Bandwidth Enhancement of Wavelength Selective Switches to Improve Transmission Performance of Subcarrier-Multiplexed Optical Links

The analysis presented in this thesis revolves around optical network where signals are routed from the source to the destination using devices known as Reconfigurable Optical Add-Drop Multiplexers (ROADMs). These devices act as both multiplexers and demultiplexers, capable of selecting specific wavelengths and route it to a local RX (Drop) or to choose new traffic to route from a TX (Add) eliminating the need for conversion to and from electrical domain. The core component of the ROADM is the Wavelength Selective Switches (WSS), which acts as filters, ultimately affecting signal quality at the edges of the Frequency Bandwidth. Furthermore, as optical neworks become bigger and bigger, a large number of ROADMs could be required to route the signal properly. As a result, the impact of WSS filtering becomes more significant as the number of cascaded ROADMs, and therefore the number of WSS, increases. Indeed, the main goal of this thesis is to reduce its impact, following different strategies. Firstly the transmitted signal has been divided into subcarriers, each with the same Spectral Bandwidth. The use of multi-sub-carrier systems (MSCs) offers the advantage of assigning different modulation formats to each sub-carrier independently using the Frequency Domain Hybrid Modulation Formats (FDHMF) allowing traffic’s heterogeneity and the usage of optimization strategies, such as bit and/or power loading, to enhance system performance. Finally, these techniques have been combined jointly to the use of modern WSS relying on a revised transfer function. This allows for the introduction of a new technique called "Bandwidth Enhancement", which aims to better exploit Bandwidth Edges to minimize the filtering effect. In this work, standard Single-Carrier transmission is compared to MSC solution for a system operating at 800 Gbps. Specifically, two modulation format approaches are examined: Uniform Quadrature Amplitude Modulation (QAM) Constellation and Probabilistic Constellation Shaping (PCS). With Uniform QAM Constellation, all symbols are equally probable. In contrast, PCS assigns different probabilities to the symbols, shaping the initial 2^M − QAM (where M is an integer number) constellation enabling rate flexibility and increased spectral efficiency. As final outcome, the application of these techniques has shown that MSC systems generally outperform Single-Carrier one when multiple WSS are present.