Abstract
Power-digital-to-analog converters (DAC) circuits for high capacity and high spectral efficiency optical transmitters are presented in this paper. They are based on a 0.7- μ m InP DHBT semiconductor process, of which main differentiating characteristics are highlighted. We introduce the Power-DAC circuit concept, its usual architectures, and we provide design details of its latest generation. Named SPDAC, it integrates multiplexing front-end blocks composed of three 2:1-selectors. With this architecture, the input interface operates at half rate. 107-GBd operation for PAM-4 and 64-GBd for PAM-8 were demonstrated with quality suitable for direct detection and datacenter applications. 90-GBd operation in a PDM-64QAM transmitter with coherent detection was also enabled by SPDAC modules. With 3.7 to 4 V differential output voltage swing, Power-DAC circuits can be directly connected to different types of E/O modulators (Mach–Zehnder, EAM). We show in particular how the SPDAC circuit enables single-carrier transmission at 100-GBd in PAM-4 IM/DD modulation for datacenters. In this experiment, the feed-forward equalizer combined with stacking a <3-symbol memory sequence estimator demonstrated signal recovery for 107-GBd PAM-4 in back-to-back configuration, after 500 m and 1 km of transmission over standard single-mode fiber for 100- and 84-GBd PAM-4 configuration, respectively.
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