# Integrated Components and Solutions for High-Speed Short-Reach Data Transmission
## 1 Introduction  
In recent years, with the applications of various multimedia and data services (e.g., Internet of Things, cloud computing, remote surgery, the construction of 5G, and beyond 5G networks), global network traffic has presented explosive growth over the past decade [1]. Since massive data needs to be stored, transmitted, and processed in a data center, the corresponding traffic also grows rapidly. As the intra- and inter-data center optical interconnects in data center application scenarios [2] require a large number of transmitter and receiver components between servers and racks, the component cost plays a critical role in optical interconnects [3]. Even though the coherent solution with IQ modulation and coherent detection beyond 100-Gbit/s is relatively mature, its transmitter and receiver components with high cost and large footprint size cannot be directly transplanted to short-reach transmission systems. Considering the cost and footprint size, 100-Gbit/s short-reach transmission systems prefer intensity modulation and direct detection (IM/DD) technology [4]. In recent years, numerous 100-Gbit/s IM/DD-based solutions with different transmitter and receiver components have been demonstrated. In these solutions, the transmitter components mainly arevertical-cavity surface-emitting laser (VCSEL) [5; 6; 7], directly modulated laser (DML) [8; 9], Mach-Zehnder modulator (MZM) [10; 11], integrated electro-absorption modulated laser (EML) [12; 13; 14], and monolithic integrated transmitter [15; 16]. The receiver components usually contain single-end photodetector (PD), Kramers-Kronig receiver [17; 18], Stokes vector receiver [19; 20; 21], and monolithic integrated receiver [22; 23]. Here, it is commonly considered that monolithic integrated transmitter and receiver under low-cost and small footprint size have great potential to be the candidate transmitter and receiver component for next-generation high-speed short-reach transmission systems. In order to further increase the transmission capacity of IM/DD systems, advanced multi-level modulation formats, polarization division multiplexing, and powerful digital signal processing algorithms have been introduced to support high-speed short-reach transmission systems [24]. However, when a lane rate increases from 100-Gbit/s to 400-Gbit/s, a smooth transition from the IM/DD technology to digital coherent technology will be in progress; of course, there are still many challenges. The conventional transmitter and receiver components in digital coherent technology are not cost-effective [25], so that these components cannot be directly transplanted to short-reach transmission systems. In addition to the digital coherent technology and the traditional IM/DD technology, some advanced direct detection technologies such as Kramers-Kronig receiver [17; 18] and Stokes vector receiver [19; 20; 21] have been proposed and studied extensively. Since these techniques can reconstruct the complex domain of the signal, rate-distance product can be further improved by combing impairments compensation algorithms and advanced modulation technology. These advanced techniques can be treated as a compromise between the digital coherent technology and the traditional IM/DD technology. Recently, an effective solution, a transceiver integration scheme based on the digital coherent technology [26; 27], has been reported and shows the potential to reduce cost and footprint size of the transmitter and receiver components dramatically. Subsequently, the transceiver integration scheme for multiple parallel IM/DD channels [28] has also been demonstrated. The rest of this paper is organized as follows. We firstly review mainstream transmitter components (e.g., VCSEL/DML/EML/MZM/monolithic integrated transmitter) in Section 2, and then review mainstream receiver components (e.g., single-end PD /Kramers-Kronig receiver/Stokes vector receiver/monolithic integrated receiver) in Section 3.