## 3 Short-Reach Systems Classification and Enabling Technologies
### Mach-Zehnder Modulator  
The Mach-Zehnder modulator (MZM) has been widely employed in IM-DD optical communications for external modulation. It operates through the combination of two-phase modulators within a Mach-Zehnder interferometer structure to accomplish intensity modulation. Furthermore, for effective support of high-speed communications using advanced modulation formats, significant efforts have been made regarding the realization of small-sized, but high-performance MZMs. It is noteworthy that commercial MZMs are usually made from lithium niobite (LiNbO\({}_{3}\)) and are capable of supporting high-capacity transmissions [94]. Nevertheless, they are typically packaged into bulky modules that are not only expensive, but also energy-intensive. Therefore, their application for client-side optical interfaces such as pluggable optical transceivers is hindered. Based on this, they might not be able to meet the usual growing requirement in the optical communication links [42; 95]. These challenges can be addressed by the employment of monolithicallyintegrated LiNbO\({}_{3}\) electro-optic modulators [95]. Energy-efficient integrated nanophotonic LiNbO\({}_{3}\) MZMs with a high bandwidth can also be employed. Likewise, MZMs that are based on InP can be produced at a low cost and be monolithically integrated into a small size [42; 94]. For instance, a distributed feedback (DFB) laser that is based on InP can be monolithically integrated with the MZM. Similarly, another good example is the MZM laser that is based on InP being integrated with a capacitance-loaded traveling-wave electrode (CL-TWE). Additionally, SiP-based MZMs fabricated with wafer-scale technology are also viable solutions [94]. In this context, there have been intensive research efforts on multi-electrode MZMs (ME-MZMs) [96; 97] and SiP-based traveling-wave MZMs (TW-MZMs) [98]. It is noteworthy that MZMs based on SiP and InP are now in the industrial development stage. MZMs based on silicon-organic hybrid (SOH) platforms have been presenting attractive features to support high data rates [99]. Electro-absorption modulated lasers (EMLs) are based on a well-developed InP technology and have been extensively employed for short-reach links because of the associated advantages such as the high bandwidth, small form factor, and large extinction ratio [100]. Typically, an EML has just one modulated output, and the other output is normally employed for monitoring. In a double-sided EML (DS-EML), two EAMs are integrated on both sides of the laser. Consequently, two independent optical modulated signals can be offered at the same wavelength by a DS-EML. It is noteworthy that it is relatively challenging to generate polarization division multiplexed signals by employing two discrete EMLs. This is owed to the relative wavelength drifting. In this regard, a DS-EML is an ideal solution for polarization division multiplexed signal generation due to the two offered independent modulated outputs at the same wavelength [35; 100]. In the commercial 10G and 25 G transceiver applications, the distributed feedback laser with electro-absorption modulator EAM (EA-DFB) has been widely employed [101]. Compared with the DML, the EA-DFB lasers offer notable advantages such as an improved extinction ratio, better modulation linearity, and enhanced bandwidth. With respect to the external MZM, it has a smaller size and requires a lower driving voltage at a lower cost, making it an attractive candidate for future high-speed applications. Moreover, another promising and advanced device is the distributed feedback laser that is monolithically integrated with a traveling-wave EAM (DFB-TWEAM). Apart from the high linearity of its phase response, the DFB-TWEAM device has a broad bandwidth that is capable of supporting several high-speed transmissions [42; 102]. In general, short-reach transmission based on PAM-4 or -8, with either separated or integrated modulators and lasers, can support 100 Gb/s per lane.