applied sciences Review A Review of Self-Coherent Optical Transceivers: Fundamental Issues, Recent Advances, and Research Directions Isiaka Alimi 1,* , Romil Patel 2 , Nuno Silva 1 , Chuanbowen Sun 3, Honglin Ji 3, William Shieh 3 , Armando Pinto 2 and Nelson Muga 1 1 Instituto de Telecomunicações, Campus Universitário de Santiago, University of Aveiro, 3810-193 Aveiro, Portugal; [email protected] (N.S.); [email protected] (N.M.) 2 Department of Electronics, Telecommunications and Informatics, Instituto de Telecomunicações, Campus Universitário de Santiago, University of Aveiro, 3810-193 Aveiro, Portugal; [email protected] (R.P.); [email protected] (A.P.) 3 Department of Electrical and Electronic Engineering, University of Melbourne, Melbourne, VIC 3010, Australia; [email protected] (C.S.); [email protected] (H.J.); [email protected] (W.S.) * Correspondence: [email protected]; Tel.: +35-123-437-7900; Fax: +35-123-437-7901 Abstract: This paper reviews recent progress on different high-speed optical short- and medium- reach transmission systems. Furthermore, a comprehensive tutorial on high-performance, low-cost, and advanced optical transceiver (TRx) paradigms is presented. In this context, recent advances in high-performance digital signal processing algorithms and innovative optoelectronic components are extensively discussed. Moreover, based on the growing increase in the dynamic environment and the heterogeneous nature of different applications and services to be supported by the systems, we discuss the reconfigurable and sliceable TRxs that can be employed. The associated technical Citation: Alimi, I.; Patel, R.; Silva, N.; challenges of various system algorithms are reviewed, and we proffer viable solutions to address them. Sun, C.; Ji, H.; Shieh, W.; Pinto, A.; Muga, N. A Review of Self-Coherent Keywords: 5G network; coherent detection; data center network; digital signal processing; direct Optical Transceivers: Fundamental detection; intensity modulation; Kramers–Kronig algorithm; minimum-phase signals; self-coherent Issues, Recent Advances, and detection scheme; short-reach communication; Stokes vector detection Research Directions. Appl. Sci. 2021, 11, 7554. https://doi.org/10.3390/ app11167554 1. Introduction Academic Editor: Rastislav Róka The advent of the fifth-generation (5G) network will impact different network entities, technologies, and the corresponding system as a whole. In this perspective, how the access Received: 20 July 2021 networks, metro, core networks, high-performance cloud data center networks (DCNs), Accepted: 9 August 2021 Published: 17 August 2021 as well as the associated edge services and Internet of Things (IoT) devices are designed and deployed will be affected [1–3]. Furthermore, the emergence of IoT devices has brought Publisher’s Note: MDPI stays neutral about the unprecedented expansion of Internet connections [4,5]. Their proliferation and with regard to jurisdictional claims in effective support for several innovative applications and services such as cloud computing, published maps and institutional affil- augmented reality (AR), massive machine-to-machine communication, and emerging web iations. applications have considerably contributed to the growing increase in machine- and user- generated data traffic. In this regard, there has been a significant increase in the workload of the underlying DCNs that support the applications and services [6]. For effective service de- livery, the ensuing traffic transportation requires a consistent increase in bandwidth both in the metro and long-haul optical fiber networks. Consequently, this imposes unprecedented Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. bandwidth requirements on the transport networks. In this perspective, to satisfy diverse This article is an open access article requirements such as robustness, flexibility, high bandwidth, and relatively reduced cost distributed under the terms and per unit bandwidth, bandwidth-efficient and cost-effective transmission systems have to be conditions of the Creative Commons adopted [2]. One such effective solution is the intensity modulation with direct detection Attribution (CC BY) license (https:// (IM-DD) transceivers (TRxs) scheme. creativecommons.org/licenses/by/ In addition, IM-DD-based transceivers offer various advantages such as low cost and 4.0/). ultra-low complexity. For instance, they require fewer optical components and present Appl. Sci. 2021, 11, 7554. https://doi.org/10.3390/app11167554 https://www.mdpi.com/journal/applsci Appl. Sci. 2021, 11, 7554 2 of 85 relaxed laser wavelength stability, without the need for complex digital signal processing (DSP) such as frequency offset compensation and carrier phase compensation [7]. These advantages make