Michela Svaluto Moreolo

The continuous growth of data traffic and emerging 6G services will drive the evolution of current optical transport networks to support higher capacity and enhance scalability and programmability. Key innovative technologies, including multiband over spatial division multiplexing (MBoSDM) and software-defined optical networking (SDN), will enable the efficient increase of spectral and spatial resources and support dynamic and automated network operation. Hence, advanced transmission and switching solutions should be investigated that are capable of fully exploiting these technologies toward enabling dynamic, adaptive, and high-capacity future optical networks. In this paper, a next-generation MBoSDM scenario including an MB(oSDM) sliceable bandwidth/bit-rate variable transceiver (S-BVT) and an MBoSDM node prototype is experimentally validated in the ADRENALINE testbed and integrated within an SDN control plane considering different network use cases. Full programmability of the integrated solution is achieved through dedicated SDN agents and a unified SDN controller, demonstrating the provisioning of MB flex-grid and SDM connections. The proposed transmission and switching approaches significantly enhance network capacity, flexibility, and reconfigurability, offering a scalable pathway for future 6G optical transport infrastructures.

Optical transport networks are evolving toward unprecedented capacity, sustainability, and security requirements. We present an SDN-enabled architecture for integrating continuous variable quantum key distribution (CV-QKD) into programmable optical networks, adopting sliceable bandwidth/bitrate variable transceivers (S-BVTs) and/or pluggable modules. The proposed solution combines a TeraFlowSDN-based SD-QKD network controller with a reconfigurable CV-QKD system through interfaces compliant with ETSI GS QKD 015. Experimental validation demonstrates real-time monitoring and dynamic wavelength (re)configuration, facilitating coexistence. Alternative dense wavelength-division multiplexing (DWDM) co-propagation scenarios are presented, and flexible channel allocation strategies are analyzed and discussed. Simulation results further evaluate evolutionary scenarios involving coexistence with 400G pluggable modules at 0 dBm launch power. The analysis identifies the maximum number of supported classical channels (up to 20 over a 20 km fiber link) and suitable spectral allocation in accordance with the experimental results. Furthermore, we compare the coexistence performance of CV-QKD adopting discrete components and an early version of a photonic integrated circuit (PIC) receiver, supporting half the number of 400G channels. Performance enhancement can be envisioned with further PIC design improvement and optimization. The obtained results confirm the viability of SDN-controlled CV-QKD as an enabler for quantum secure operation in future optical transport networks and a promising evolutionary path toward the adoption of this technology.

Future optical networks, envisioning the support of 6G services and related demanding requirements, should provide ultra-high-capacity and reliable connectivity, ensuring sustainability and security. Quantum key distribution (QKD) is a technology to address the limitations of classical cryptography, enabling quantum secure communications. Continuous variable QKD (CV-QKD) offers potential cost savings and enhanced compatibility with classical systems. This facilitates the integration within the network infrastructure, particularly in synergy with software-defined networking (SDN), further promoting interoperability and resource sharing/saving. For the adoption and practical implementation of CV-QKD, it is relevant to consider composable security and a finite number of symbols employed in the protocol (block size). In this work, we present the comparison of two models, to provide a figure of the required block size. We also assess the results using a simulation software. Furthermore, we explore a coexistence scenario considering a flexible grid, to exploit the CV-QKD capability of arbitrarily tuning the operating wavelength. This is a relevant feature enabling flexible allocation of the quantum channel to mitigate impairment and saving spectral resources. Considering realistic parameters aligned to commercially available CV-QKD systems and finite block size, we show an improvement with respect to our previous results. A minimum quantum-classical channel spacing of 112.5 GHz is required in a coexistence scenario with 8×200G transceivers and 9 dBm of total power over a 15 km link. In case of 10 dBm of total power, 125 GHz quantum-classical channel spacing is required to generate keys for practical implementation over the analyzed links. Finally, we discuss SDN aspects relevant for dynamic quantum channel allocation and flexible network management, enabling coexistence and efficient resource sharing, facilitating QKD technology integration in deployed networks. The presented results and envisioned potentialities of CV-QKD for practical implementation in optical networks pave the way for its adoption in the network infrastructure toward future quantum secure communications.

