration

We discuss the general design concept of lattice optical modulators and detailed characteristics of a flexible-format modulator with a lattice configuration. Compared to the conventional modulators, the lattice modulator provides smaller intrinsic signal-synthesis losses because it utilizes both of the two complementary quadrature-phase-shift-keying (QPSK) signals to synthesize higher-order signals, although in practice the decrease in the intrinsic loss may be canceled out with the increase in the propagation loss. The flexible-format modulator, which we fabricated with a hybrid configuration of silica planar lightwave circuits and a LiNbO₃ chip, enables us to select the format of the output signal from QPSK, 8-level PSK (8PSK), 8-level quadrature amplitude modulation (8QAM) and 16-level QAM (16QAM) just by using static tunable couplers and a phase shifter, without electronic digital-to-analog converters.