## 1 Introduction  
To meet the ever-growing demands on the internet bandwidth, the Ethernet speed has evolved at an astonishing pace, from 10 Megabit Ethernet to 100 Gigabit Ethernet (100 GbE) within the past 30 years [1]. In 2017, 400 GbE standard was ratified by the IEEE P802.3 bs Task Force [2]. This built the foundation for industrial deployment of 400 GbE in the global network. It is anticipated that 400 GbE will be rapidly adopted over the next few years [3]. Recently, 400 GbE pluggable optics is under development to satisfy the demand on bandwidth-consuming applications, including data-center-based cloud services [4; 5; 6].  
For intra data center interconnect (DCI) application, optical direct detection with four-level pulse amplitude modulation (PAM-4) has been widely adopted to enable the low cost 400 GbE optical transceivers, which can carry up to 400 Gb/s Ethernet data over either parallel fibers or multiple wavelengths. Direct detection PAM-4 optics has also been standardized by Multi-Source Agreement (MSA) groups to achieve link distance from 500 m to 10 km (e.g., 400G-FR8, 400G-DR4, 400G-LR4, etc.) [7]. For inter DCI application, pluggable 400G digital coherent optics (DCO) with single-carrier dual-polarization quadrature amplitude modulation (DP-QAM) signal is being proposed to achieve 40 km and beyond fiber link [8; 9]. PAM-4 signal with direct detection only uses the intensity of the light to carry information. Instead, DP-QAM signal with coherent detection technology can utilize a few degrees of freedom for photons, including polarization, intensity, and phase. Consequently, coherent technology can transmit a single-carrier 400 Gb/s signal with significantly improved spectral efficiency. Furthermore, its receiver sensitivity can be significantly improved over the one of intensity modulation with direct detection (IM/DD) systems.  
In this article, we first introduce the application scenarios of pluggable optics in DCI applications. Next, we discuss different perspectives on the 400G pluggable module, including form factor,architecture, digital signal processing (DSP), and module power consumption. Then, we experimentally investigate the fiber transmission performance of 16-QAM and 64-QAM single-carrier signals at various baud rates, aiming at 400 Gb/s DCI applications with unamplified link. Here, we consider 25% overhead (OH) soft-decision forward error correction (SD-FEC), which gives a \(4.2\times 10^{-2}\) bit error ratio (BER) threshold [10]. For 64 Gbd 16-QAM signals with 12 dB and 18 dB link margins, it is possible to transmit 400 Gb/s net data rate over 40 km and 80 km unamplified fiber link, respectively. With 18 dB link margin, 64 Gbd 64-QAM signal can produce greater than 600 Gb/s net data rate for an unamplified link as long as 40 km. We further study the impact of transmitter (Tx) side power on the transmission performance and obtain a high-baud-rate QAM signal transmission matrix in unamplified link. For various signals, extra 6-dB more power can extend the fiber transmission reach to approximately 30 km. It is thus critically important to increase the Tx output power or improve the Rx sensitivity, under the available power and space constrains of the DCO pluggable module.