# High-Speed VCSEL-Based Transceiver for 200 GbE Short-Reach Intra-Datacenter Optical Interconnects
## 1 Introduction  
Rapidly expanding new technologies, video services, artificial intelligence [1], and cloud applications have caused a relentless growth in IP traffic. Predictions indicate that by the end of 2021, the annual global datacenter IP traffic will exceed 20.5 ZB, a threefold increase from 2016 [2]. Moreover, the nature of these applications and the intrinsic characteristics of networks mean that an enormous proportion of this traffic stays within the datacenter, accounting for 71.5% of the total. In such a challenging landscape, operators struggle to keep pace with the increasing demand for speed, cost, and energy efficiency [3, 4]. Optical interconnects are in the spotlight as an enabling technology that will accommodate the leap forward to the next decade. While shipments of 100 GbE devices are growing exponentially and will continue to proliferate until 2023 [5, 6], replacing the respective 40 GbE in datacenters, the next upgrade step is nearby [7]. For this reason, different techniques (space division multiplexing,polarization multiplexing, advanced modulation formats) and technologies (modulator-based transmitters, vertical-cavity surface-emitting laser (VCSEL)-based transmitters) are used to meet these requirements for intra-datacenter connectivity. Augmenting the number of parallel optical lanes offers a straightforward path towards scaling the link capacity but lacks in power efficiency and footprint [8]. Polarization multiplexing as a means of generating four-level signals in the optical domain was shown to achieve a 28 Gb/s transmission rate [9]. A four-channel compact and a low power transmitter module were reported in [10] to exhibit a limited bitrate of 25 Gb/s per lane. An eight-channel hybrid transmitter based on InP lasers, SiP modulators, and parallel single-mode fibers was shown in [11] to reach 56 Gb/s per lane. In [12], the hybrid integration of electronics and SiP integrated circuits resulted in the realization of a 56 Gb/s transceiver for a transmission distance of up to 2.2 km. Scaling the bitrate of each optical lane has also been demonstrated using discrete multitone formatting and complex digital signal processing [13]. Especially in the field of short-reach optical interconnects, vertical-cavity surface-emitting lasers (VCSELs) over multimode fiber (MMF) links are a dominant technology in the market since they have several beneficial characteristics, namely, low manufacturing and packaging costs, small footprint, low power consumption, high reliability, and high bandwidth density [14; 15]. Several attempts have been made to use VCSEL-based transmitters that operate at 850 nm in combination with MMF to achieve bitrates of up to 60 Gb/s [16; 17; 18; 19; 20; 21; 22]. High-speed transmitters of up to 71 Gb/s were demonstrated but with a small bit-rate-distance product [23; 24]. Single-mode VCSELs reduce the effect of modal and chromatic dispersion in the link and, as a result, extend the reach capability of the transmitter. Besides, datacenter operators have already started to replace multimode fibers with single-mode fibers in their datacenter environments, shifting from parallel optics to WDM (Wavelength-Division Multiplexing) solutions. The driving force for this trend is the continuous requirement to upgrade the type of installed multimode fibers to support higher bandwidth and longer reach as the datacenter grows in both size and traffic. Facebook is a prominent example that has come up with using low-cost single-mode fibers for short distances in the datacenter environment, a trend supported by other hyper-scale datacenter operators like Microsoft and Amazon [25]. Data rates of 30 Gb/s at 1270 nm and 40 Gb/s at 1525 nm were reported to obviate the need for equalization [26; 27]. Using the transmitter- and receiver-side equalization, a 56 Gb/s C-band VCSEL was showcased to have the ability to transmit only NRZ (Non-Return-to-Zero) signals [28]. A 56 Gb/s driver-integrated circuit for VCSEL transmitters with on-chip equalization functionalities was reported in [29].