## 2 Special Issue Papers  
Following SOA's huge practical potential, this Special Issue was introduced and prepared with a two-fold aim: on one hand to address, present, and investigate modern applications of SOAs, while, on the other hand, to explore and highlight trends, challenges, and perspectives for motivating efforts toward continuous exploitation of these active modules in a feasible, innovative, and global manner. The submitted papers have successfully contributed to achieving these goals by treating cutting-edge topics that have led to the derivation, collection, and publication of significant results on SOA-enabled applications. In this manner, it has been possible, through reaching comprehensive conclusions, to advance knowledge and open new horizons in the target field of research. More specifically:  
The research field of integrated optical memories is covered by [PERSON], [PERSON], [PERSON], [PERSON], and [PERSON] in Ref. [20]. This paper describes a novel application scenario of optical memories based on monolithically integrated SOA and Mach-Zehnder interferometer layouts/arrangements. In this context, it proposes an alternative path to circumvent the mismatch between the rapidly growing optical transmission line rates and the electronic processing speeds towards facilitating technological evolution in the era of the Internet of Things. The use of SOAs for optical signal amplification applications is reported in two papers. [PERSON], [PERSON], and [PERSON] present in Ref. [21] a theoretical analysis and benchmarking of two basic optical notch filters employed to compensate for the SOA pattern effect. The comparison reveals the performance merits of each filter which allow it to favorably compete against the other for efficiently addressing the pattern-dependent operation of SOAs and assisting the latter in serving linear amplification applications with improved performance. [PERSON], [PERSON], and [PERSON] propose in Ref. [22] multi-section SOAs as pre-amplifiers for short range optical communication linkswithin datacenters. By creating and employing a simplified multi-section SOA model, they evaluate the performance benefits offered by these special type SOAs, which are shown to exhibit better input power dynamic range than conventional single-section SOAs when amplifying single- as well as multi-channel signals of advanced modulation format (4-level pulse amplitude modulation/PAM4). Reflective semiconductor optical amplifiers (RSOAs) form the subject of two papers. [PERSON], [PERSON], [PERSON], [PERSON], and [PERSON] in Ref. [23] conduct a comparative study of the capacity increase brought in Wavelength Division Multiplexing (WDM) passive optical networks (PONs) architectures by exploiting bit- and power-loading discrete multi-tone (DMT) modulation in upstream colorless transmitters using RSOAs. Three different RSOA-based schemes are compared against appropriate evaluation criteria whose measurement and analysis allows to specify and highlight performance and cost trade-offs. Concurrently, record high line rates in both back-to-back and transmission experiments are obtained and reported for all three configurations. [PERSON] and [PERSON] in Ref. [24] demonstrate the feasibility of using a single microring resonator (MRR) as optical notch filter for enabling RSOA direct modulation at extended data rate than is possible with the RSOA alone. To this aim, it has been investigated and specified how the MRR should be designed to improve the encoded signal characteristics for RSOA direct modulation applications. The exploitation of SOAs for photonic switching purposes is addressed in two papers. [PERSON], [PERSON], [PERSON], and [PERSON] in Ref. [25] present a novel photonic WDM optical cross-connect node based on SOAs that allows switching data signals in wavelength, space, and time for fully exploiting statistical multiplexing.