# Review of CMOS Integrated Circuit Technologies for High-Speed Photo-Detection
## 2 Silicon Photonics for High-Speed Data Communications
### Silicon Photonics  
In this section, an overview of silicon photonics is presented and silicon photonic technologies are discussed with some examples of silicon photonic transceivers. The current status of silicon photonics and the underlying problems to be solved in the future are addressed. As explained in [34; 35], outstanding progress in silicon photonics was first observed in the early 2000s. Although silicon photonics pursues the realization of on-chip optical interconnections, they may find immediate application in fiber-optic communications as discussed in the previous section. Therefore, extensive research on silicon photonics include not only lasers, photodiodes, modulators, and waveguides but also waveguide-to-fiber couplers. Furthermore, all the photonic devices necessary to build a complete photonic transceiver were already viable in 2000s except for the laser source. A more recent review work states that the implementation of silicon lasers is still challenging; however, a number of innovations using bonding and epitaxial growth of III-V materials on silicon have been accomplished, so called a hybrid silicon laser [36]. In the case of photodetectors, silicon by itself cannot be a good absorber for telecommunication wavelengths due to its large bandgap energy. Among the numerous silicon-based photodiodes, a germanium-introduced photodiode is the most popular photodetector; moreover, recent germanium photodiodes have achieved very high bandwidths of several tens of GHz [37; 38]. Silicon-based photodiodes will be discussed again in the following section. Silicon modulators are typically based on plasma dispersion effect that the refractive index of the waveguide is changed by the density of free carriers, thereby enabling the amplitude modulation with Mach-Zehnder interferometer (MZI) structures. Modern silicon MZI modulators are capable of handling data rates higher than 50 Gb/s [39; 40]. Another type of silicon modulators is a ring resonator which exhibits a small footprint and better energy efficiency than the MZI structures [41; 42]. Furthermore, a wavelength-division-multiplexing (WDM) is readily applicable to the ring-resonator-based architectures, which is a desirable feature for future silicon photonic transceivers [43]. Much progress has also been made in silicon waveguides and couplers. Depending on the type of the waveguide, typical propagation loss can be several dB/cm or less than 1 dB/cm [36]. A coupling technique is also of great importance for successfully interfacing with an off-chip optical fiber. Fortunately, a number of good solutions such as holographic lens have been proposed so far, ideally exhibiting an extremely low-loss condition which is comparable to that of the conventional fiber connector [6]. Despite the brilliant success of silicon photonic ICs (PICs), compatibility with electronic ICs (EICs) or a monolithic integration of the PICs and the EICs is still challenging. Despite many efforts to realize electronic and photonic ICs (EPCs) on a bulk CMOS platform, the overall performance has yet to be  
Figure 5: Summary of communication standards, Ethernet. improved. Alternatively, an SOI CMOS platform is generally accepted as an ideal platform for the implementation of both silicon waveguides and active transistors. In a standard 0.13-\(\upmu\)m CMOS silicon-on-insulator (SOI) platform, the first fully integrated optoelectronic transceiver was realized by Luxtera in 2006 [44]. The implemented chip achieves a total throughput of 20 Gb/s with a dual-channel architecture using an SMF for long-reach communication. The overall block diagram of the transceiver is illustrated in Figure 6. The TX converts an electrical signal into a data-modulated optical signal and the RX operates in the other way around. The TX equalizer in the front compensates for the frequency loss of the electrical channel prior to the TX input. Following the equalizer, the TX-side clock and data recovery (CDR) retimes the data to reduce jitter and produce a clean data output.