Contents 1. Introduction
Driven by the explosive growth of mobile and cloud services, the demand for greater capacity in data communications systems is rapidly growing. In the most bandwidth-hungry application fields, such as inter-and intra-data-center networks, compact and low-cost transceivers with a data rate of 400 Gbit/s are increasingly demanded. Forthcoming 400-Gbit/s Ethernet (400GbE) transceivers are likely to be implemented with an -Gbit/s 4-level pulse amplitude modulation (4-PAM) system [1], [2]. However, systems with smaller numbers of optical lanes (higher per-lane data rates) are strongly desired to reduce the complexity, size, power consumption, and cost of the transceivers. As straightforward solutions to this challenge, 100-Gbit/s/ 4-PAM transmitters have attracted much attention as promising candidates for 4-lane 400GbE systems [1], [3]. Even higher data rates of around 150 Gbit/s/ have also been reported with 4-PAM and 8-PAM [4], [5]. However, a data rate of 200 Gbit/s/ , which may enable a two-lane 400GbE system, was yet to be reached with a simple single-polarization intensity-modulated direct-detected (IMDD) configuration.