In this paper, we explored various physical-layer parameters affecting the coexistence of continuous-variable quantum key distribution (CV-QKD) and classical channels (CCs) within dense wavelength division multiplexing (DWDM) in the C-band. These parameters include the number of transmitted quantum symbols, power levels of CCs, quantum channel (QC) allocation within the C-band, power distribution among CCs, channel spacing between CCs and the QC, and CC power ranges suitable for both CCs and the QC. These insights provide valuable guidance for a software-defined networking (SDN) control system to manage CV-QKD and CC coexistence effectively. We observed that, for short distances and lower key rates, the number of quantum symbols exchanged does not need to reach the asymptotic convergence limit. However, achieving the asymptotic regime on a 10 km link requires transmitting approximately 1×10¹³ quantum symbols. CC power significantly impacts CV-QKD performance. For example, in a DWDM setup with 37 dummy CCs on a 100 GHz grid, CV-QKD can support up to 11 dBm total launch power over a 10 km link when the QC is positioned at 193.2 THz and a 200 GHz channel spacing between the QC and CCs is maintained. Strategic placement of the QC within the C-band also enhances performance. Placing the QC at higher frequency channels relative to CCs improves both the secret key fraction and the maximum supported power of CCs. For instance, placing the QC at 195.9 THz provides better performance than positioning it at 193.2 or 192.2 THz. However, when the QC is placed at 192.2 THz, the maximum supported CC power reduces to 9.8 dBm. Additionally, the relationship between power distribution and power concentration is complex and requires careful consideration. It depends on the number of active CCs, the power of CCs, and their spectral placement relative to the QC. We also analyzed the case of 8×200G modulated CCs and found that placing them 100 GHz from the QC introduces nonlinear noise but still allows a positive secret key fraction. Increasing the spacing to 200 GHz further improves CV-QKD performance. Joint performance analysis revealed that, at a 100 GHz spacing and a 30 dB optical signal-to-noise ratio for CCs, a launch power range of −16.96 to 9.03 dBm supports both error-free CC recovery and positive key rate generation over a 10 km link. This comprehensive analysis highlights the interplay between CV-QKD and CCs, offering practical strategies for optimizing their coexistence in DWDM systems through an SDN control system.

To meet the increasing demand for higher data rates and energy-efficient optical communication networks, this study investigates the unique characteristics of directly modulated (DM) vertical-cavity surface-emitting laser (VCSEL)-based bandwidth/bitrate variable transceivers (BVTs), identifying the critical impact of chirp on system performance. The core contribution of this paper lies in the introduction and exploration of trained Hammerstein-Wiener system model as a novel approach for the chirp compensation in DM VCSEL-based BVTs by considering tailored experimental adaptive digital signal processing blocks in the numerical simulations.With this approach, the system performance is evaluated with direct and coherent reception up to 80 km standard single mode fiber. The DM VCSEL-based BVT with modulation bandwidth of 20 GHz provides an energy-efficient solution with required optical signal-to-noise ratio values between 32.6 and 24 dB for the transmission of double-sideband and single-sideband signals over 40 and 80 km with achieved capacities of 50 and 45 Gbps at 4.62·10 ^-3 bit error rate with 7% overhead hard decision-forward error correction limit.

This Special Issue contains a collection of twelve papers on the impact of photonic technologies on future optical networks, including extensions of selected works presented at the SBMO/IEEE MTT-S International Microwave and Optoelectronics Conference (IMOC) held on 5–9 November 2023 in Castelldefels, Spain. We present a brief introduction followed by an overview of the papers.

This Special Issue contains a collection of twelve papers on the impact of photonic technologies on future optical networks, including extensions of selected works presented at the SBMO/IEEE MTT-S International Microwave and Optoelectronics Conference (IMOC) held on 5–9 November 2023 in Castelldefels, Spain. We present a brief introduction followed by an overview of the papers.

Innovative transceiver and switching approaches should be explored with special focus on flexibility, energy efficiency, sustainability, and interoperability to be adopted on next-generation 6G optical networks driven by the diverse landscape of emerging applications and services and increasing traffic demand. In this regard, multiband (MB) and spatial division multiplexing (SDM) technologies arise as promising technologies for providing suitable network capacity scaling while fulfilling the stringent requirements of the incoming 6G era. In this paper, innovative MB over SDM (MBoSDM) switching node and sliceable bandwidth/bit rate variable transceiver (S-BVT) architectures with enhanced capabilities and features are proposed and experimentally validated. Different network scenarios have been identified and assessed, enabling up to 180.9 Gb/s S+C+L transmission in back-to-back (B2B) configuration. A MBoSDM scenario including both transceiver and switching solutions is demonstrated, including a 19-core multi-core fiber (MCF) of 25.4 km. Thanks to the transceiver modular and scalable approach, higher capacities can be envisioned by enabling multiple slices working in the different bands beyond the C-band. A power efficiency analysis of the proposed transceiver is also presented, including a pathway towards the integration with a software defined networking (SDN) control plane assisted by energy-aware artificial intelligence (AI)/machine learning (ML) trained models.

The emergence and consolidation of increasingly programmable optical devices such as transceivers, amplifiers, multiplexers, or ROADMs—which allow their remote configuration and control by adopting software-defined networking principles such as model-driven development—is enabling the evolution toward gradually more autonomous networks. Such networks leverage device programmability and are able to adapt and react to traffic and network condition changes, e.g., changing modes of operation or reconfiguring the network state, paving the way for the increased adoption of AI/ML models in support of enhanced network operation. In this paper, after a short review of some key elements in the control and orchestration systems of optical networks in support of autonomous networking, we present in detail a proof-of-concept validation of autonomous, closed-loop dynamic adaptation of transceiver operational modes. This includes (i) the design and development of an SDN agent of a multi-band sliceable bandwidth variable transceiver, based on extended OpenConfig terminal device data models; (ii) an SDN controller that performs discovery and management of transceivers’ operational modes and maps to transport API (TAPI) profiles enabling efficient physical layer impairment-aware path computation; (iii) a dedicated externalized path computation element/digital twin that performs adaptation recommendations; and (iv) an MQTT-based telemetry platform for publisher/subscriber based state synchronization between the control plane functional entities to avoid systematic polling.