Optical DACs for Ultra-High-Speed Green Photonic Interconnects

We review the principles of operation and latest conceptual advances in optical DACs for next-gen Ultra-High-Speed (UHS) photonic interconnects. Optical DACs ‘done right’ enable UHS optical transmitters at high-resolution, high photonic-efficiency and high energy-efficiency. An oDAC is defined as a Transmit-Optical-Sub-Assemby (TOSA) unit generating a constellation of C=2
^B
levels, comprising a collection of B modulators with static or slowly-adjustable ‘glue optics’; the admissible drives of the B modulators are specified as either two-level (NRZ, OOK) or 4-level (unipolar or bipolar PAM4); neither higher-order electrical driving-DACs, nor power-hungry digital encoders at baudrate are allowed.We model and compare serial (segmented-MZM) vs. multi-parallel oDAC architectures and their win-win combination. We present analytical models of serial and multi-parallel oDACs based on the code matrix concept. We introduce suitable metrics for oDACs characterisation. A ‘perfect’ oDAC features a max-full-scale (inherently optical-loss-free) uniform constellation (having equispaced levels), minimizing BER. We prove that for coherent (COH) detection links it is possible to design ‘perfect’ oDACs in the optical-field domain, but for a direct-detection link there exists no C>4 ‘perfect’ optical-power domain oDAC. We show how oDACs are assembled as building blocks in complete IQXY optical transmitters for coherent transmission links, at unprecedented performance, energy-efficiencies, reconfigurability.