We provide design guidelines for the integration of quantum key distribution (QKD) into legacy passive optical networks (PONs). Our study addresses the challenge of quantum coexistence with various PON standards in different network topologies. We develop a novel theoretical model to assess the impact of the passive optical distribution network on both the quantum signal and the generation of scattered Raman photons from classical signals. This work also includes an extensive Raman efficiency experimental evaluation over a wide frequency range. The study evaluates the integration of upstream QKD systems in 32- or 64-user PON scenarios, determining quantum-available bandwidths and maximum supported PON lengths. In single-fibre architectures the coexistence with standards having upstream and downstream classical channels in different telecommunication fibre windows (e.g. XG-PON) is extremely challenging due to the pervasive spectrum placement of the spontaneous Raman scattering noise. However dual-feeder architectures (i.e., placing the QKD receiver at the OLT-side of the protection feeder fibre) can significantly enhance QKD performance and support the key-exchange over the entire standardized PON length for most PON standards and even in case of two-PON standard presence. Finally, the dual-fibre topology allows the quantum coexistence in access networks where three classical standards share the same infrastructure.

Tb/s transmission matching the requirements of future beyond 5-G mobile systems and high-throughput services in the future sustainable metro/access network is demonstrated. An innovative photonic transmitter module of a bandwidth/bitrate variable transceiver has been designed and developed integrating multiple directly modulated long-wavelength InP vertical cavity surface emitting lasers (VCSELs) in a silicon-photonics platform. Multicarrier direct modulation of each VCSEL combined with dense WDM multiplexing with a modular approach provides flexibility in the transported capacity to match dynamic bandwidth allocation, fine granularity, and scalability according to a pay-as-you-grow scheme. The use of VCSELs directly modulated with discrete multitone (DMT) signals is a sustainable technological solution in terms of cost, power consumption and footprint. Moreover, the adoption of multi core fibers (MCFs) allows to also use the space dimension for multiplexing, perfectly matching the proposed modular approach and coping with the mobile systems increasing throughput in an effective way. Here we experiment the propagation over 129 km, including 21 km of a 7-core MCF, with 57 Gb/s capacity per DMT -modulated VCSEL channel and 25 GHz WDM spacing, demonstrating more than 1.2 Tb/s total throughput thanks to the proposed modular solution.

We consider long-wavelength vertical cavity surface emitting lasers (VCSELs) directly modulated with discrete multitone (DMT) signals as a very promising solution for implementing energy-efficient transmitters for 50G passive optical networks (PONs). Two scenarios based on single sideband and dual sideband modulation are taken into account. Preliminary experimentation with already available short-cavity VCSELs operating in the third window is presented to show the PON performance as a function of the accumulated chromatic dispersion and of the received power. Experimental results are also used to validate a simulation tool, which is further used to provide the performance evaluation of transmitters based on next-generation short cavity VCSELs with higher bandwidth (up to 20 GHz), operating in the O band. Thanks to its water-filling nature, DMT is demonstrated to enable PON flexibility: in fact, considering the statistics regarding a commercially deployed PON, a significant increase of the maximum aggregated capacity is provided, optimizing the PON resource usage with respect to the losses and dispersion impairments.

Multicore fibers (MCFs) are one of the main technological enablers for space-division multiplexing. In principle, MCFs could scale the fiber capacity by a factor equal to the number of cores, but in practice such increase is hindered by transmission impairments due to the inter-core crosstalk between adjacent lit cores. The entity of such crosstalk depends on the number of cores and on their disposition within the fiber cladding, and also on the baud rate and modulation format used for transmission. As first MCF applications are expected over point-to-point systems, in this paper we concentrate on the resource allocation over a single link. Specifically, we study the Baud rate, Modulation format, Core and Spectrum Assignment problem in a multicore flexi-grid link, considering distance-adaptive reaches for different baud rates, modulation formats, and crosstalk impairments. We show that the problem is NP-hard and provide two integer linear programs, as well as heuristic approaches to solve it over large/practical traffic instances. Our problem formulations incorporate modeling of the exact inter-core crosstalk contributions depending on the number of lit neighbor cores. Numerical results are provided in a high-spatial-efficiency 19-core fiber considering different transmission impairment conditions.

In this paper we consider a centralized radio access network (C-RAN) architecture with a fully analog fronthaul link between remote radio heads (RRHs) and baseband units (BBUs) based on the radio over fiber (RoF) paradigm. Mode division multiplexing (MDM) and frequency division multiplexing (FDM) are employed to provide an additional multiplexing signal dimension to meet the huge bandwidth requirements of next generation (5G) wireless mobile systems. The main contribution of the paper is to prove that a smart resource assignment between the radio antennas and the mode/frequency dimensions allows the communication over the RRH-BBU link at rates that are comparable to those achieved by an ideal fronthauling where BBU and RRH are assumed to be co-located, even without any complex and costly optical equalization technique. Validation is on the radio-link capabilities employing multiple antennas to meet the demand for massive MIMO technology.

In case of Differential Quadrature Phase Shift Keying transmission, the diode laser frequency instability affects the quality of the received signal. Free-running transmitters are employed to experimentally evaluate the tolerance of the interferometric DQPSK receiver to this instability. Heterodyne measure of the frequency offset is discussed for different transmitters. Experimental data are related both with BER performance and power penalty. Polarization-dependent delay impairment is isolated as well.

The Resonant Router (ReR) is an integrated optics crossbar matrix based on ring resonators. Depending on the characteristics of the rings the ReR can operate as cross-connect (X-ReR) or as wavelength router (λ-ReR). The analysis of the device, the spectral characteristics and the impact on a high bit rate signal are presented and discussed both on theoretical and experimental basis. The ReR can add flexibility and extend the capabilities of a node of a transparent optical network.