## 3 400G Pluggable Module Form Factors and Power Requirement  
Due to the emerging bandwidth-hungry applications, the photonics communications industry is unceasingly driven towards higher-capacity, smaller-size, and lower-power solutions. Standardization organizations, including the Institute of Electrical and Electronics Engineers (IEEE) and the Optical Internetworking Forum (OIF), are leading the industrial transformation of photonics communications society from using discrete optical components to utilizing photonics integration enabled packet  
Figure 1: Typical application scenarios of 400 Gb/s pluggable optics for data center interconnect. **(a)** Unamplified point-to-point applications for 40 km reach; **(b)** amplified dense wavelength division multiplexing (DWDM) applications for 80–120 km reach. optics [11; 12]. Lately, line and client-side industrial trials on interoperability have been demonstrated successfully using 100 Gb/s CFP2 and 400 Gb/s CFP8 pluggable optical modules, respectively [13; 14; 15; 16; 17]. Nowadays, the industry is marching towards 400 Gb/s pluggable optical modules with even more compact form factors. The first debuts of 400 Gb/s client and line-side pluggable QSFP modules are planned for year 2019 and 2020. Based on the DSP's physical location, there are two types of pluggable coherent modules: analog coherent optics (ACO) and DCO. For 400G pluggable coherent transceivers, the industry has reached an agreement on implementing DCO, which has the DSP inside the module. A few form factors have emerged recently as the candidates for 400G DCO pluggable modules, such as CFP2, CFP8, OSFP, and QSFP-DD [18; 19; 20; 21; 22], whose mechanical dimensions and power consumptions are shown in Figure 2. These transceivers are equipped with eight pairs of differential signal pins, which can support 400G transmission speed using 50G PAM-4 electrical interface [23]. One can see that CFP2 and CFP8 have larger physical dimensions than those of OSFP and QSFP-DD, thus, they have more room for additional components and allow a larger power budget. Consequently, OSFP and QSFP-DD are targeted for the DCI application, while CFP2 and CFP8 are planned for the long-haul applications. The line-card fitting into one rack unit (RU) space can accommodate 36 QSFP-DD pluggable modules. The total capacity of the line-card is 14.4 Tb/s. Currently, the industry is converging behind QSFP-DD due to its high port density in the front panel, its backward compatibility with QSFP28/QSFP+, and thus a larger ecosystem. Figure 3 illustrates the typical architecture of a 400G pluggable coherent transceiver. There are three functional blocks inside the module, one DSP chip, one transmitter and receiver optical subassembly (TROSA), and one power/control unit. Besides the power and control signal, the host side also provides up to 400 GbE client traffic to the pluggable coherent module through eight pairs of differential signal pins. 400 Gb/s single-carrier signal (>60 Gbd 16-QAM) can thus be generated out of the Tx output. The other port of receiver (Rx) input captures the received signal, and coherent detection is performed by the TROSA and DSP. The pluggable coherent optics starts from 100 Gb/s CFP. During the past few years, its evolution of density has been quite amazing. Meanwhile, a lot of challenges to the whole industry have been triggered. Power consumption is always one of the key constrains for industrial applications. As shown in Figure 2, the requirement on power consumption for 400 Gb/s QSFP-DD DCO is less than 15 W. Over the years, the power consumption of the coherent DSP has reduced significantly [24; 25; 26]. Table 1 illustrates the evolution of the power consumption of coherent DSP. One can see that, with 40 nm complementary metal-oxide-semiconductor (CMOS) process, 100G coherent traffic consumes approximately 50 W power. Nowadays, with the latest 7 nm CMOS technique, the industry can bring  
Figure 2: 400G pluggable coherent transceiver form factors.