## 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.  
[PERSON] 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.  
[PERSON] 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 [PERSON] and coworkers [8] and by [PERSON] 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.  
[PERSON] 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 onPAM-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, [PERSON] 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.  
The editor would like to thank all the authors for their valuable contribution to the scientific community, all the peer reviewers for taking the time to improve the quality of the submitted manuscripts and, by extension, of the whole Special Issue and editorial team of _Applied Sciences_ for their hard work. My gratitude goes especially to Section Managing Editor [PERSON] for the dedication and continued support shown.  
The authors declare no conflict of interest.