Dual-Carrier Dual-Polarization IQ Modulator Using a Complementary Frequency Shifter

We discuss the detailed characteristics of a dual-carrier dual-polarization (DP) in-phase-and-quadrature (IQ) modulator. As the dual-carrier generator, we used a complementary frequency shifter (CFS), which generates two frequency-spacing-locked optical subcarriers and outputs each of them from different output ports without any optical demultiplexers. We fabricated the modulator, which integrates a CFS, a quad-parallel IQ modulator, and a polarization-multiplexing circuit, with a hybrid configuration of silica planar lightwave circuits and a LiNbO₃ chip. With the fabricated modulator, we demonstrated that the CFS enables us to flexibility change the relative and absolute optical frequencies of the subcarriers with a high isolation ratio of about 40 dB. We generated a 400-Gb/s dual-carrier DP 16-level quadrature amplitude modulation (16QAM) signal by driving the modulator with high-speed digital-to-analog converters.