IM-DD-based schemes good candidates in the short-reach applications where a considerable amount of transceiver deployment is demanded. As a result, the as- sociated cost of the short-reach transceivers is one of the main factors for network design and deployment, making it deserve significant consideration [8,9]. Moreover, the IM-DD receiver (Rx) sensitivity is independent of the state of polariza- tion (SOP) and the carrier phase of the signal. This is advantageous as the polarization state randomly fluctuates in real system scenarios [10]. Nevertheless, the IM-DD system’s scalability and its spectral efficiency (SE) are significantly limited when the achievable infor- mation rate (AIR) is to be upgraded beyond 100 gigabits per second (Gb/s) per wavelength. This is mainly due to the associated one-dimensional (1D) modulation and detection con- straints [8,11]. For instance, one of the fundamental bottlenecks of the conventional IM-DD system is the second-order nonlinearity. This is caused by the square law photodetec- tion, and it limits the system capacity significantly. As a result of the lack of the phase diversity of the system, the transmission distance will be limited by chromatic dispersion (CD)-induced power fading [9] since the information is modulated on the optical intensity rather than the field [12]. Therefore, the conventional IM-DD system is incapable of field recovery (FR), which can support the digital compensation of fiber linear impairments and multidimensional modulation. On the other hand, the long-haul-based optical networks have attained a considerable AIR advancement for the multiterabit transmission with coherent (COH) transceiver system’s revitalization [8,11]. Furthermore, the polarization diversity feature of light can be exploited in the COH system to maximize the system SE and optimize the AIR [8]. With the aid of COH detec- tion, optical receivers can perform field recovery of both the intensity and phase of the optical signal that is propagating over the fiber [8,13]. Furthermore, the coherent receiver (Co-Rx) is capable of the linear recovery of the signal with phase diversity. Fiber linear impairments can be effectively compensated through DSP. Other attractive features of the COH technology that facilitate multiterabit per second (Tb/s) capacity are the support for polarization multiplexing (PolMux), high-order modulation formats, dense wavelength division multiplexing (DWDM), and superchannels [12]. Apart from the offered increase in the degrees of freedom (DoFs) for optical modulations, the Co-Rx facilitates the digital compensation of fiber dispersion. This helps prevent the need for the complicated optical dispersion management [8,13]. It should be noted that polarization-diverse Co-Rx usually performs four-dimensional (4D) detection. This includes the signal in-phase (I) and quadrature (Q) of both polarizations X/Y. As a result, the offered dual-polarization (DP) field recovery by the implementation helps maximize the detection dimension in the single-mode fiber (SMF). Under random po- larization variations of the fiber channel, it ensures a closed multiple-input multiple-output (MIMO) space without information loss. Nonetheless, the associated system complexity of the conventional Co-Rxs has been limiting its implementation to long-haul-based networks due to the cost, footprint, and power consumption of the transceiver [8,11,13]. In addition, based on the exhausting multiplexing dimension in the conventional direct-detection-based system and the consideration of the enhanced sensitivity of the COH detection along with its inherent WDM functionality, research attention has been shifting towards low-complexity implementations of COH systems. Consequently, to enhance the system AIR while maintaining the related cost-effective benefit, the short-reach networks should explore an appropriate compromise between 1D-based IM-DD and 4D-based COH detection schemes. In this perspective, some simplified and low-cost optical transceivers with good tradeoffs between the COH and the direct-detection-based systems have been presented [8,11,13]. In general, a promising system should be able to offer low-latency, low-cost, and small-footprint optical transceiver components that can support larger bitrates and operate over a considerable reach compared with the conventional IM-DD systems, while still Appl. Sci. 2021, 11, 7554 3 of 85 being power efficient. To explore a suitable compromise between the cost-effective direct detection and spectrally efficient COH configurations, different potential solutions have focused on the optimization of the required components. For instance, some prevent the implementation of a local oscillator (LO) laser and extensive DSP that are normally employed in