Optical Digital‐To‐Analog Converters (oDACs): A Solution to Overcome the Limitations of Conventional High‐Speed Optical Transmitters

The continuous increase of data traffic demands necessitates evolution of optical communication systems towards larger capacities enabled by highly performing optical transmitters that generate multilevel and multidimensional ultrahigh data rate signals. Commercially deployed transmitters rely on high-order electronic digital-to-analog converters (eDACs) that drive optical Mach–Zehnder Modulators (MZMs) under particular biasing/driving conditions. However, these conventional transmitters face bandwidth and power consumption limitations arising from the eDACs and the nonlinearity of MZMs transfer function among others. Recently, alternative transmitter architectures have been proposed to perform direct digital to optical conversion, by relying on the utilisation of lower-order eDACs/drivers in combination with an increased number of MZMs operated with different biasing/driving conditions compared to the conventional transmitters, which are configured in either serial cascade or parallel stacking configurations. These novel transmitter architectures are referred as serial or parallel ‘Optical DACs’ (oDACs) enabling ultrahigh data rate and improved signal quality. In this article, we first discuss the issues of conventional optical transmitters and subsequently analyse selected oDAC architectures that solely rely on simple NRZ drivers or PAM4 eDACs. Simulation studies demonstrate their superior bit rate scalability (up to 3.2 Tbps) and analysis of power consumption reveals savings of about 40% comparing with conventional architectures.