Digital-Preprocessed Analog-Multiplexed DAC for Ultrawideband Multilevel Transmitter

High-speed digital-to-analog converters (DACs) are essential in advanced optical transmission systems, utilizing multilevel modulation formats and pulse-shaping technologies. Analog bandwidth of DACs is one of the major limiting factors for the system capacity. In this paper, we present an idea to extend the analog bandwidth of DACs by utilizing a digital preprocessor, two sub-DACs, and an analog multiplexer (AMUX). The AMUX is a high-speed linear selector, and a frequency-domain interpretation of the AMUX's function has led us to devise the digital preprocessing for a seamless extension of the analog bandwidth. This configuration, which we call a digital-preprocessed analog-multiplexed DAC, provides nearly doubled analog bandwidth compared to that of each subDAC with a symmetric configuration. The idea was verified by a transmission experiment with 160-Gb/s 80-Gbaud Nyquist four-level pulse amplitude modulation. In the experiment, we used subDACs with an analog bandwidth of ~20 GHz in combination with an AMUX fabricated with InP heterojunction bipolar transistor technology.