# Review of CMOS Integrated Circuit Technologies for High-Speed Photo-Detection
## 2 Silicon Photonics for High-Speed Data Communications  
### Overview of Optical Link  
Links or I/O circuits have experienced remarkable advancements and played a major role in the development of data communications systems. However, the bandwidth limitation of copper-based links was already predicted a long time ago and engineers have attempted to determine alternative ways to handling the ever-increasing bandwidth requirement. The optical link based on silicon photonics continues to be the best solution to successfully replace the conventional electrical link [2]. Accordingly, many research groups at not only the leading companies such as Intel and IBM, but also universities and national institutions have focused on developing silicon photonic devices including both the optical and electrical parts [3; 4; 5]. The results of massive research over several decades appear to be successful and some remarkable results are already applicable to the industrial world [6]. Optical links are gradually replacing their electrical counterparts in long-haul and high-speed applications. Moreover, the ultimate goal of silicon photonics is defined and envisioned as a \"macro  
Figure 3: (**a**) Power breakdown of data centers and (**b**) forecast of IP traffic contribution [1]. chip\" [7] or an \"on-chip server\" [8], indicating that the optical links may hopefully substitute most of the electrical links even in the inter-/intra-chip scale. Nevertheless, it appears that the industry is hesitating at adopting silicon photonics as a primary interconnection method for some practical reasons. In the case of very short-reach applications ranging from several centimeters to meters, electrical links are still dominant, despite the bandwidth limit of copper-based interconnection. The proliferation of copper-based links in this area can be mainly attributed to the advancements of CMOS technologies and circuit techniques. By utilizing advanced CMOS technologies and equalization techniques, operations at higher than 25 Gb/s even in severe loss conditions of more than 40 dB were achieved [9; 10; 11]. Recently, circuit designers attempt to overcome the limitations of copper by introducing a pulse-amplitude-modulation (PAM) signaling that can enhance the effective data rate for the same loss condition as the conventional binary signaling [12; 13; 14; 15]. Figure 4 summarizes the binary and PAM-4 transceivers which recorded the fastest operating speed each year over the past 10 years [16; 17; 18; 19; 20; 21; 22; 23; 24; 25; 26; 27; 28; 29]. Notably, the electrical links face the challenge of bandwidth limitation and the maximum available speed would saturate in the immediate future. Therefore, designers are focusing on developing PAM-4 transceivers in recent years. However, despite the innovations in electrical links, one can expect that the capacity of the copper-based links would saturate eventually because of both the circuit and device limitations. The optical links can be configured with either a multi-mode fiber (MMF) or a single-mode fiber (SMF). An MMF-based link features a distinctly low cost whereas the communication distance is strictly restricted due to modal dispersion. A vertical-cavity surface-emitting laser (VCSEL) is commonly used as a light source in the MMF-based links with a typical wavelength of 850 nm. Owing to the low cost of MMF-based links, short-reach applications ranging up to several hundreds of meters mostly rely on them. On the other hand, an SMF-based link is free from modal dispersion, thereby providing a much longer communication distance and is only limited by chromatic dispersion and other loss mechanisms such as Rayleigh scattering or electronic absorption. Typical communication standards based on SMF support a communication distance of up to several tens of kilometers with wavelengths of 1310 or 1550 nm. Figure 5 summarizes the communication trends from Ethernet standards: 10 GbE, 40 GbE, and 100 GbE [30; 31].