We report a model to evaluate the performance of multiple quantum key distribution (QKD) channel transmission using spatial division multiplexing (SDM) in multicore (MCF) and few-mode fibers (FMF). The model is then used to analyze the feasibility of QKD transmission in 7-core MCFs for two scenarios of practical interest. First for transmission of only QKD channels, the second for simultaneous transmission of QKD and classical channels. In the first case, standard homogeneous MCFs enable transmission distances per core compatible with transmission parameters (distance and net key rate) very close to those of single core singlemode fibers. For the second case, heterogeneous MCFs must be employed to make this option feasible. IntroductionQuantum key distribution (QKD) provides an intrinsically secure way to distribute such secret keys between remote parties [1][2][3]. The secrecy of the keys distributed by QKD is verifiable [4], since it relies upon quantum mechanical principles featuring resilience against an eavesdropper. Early QKD experiments focused upon the feasibility of the technology, starting from proof-of-principle laboratory experiments. Subsequent developments in the field dramatically improved the security, performance, accessibility and reliability of the QKD technology. Its security can be rigorously proven even when implemented with practical components only, such as attenuated lasers [5]. The secure bit rate has been increased by three orders of magnitude to 1 Mb/s over 50 km fiber thanks to the development of efficient QKD protocol [6] and high-speed single photon detectors [7].A second evolution step has been connected to the integration of QKD systems into telecommunication networks. In a first stage, the effort concentrated in the development of backbone and metropolitan QKD network demonstrators to enable multi-user connectivity in Japan [8], the US [9], Europe [10], and China [11,12]. A nodal network of point-to-point (P2P) QKD links can then be used to relay a global key between any two distant locations in the network [8][9][10][11]. Alternative approaches based on active routing of optical signals were reported [12,13]. A second stage has focused on the integration of QKD systems into access networks. Here, a point-to-multiple-point (P2MP) architecture is more suitable to allow simultaneous access by multiple users rather than resorting to P2P QKD links. For instance, researchers at Toshiba demonstrated a quantum access network (QAN) of this type allowing a high-speed detector to be simultaneously used by up to 64 users [14]. This QAN was designed for resource and cost sharing where the most expensive components, the single photon detectors, were placed in the central location to be shared by multiple users.The full capacity of QKD can be unleashed by incorporating the spatial division multiplexing (SDM) domain on top of the existing wavelength division multiplexing (WDM) layer. SDM is now being considered the route towards capacity upgrade for current core optical communication networks [...

In this paper we provide the theoretical and experimental evaluation of fiber bending and twisting effects on the group delay performance of a homogeneous 7-core fiber. We have experimentally evaluated the differential group delay between the central and outer cores for different curvature radii and twisting conditions, demonstrating that fiber twisting counteracts the degradation introduced by the curvature itself. These findings are generally applicable to time-sensitive application areas such as radio-over-fiber distribution and microwave photonics signal processing in fiber-wireless access networks, as well as highcapacity long-haul digital communications where digital multiple-input multiple-output processing may be required.

Beyond playing a primary role in high-capacity communication networks, multicore optical fibers can bring many advantages to optical and microwave signal processing, as not only space but also chromatic dispersion are introduced as new degrees of freedom. The key lies in developing radically new multicore fibers where the refractive index profile of each individual core is tailored properly to provide parallel dispersion-diversity signal processing with application in a variety of scenarios such as parallel channel equalization, analogue-to-digital conversion, optical computing, pulse generation and shaping, multiparameter fiber sensing, medical imaging, optical coherence tomography, broadband measurement instrumentation, and next-generation fiberwireless communications. Here, we experimentally prove, for the first time to our knowledge, reconfigurable two-dimensional dispersion-managed signal processing performed by a novel dispersiondiversity heterogeneous multicore fiber. The fiber comprises seven different trench-assisted cores featuring a different refractive index profile in terms of both radial geometry and core dopant concentration. As a representative application case, we demonstrate reconfigurable microwave signal filtering with increased compactness as well as performance flexibility and versatility as compared to previous technologies.

We experimentally demonstrate for the first-time to our knowledge distributed radiofrequency signal processing performed by a heterogeneous multicore fibre link. A trench-assisted 7-core fibre, where each core presents a different chromatic dispersion behaviour, is custom-engineered to operate as a 2D sampled true time delay line.

We experimentally demonstrate, for the first-time to our knowledge, optical beamforming for microwave phased array antennas implemented with a heterogeneous multicore fiber link. The multicore fiber has been engineered to act as an optical sampled true time delay that allows to implement radiofrequency signal processing in a distributed way. It comprises 7 trench-assisted cores where each core is fabricated with different dimensions and core dopant concentration, as to feature a different group delay and chromatic dispersion behavior. We emulated different radio beamsteering scenarios where the beam-pointing angle is modified by tuning the optical wavelength in a 20-nm range, while squint-beam effects are avoided.