## 3 Short-Reach Systems Classification and Enabling Technologies
### Pulse Amplitude Modulation
#### 3.3.2 Advanced Optoelectronic Devices for High-Speed DCIs  
Apart from the advanced formats that have been discussed so far, there are different research efforts on optoelectronic devices and components to support high-speed DCIs. This subsection focuses on different devices for short-reach transmission systems. Vertical-Cavity Surface-Emitting Lasers  
Cost-effective support of the unprecedented data traffic demands the use of inexpensive laser diodes at the transmitter. The laser should be able to offer sufficient output power and high modulation bandwidths, and emit in the nearinfrared [86]. Directly modulated VCSELs have gained significant attention in high-speed applications because of the associated benefits such as the small footprint, energy efficiency, and cost effectiveness [87; 88]. For instance, gallium arsenide (GaAs) 850 nm multimode (MM) VCSELs in conjunction with MMF are widely employed in the commercial short-reach intra-DCI. However, one of the main performance limitations of this implementation is the modal dispersion, which is instigated by the differential modal delay (DMD) in the MMF [42]. The modal dispersion effect can be mitigated by employing a single-mode or few-mode operation. Based on this, SMF deployment will be a promising option to support the evolving hyperscale DCNs. Similarly, the adoption of single-mode (SM) VCSEL-based technologies that are capable of mitigating the chromatic dispersion effects and supporting spectrally efficient transmissions over an extended reach are also an attractive option [76; 89; 90]. Nevertheless, the major weaknesses of single-mode VCSELs are that they usually have limited output power and demand further complex optical alignment. In order to prevent the high-order transverse modes' lasing and achieve single-/quasi-mode lasing, the aperture size can be minimized by means of oxidation or through the integration of a mode filter that is generated through a surface relief [42]. Furthermore, VCSELs emitting at 980 nm, 1060 nm, and 1100 nm with a negligible increase in fabrication complexities and significantly high-modulation bandwidths have been studied in the upper limit of GaAs technology [42; 89; 90]. Moreover, considerable efforts have been presented on high-speed VCSELs for the 1.3 \(\upmu\)m and 1.55 \(\upmu\)m wavebands to extend the transmission range [86; 88]. In this context, several device models such as nitrogen-containing quantum wells (QWs), wafer fusion, and buried tunnel junction (BTJ) have been attracting research attention in the 1.3 \(\upmu\)m waveband. On the other hand, for 1.55 \(\upmu\)m, only BTJ VCSELs in which a short-cavity design is exploited offer exceptional performance. An instance of such a design is based on the indium gallium aluminum arsenide (InGaAlAs) material. The associated optical, electrical, and thermal design of such devices and the relative design improvements were presented in [86]. Furthermore, InP-based VCSELs are capable of emitting at relatively longer wavelengths such as 1.3 \(\upmu\)m and 1.55 \(\upmu\)m. They have been attracting considerable attention owing to the associated lower losses and lesser power consumption. For instance, single-mode 1.5-\(\upmu\)m VCSELs that are InP based with two dielectric distributed Bragg reflectors (DBRs) and a 1.5-\(\lambda\)-long semiconductor cavity have been demonstrated [91]. The configuration offers superior dynamic characteristics and does not require FEC, DSP, or any equalization. A directly modulated laser (DML) typically generates high output power and has been considered to be a more power- and cost-efficient approach compared with the external modulation solutions. This is owed to the fact that the laser bias current is driven directly by the modulated signal. Furthermore, due to their compactness, they can be easily integrated with other devices. Based on the associated features, DMLs have been considered as attractive solutions for cost-sensitive DCNs and access networks.