Advances in Optical Fiber Communications

1. Introduction

Given the increasing importance of a globally interconnected world, driven by modern digital services and the need for fast and reliable access to digital resources, communications networks are one of the key infrastructures in today’s society. Telecom operators strive to make networks increasingly efficient and low-cost at every step of the technology’s development. In this scenario, fiber optics and optical devices play a leading role, as they allow for an unprecedent growth in our capacity to cope with the ever-increasing traffic demand.

Currently, most worldwide communications rely on optical technologies [1], and most research and industrial efforts focus on achieving the optimum trade-off between transmission speed and cost per bit. This goal is usually pursued by acting on three main aspects: the manufacture of low-cost devices [2], the introduction of digital solutions to help overcome some of the physical limitations of optical communications systems [3] and the optimization of network design [4].

2. Advances in Optical Fiber Communications

Contributions to this Special Issue address the three aforementioned subjects and bring valuable insights into the optical fiber communications world.

Li and coworkers [5] analyze in detail how substrate misorientation affects the structural and optical properties of Quantum Well (QW) lasers with large lattice mismatch between the InGaAs QW and the GaAs substrate. Understanding how QW lasers work is vital as they are a potential candidate for low-cost semiconductor lasers with high efficiency in uncooled operations [6]. In this manuscript, a mechanism elucidating the effect of substrate miscuts on the structural and optical properties of QWs is proposed and verified.

De Farias and coworkers [7] propose two heuristic strategies to solve the regenerator placement problem (RPP) in translucent networks under dynamic traffic. The translucent architecture is opposed to the fully transparent (all-optical) and fully opaque (optical–electrical–optical regeneration at every intermediate node) network configurations and uses a limited number of regenerators. This is achieved by solving the regenerator placement problem to reduce capital expenditure, while keeping performance at the desired level. The proposed dynamic load distribution (DLD) algorithm outperforms previously proposed heuristics, yielding lower blocking probability for the same number of regenerators, regardless of the topology, traffic pattern or intensity, and considered transmission reach.

The digital aid is introduced by Hadi and coworkers [8] and by Wang and coworkers [9]. In the former paper, digital predistortion is applied in the radio over fiber (RoF) scenario, using either conventional Volterra-based or deep learning methods to compensate for nonlinear impairments in optical front haul-based RoF systems. Both proposed methods which reduce the adjacent channel power ratio and the error vector magnitude. A considerable complexity reduction was also achieved by using a novel magnitude selective affine (MSA) method for digital predistortion.

Wang and coworkers [9] apply digital pre-compensation to the passive optical network scenario, showing a 29 dB power budget in a 200 Gbps communication system based on PAM-8 modulation over 20 km of standard single mode fiber. Pre-distortion at the transmitter is combined with three different digital signal processing techniques, with the receiver analyzing transmission performance as a function of overall digital complexity. A Volterra nonlinear equalization combined with a square-root-like technique was demonstrated to be the best among the considered options.

Lastly, in even-shorter-reach scenarios of data center interconnections only up to a few hundred meters, Ozolins and coworkers [10] show up to 100 GBaud capacity over 400 m using OOK and PAM-4 modulation and a monolithically integrated, externally modulated laser with 100 GHz 3 dB bandwidth. Equalizer complexity is also taken into account, studying its evolution as a function of the baud rate. They also show that around 1 km dispersion uncompensated 100 GBaud transmission can be achieved with a simple decision feedback equalizer with a 1 dB received power penalty with respect to back-to-back configuration.