Quantum key distribution (QKD) is a secure communication method for sharing symmetric cryptographic keys based on the principles of quantum physics. Its integration into the fiber-optic network infrastructure is important for ensuring privacy in optical communications. Multi-core fibers (MCFs), the likely building blocks of future high-capacity optical networks, offer new opportunities for such integration. Here, we experimentally demonstrate, for the first time, the coexistence of discrete-variable QKD and high-throughput classical communication in the C-band over a field-deployed MCF with industry standard cladding diameter of 125 μm. Specifically, we demonstrate successful secure-key establishment in one core of a 25.2-km uncoupled-core MCF, while simultaneously loading the remaining three cores with full C-band counter-propagating classical traffic at an aggregate net rate of 110.8 Tb/s. By proposing and experimentally validating an improved analytical model for inter-core spontaneous Raman scattering noise, we find that this configuration is optimal for our deployed MCF link as it is immune to four-wave mixing, that becomes relevant when the quantum and classical signals are propagating in the same direction. Our findings make an important step forward in demonstrating the integration of QKD and classical transmission in uncoupled-core multi-core fibers for next-generation optical communication networks.

The optical fiber deployed in the metropolitan infrastructure carrying high-speed WDM traffic data is used also for network surveillance, identifying and localizing the onset of vibrations and dynamic stress events, potentially dangerous for the integrity of the fiber telecom asset. A coherent approach is adopted in a counter-propagating interferometer to develop a sensing system exploiting the ring layout of the metro net- work. With respect to usual dual Mach-Zehnder interferometric sensors, where typically the reference arm is kept isolated, in our arrangement we use as reference arm a fiber inside the same deployed cable, minimizing the impact of the city strong environmental noise. Preliminary assessments of the proposed sensing solution have been experimented in a metro ring of 32 km standard single-mode fiber deployed in the city of Turin, Italy, devoted to telecom applications. The sensor is demonstrated to operate preserving the coexistence with the high-speed WDM telecom traffic. With respect to complex and expensive sensing solutions based on pulsed phase-OTDR techniques, in the present case continuous-wave counter-propagating optical signals are exploited and the spatial resolution in dynamic event localization depends only on the receiver sampling rate: in our implementation, by adopting off-the shelf low-cost commercial 20MS/s sampling boards, 10-meter spatial resolution is experimentally achieved. The proposed sensing solution provides significant added value to the deployed metro fiber infrastructure and the same interferometric approach based on the counter-propagating layout can be easily exploited in metro networks even in case of presence of optical nodes equipped with reconfigurable optical add-drop multiplexers useful for WDM traffic routing.

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.

Few Mode Fibers (FMF) and Space Division Multiplexing (SDM) are an attractive solution to offer high capacity in optical networks. Although transmission along FMF presents several issues mainly due to the cross-talk among modes, the use of Multiple Input Multiple Output (MIMO) coherent receivers permits to limit the impact of such physical impairment. However, the complexity of MIMO is not negligible (especially with a large number of modes) and the spatial modes must cover the same path, thus limiting network flexibility, e.g. routing modes along different paths is not admitted. In this paper we exploit the concept of Mode-Group Division Multiplexing (MGDM) and we investigate the network architecture and provisioning supporting MGDM. Modes are divided in groups: the modes within a group are co-routed and received with a reduced-complexity MIMO receiver, while the different groups can be routed along different paths. Different node architectures supporting MGDM are presented taking in account state-of-the-art components and devices, even commercially available on the market. A quality of transmission (QoT) model is also presented accounting to the inter mode group crosstalk, that cannot be neglected in the reach evaluation. QoT is exploited by a proposed connection provisioning scheme for MGDM. Simulations are carried out in metro/rural network scenarios with different link span lengths. Simulations show high throughput increase while limiting the complexity of receivers.

Transmission based on mode-division multiplexing can enhance node flexibility in metropolitan networks by exploiting mode-group routing. In fact, each mode group, constituted by degenerate spatial modes, can act as a single modal super-channel: since the inter-group crosstalk remains limited or even negligible among different mode groups for sufficiently short distances, typical of urban scenarios, the mode groups can be optically multiplexed and demultiplexed at the network nodes. Moreover, each group can cover an optical path independent from the propagation of the other groups, and be received by means of a so-called partial-MIMO processing, handling just its own degenerate modes, with a significant reduction of the necessary digital resources. In this paper we experimentally assess the capabilities of such a mode-group approach using a few-mode fiber, supporting 5 mode groups, already deployed in the city of L'Aquila, Italy. Transmission throughput versus DSP resources (in terms of number of taps required to achieved a proper GMI) is analyzed, experimenting different modulation formats (at 50 Gbaud), in case of 75-GHz spaced WDM transmission. The large number of spatial modes supported by the few-mode fiber employed in the field-trial allows to explore several combinations of transmitted mode groups in order to better understand how the inter-group cross-talk affects the capacity performance. Almost 2-Tb/s capacity per WDM channel is demonstrated for 6 WDM channels over 6.1-km few-mode fiber reach, thanks to partial MIMO. Finally, multi-hop routing is also performed in a 4-node scenario, in particular taking into account the impact of the optical mode multiplexer/demultiplexer employed in the field trial on the routed throughput per wavelength.

Spatial division multiplexing (SDM) can support the capacity demand increase in short-range optical link, but the communication is affected by the intermodal crosstalk. In order to target a low-cost energy-efficient solution we proposed to exploit direct modulation and direct detection without MIMO post-processing in a mode division multiplexed (MDM) system. We theoretically analyze the frequency distribution of the crosstalk contributions taking into account that directly modulated sources at the same nominal wavelength are actually characterized by a certain frequency shift. Our model demonstrates that this shift originates a non-uniform beating noise in the frequency domain due to the intermodal crosstalk. We proposed to exploit discrete multitone (DMT) modulation and a guard-band in the multicarrier spectrum to cope with the non-uniform crosstalk issue for typical crosstalk values due to present commercial multi-plane light conversion MUX/DEMUX devices and propagation in few few-mode fiber (FMF) kilometers. The model is validated by comparing simulations and experimental results in the case of a 2-mode MDM system. We demonstrate in a pure SDM scenario that with respect to single mode transmission, the capacity can be increased of 2.3 times in a 10-km FMF link with 4-mode exploiting direct detection, limited-bandwidth directly-modulated lasers and commercial mode MUX/DEMUXs.