HIGH-CAPACITY SUSTAINABLE OPTICAL COMMUNICATION SYSTEMS FOR SMART NETWORKS
The ongoing traffic growth that we are witnessing in these last years in urban environments represents quite a challenge for the optical network, especially on a metro and access level. This is strongly related to the kind of approach mainly adopted in these contexts, based on an intensity modulation at the transmitter and a direct detection at the receiver. On the other hand, this working scheme is extremely simple and cost-effective. For this reason, it is fundamental to find innovative solutions applicable in the optical metro and access network and able to increase the network performance (in terms of capacity and reach) keeping the previous simple system scheme. This theme is of utmost importance also thinking about the future, since the optical network has to be ready to support the huge throughput demand requested by the future 6G radio network and database interconnections. There are several ways of acting on the system that can lead to such result.A huge throughput increase (with respect to single-carrier modulations) can be obtained by using innovative multi-carrier modulation formats, such as Discrete Multitone (DMT), thanks to the parallel orthogonal channels availability and its water filling principle. The DMT modulation is already extremely effective with the current devices, so, considering also the technology development, it will be part of the future optical network for sure. The kind of study that must be carried out on this modulation is where and how to apply it in order to obtain the best results. A further (and relevant) capacity increase is achievable thanks to the usage of multiplexing techniques (such as the Space Division Multiplexing), whose realization is for sure a future challenge for the research under many points (special fibers development, crosstalk reduction among signals, etc…). For what concerns the network reach, it can be extended using Single Sideband (SSB) signals instead of the currently used Dual Sideband (DSB) signals. This is because these latter ones are affected by the power fading issue caused by the fiber chromatic dispersion (that strongly limits their performance already after a few tens of kms), something that doesn’t happen for SSB signals because of their nature. Obviously, a cost and energy effective, smart and simple way to generate this kind of signals is a real challenge. A couple of generation methods that include an IQ Mach-Zehnder Modulator or an optical filter are already available and they are very simple from a conceptual point of view, but they are characterized by some issues when passing to an actual implementation (especially in terms of integration and/or energy consumption). For this reason, an innovative SSB signal generation method that guarantees simplicity, low cost and high energy efficiency is an important topic to develop. These capacity and reach growths can go along with the employment of innovative and sustainable laser light sources at the transmitter side, in particular VCSELs, which are cost-effective sources characterized by an extremely low dimension and a high energy efficiency, fundamental requirements for the light sources of the future optical network. Moreover, a VCSEL also allows the application of a direct modulation, which leads to the possibility of assigning part or even all the light modulation process to the laser itself. In particular, related to this point, it should be analyzed how to exploit the semiconductor laser chirp with direct modulation in a clever way in order to realize a good phase modulation. Such working scheme means a reduction of the transmitter complexity, especially thanks to the opportunity of removing external modulator. On the receiver side, instead, capacity and reach can be increased with the employment of high-level receivers still relying on a direct detection process but adding also other features able to increase the received signal quality. The Kramers-Kronig receiver is probably the most relevant example, as it is based on direct detection but allows to perform a chromatic dispersion compensation via digital signal processing, that is exactly the same operation of a coherent receiver. This leads to conclude that the exploitation and/or the development of such receivers is another topic of utmost importance when speaking about future sustainable optical network. All these improvement methods can be put together, but it’s fundamental to study how it can be done and to perform lots of experimental tests that confirm the feasibility of the solutions that will be developed starting from these elements. Moreover, it must not be forgot that optical communications are not the only kind of signal existing in the optical network. In fact, the development of quantum communications and, in particular, the Quantum Key Distribution (QKD) for the exchange of communication security keys is giving impetus to the installation of new quantum networks. At the same time, as a consequence, this process is emphasizing the problem of the allocation of quantum channels and their co-existence with classical communications. This issue is considerably relevant due to the characteristics of quantum communications (e.g., the single photon transmission is very delicate). Together with QKD, all the processes related to monitoring systems and sensing fibers must be taken into account too, since they could interfere with normal communications and lead to information exchange problems. Therefore, a deep and detailed analysis about the co-existence of the cited possible solutions for the optical metro and access network improvement and QKD and monitoring systems must be carried out during the research work.
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