I. Introduction
In recent years, applications such as 5G, Internet of things, high performance computing have ever been promoting the development of the edge computing and cloud edge collaboration, resulting in the huge data traffic with and within data centers. Consequently, advanced optical interconnections at the reach of several tens of kilometers are highly demanded. For high-speed short-reach optical transmissions, considering the system implementation and power efficiency, optical intensity modulation and direct detection (IMDD) systems combined with pulse amplitude modulation (PAM) format have been considered as one of the most promising solutions. However, due to fiber dispersion and square-law detection, inter symbol interference (ISI) is very severe for optical IMDD systems.