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In order to effectively keep pace with the global IP traffic growth forecasted in the years to come, Flex-Grid over Multi-Core Fiber (MCF) networks can bring superior spectrum utilization flexibility, as well as bandwidth scalability far beyond the non-linear Shannon's limit. In such a network scenario, however, full node switching reconfigurability will require an enormous node complexity, pushing the limits of current optical device technologies at expenses of prohibitive capital expenditures. Therefore, cost-effective node solutions will most probably be the key enablers of Flex-Grid/MCF networks, at least in the shortand mid-term future. In this context, this paper proposes a cost-effective Reconfigurable Optical Add/Drop Multiplexer (ROADM) architecture for Flex-Grid/MCF networks, called CCC-ROADM, which reduces technological requirements (and associated costs) in exchange of demanding core continuity along the end-to-end communication. To assess the performance of the proposed CCC-ROADM in comparison with a fully-flexible ROADM (i.e., a Fully Non-Blocking ROADM, called FNB-ROADM in this work) in large-scale network scenarios, a novel lightweight heuristic to solve the route, modulation, core and spectrum assignment (RMCSA) problem in Flex-Grid/MCF networks is presented in this work, whose goodness is successfully validated against optimal ILP formulations previously proposed for the same goal. The obtained numerical results in a significant number of representative network topologies with different MCF configurations of 7, 12 and 19 cores show almost identical network performance in terms of maximum network throughput when deploying CCC-ROADMs vs. FNB-ROADMs, while decreasing network capital expenditures to a large extent.

Conventional optical coherent receivers capture the full electrical field, including amplitude and phase, of a signal waveform by measuring its interference against a stable continuous-wave local oscillator (LO). In optical coherent communications, powerful digital signal processing (DSP) techniques operating on the full electrical field can effectively undo transmission impairments such as chromatic dispersion (CD), and polarization mode dispersion (PMD). Simpler direct detection techniques do not have access to the full electrical field and therefore lack the ability to compensate for these impairments. We present a full-field measurement technique using only direct detection that does not require any beating with a strong carrier LO. Rather, phase retrieval algorithms based on alternating projections that makes use of dispersive elements are discussed, allowing to recover the optical phase from intensityonly measurements. In this demonstration, the phase retrieval algorithm is a modified GerchbergSaxton (GS) algorithm that achieves a simulated optical signal-to-noise ratio (OSNR) penalty of less than 4 dB compared to theory at a bit-error rate of 2×10 −2 . Based on the proposed phase retrieval scheme, we experimentally demonstrate signal detection and subsequent standard 2×2 multiple-input-multiple-output (MIMO) equalization of a polarization-multiplexed 30-Gbaud QPSK transmitted over a 520-km standard single-mode fiber (SMF) span.

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Space Division Multiplexing (SDM) is a key technology to cope with the bandwidth limitations of single mode fibers. Multi-Core Fibers (MCFs) are considered as a promising candidate technology to implement SDM, due to their low inter-core crosstalk (ICXT), experimentally proven in laboratory prototypes. Among the different channel allocation options making use of the newly enabled space dimension, the so-called spatial super-channel (Spa-SCh) is the most likely solution to be implemented, given the inherent cost reduction of the joint-switching operation (i.e., jointly switching a spectrum portion in all MCF cores at once). This work targets the cost-effective Spa-SCh allocation over MCF-enabled Flex-Grid optical core networks. To this goal, state-of-the-art 22-core MCFs are assumed, although the proposed solutions are applicable to any MCF type. In particular, we propose and evaluate a partial-core assignment as a cost-effective strategy to improve spectrum utilization and save Capital Expenditure (CapEx) costs by minimizing the number of optical transceivers used per Spa-SCh. Numerical results reveal that reductions up to 44% and 33% in the number of active transceivers in the network can be obtained in national- and continental-wide backbone networks, respectively, without affecting the network Grade-of-Service (GoS), measured in terms of Bandwidth Blocking Probability (BBP). To evaluate the impact of the ICXT, we also compare the performance of the MCF scenarios under study against equivalent Multi-Fiber (MF) ones. From the obtained results, ICXT in MCF scenarios requires the utilization of less efficient modulation formats, which reduces the admissible offered network load by up to 17% for a 1% BBP target. Furthermore, this lower spectral efficiency also demands an increase of the symbol rate per sub-channel up to a 26%, a key indicator of the modulator electronic complexity.Peer ReviewedPostprint (author's final draft

In order to transition multi-core fiber (MCF) technologies from their research state to volume production, key design specifications have to be broadly agreed upon, first and foremost an acceptable level of inter-core crosstalk per unit length. Against common belief, we show that MCF crosstalk requirements per unit length are fairly independent of transmission distance in the context of modern coherent optical communication systems. As a consequence, a single value for the tolerable inter-core crosstalk per unit length can be used to specify MCFs, valid from metropolitan (100 km) to trans-pacific (10,000 km) deployment scenarios. The notion of a universal inter-core crosstalk specification allows for application-independent MCF designs (including a distance-independent optimum core density) and will facilitate the standardization and volume manufacturing of MCF.

In this paper, we propose a novel methodology for estimating the worst-case spectral efficiency (b/s/Hz) of end-to-end paths across a Multi-Core Fiber (MCF)-enabled Flex-Grid optical network, accounting for physical link and intra-node noise, together with the inter-core crosstalk impairment appearing during the transmission of optical signals over MCFs. In particular, we employ the proposed methodology to quantify the benefits of Probabilistic Constellation Shaping (PCS) versus traditional polarization-multiplexed modulation formats in two reference backbone networks. From the obtained results, PCS yields admissible offered load gains close to 13% under 1% bandwidth blocking probability.

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