# Machine Learning Applications for Short Reach Optical Communication
## 1 Introduction  
In recent years, short-reach optical communication systems have attracted intensive interests from both the industry and the academia. The criteria we use in this paper to determine whether an optical communication system is a short-reach or a long-reach is based on the transmission distance. We use the short-reach to refer to the communication system and network with a transmission distance of shorter than 100 km, which are cost-sensitive due to their large deployment scale [1]. In short-reach optical communications, in addition to broadband optical access networks, the emergence of 5G and future beyond-5G (also called 6G), new distributed edge cloud computing networks, large-scale machine-to-machine communications, inter- or intra-datacenter communications and mobile fronthaul, also impose new stringent requirements [1, 2, 3]. The typical architecture and application of the short-reach communications is shown in Figure 1. For the ease of discussion, based on the transmission distance and function, here we divide short-reach communication systems and networks into five categories: inter-datacenter networks, intra-datacenter networks, optical access networks, in-building and indoor optical wireless communications, and mobile fronthaul communications. In the past decade, large-scale data centers that promoted the development of information technology has greatly changed our lives [4]. Internet traffic within and between data centers is growing rapidly, particularly driven by the demand for bandwidth-intensiveapplications such as cloud computing, live streaming, and online games. Therefore, there is an urgent need for the design and deployment of higher capacity optical communication links in inter- and intra-datacenter networks (DCNs), where the intra-DCN link is usually up to hundreds of meters long, and the length of datacenter inter connection (DCI) link is usually up to tens of kilometers [5]. For the sake of solving the bandwidth limitation of traditional wavelength division multiplexing (WDM) networks, flexible bandwidth optical networking has been applied to intra and inter datacenter networking. Due to the low-cost requirement and the short transmission distance property, the multi-mode fiber has been widely used in intra-data center links. To enhance the channel capacity in multi-mode fiber intra-data center links, higher symbol rates and multi-level modulation formats have been used [6]. In addition, the latest standard can use four-level pulse amplitude modulation (PAM-4) to achieve a signal transmission rate of 25 Gbaud to support shortwave wavelength division multiplexing (SWDM) for intra-DCN [5]. Multi-mode fiber links with SWDM technology can help data centers to further upgrade to 40 G and 100 G Ethernet by using vertical-cavity surface-emitting lasers (VCSELs) and lasers optimized OM3/OM4 multi-mode optical fibers [7]. On the other hand, for DCI (with longer transmission distance and higher data rate requirements compared with intra-data center links), 100-Gbaud PAM-4 intensity-modulation direct detection (IM/DD) transceiver has been developed, which supports up to 200 Gb/s data rate [8]. Whilst being cost-effective, IM/DD transmission technologies struggle to further increase the capacity and reach of DCI links to meet even higher requirements, whilst completely coherent solutions are currently too costly. Gatisfyingly, in 2020, the use of single side band (SSB) direct detection reached a data rate of more than 400 Gb/s per channel, where 5 WDM channels each with over 400 Gb/s (64-QAM at 85 Gbaud/s) have been realized for the first time [9]. The information processed and exchanged in the data center needs to be transmitted to users through the optical access network. As shown in Figure 1, passive optical network (PON) is a typical optical access network architecture that uses un-powered optical splitters  
Figure 1: Short-reach optical communication system.