We review the fundamentals of the recently proposed Generalized Droop model and highlight its use in power efficiency optimization of long-haul low-SNR space-division multiplexed submarine links.

The Generalized Droop Model for Submarine Fiber-Optic Systems

We experimentally investigated a pilot-aided digital signal processing (DSP) chain in combination with high-order geometric constellation shaping to increase the achievable information rates (AIRs) in standard intradyne coherent transmission systems. We show that the AIR of our system at 15 GBd was maximised using geometrically-shaped (GS) 2048 quadrature amplitude modulation (QAM), reaching 18.0 b/4D-symbol in back-to-back transmission and 16.9 b/4D-symbol after transmission through 100 km of a single-mode fibre after subtracting the pilot overhead (OH). This represents the highest-order GS format demonstrated to date, supporting the highest AIR of any standard intradyne system using conventional optics and 8-bit electronics. Detailed characterisation of the DSP, transceiver performance, and transmission modelling has also been carried out to provide insight into sources of impairments and directions for further improvement.

2048-QAM transmission at 15 GBd over 100 km using geometric constellation shaping.

System design issues introduced by different ownership models in undersea fiber-optic transmission systems are discussed, including design elements related to the overall cable, the individual fiber pairs, and the separate optical add-drop multiplexing bands

Design of global submarine networks

The potential to operate third-party terminals over multi-domain transparent optical networks attracts operators and customers to implement optical spectrum as a service (OSaaS). Because infrastructure information cannot always be shared with OSaaS end customers, alternatives to off-line quality of transmission (QoT) estimation tools are required to assess the performance of the spectrum slot to estimate achievable throughput. In this paper, commercially available sliceable coherent transceivers are used to assess the generalized signal-to-noise ratio (GSNR) based QoT of the OSaaS in a live production network for both narrowband and wideband OSaaS configurations. Extended channel probing based on symbol rate variability is combined with spectral sweeping and operation regime detection to characterize OSaaS implementations on 17 links with different underlying infrastructure configurations to maximize capacity and increase service margins in a low-margin operation regime. We achieve 0.05 dB estimation accuracy in the GSNR for wideband spectrum services and 0.32 dB accuracy for narrowband spectrum services. Based on the GSNR profile, spectral misalignment, spectral ripple, and operation regime are detected and service margin improvements are demonstrated. Finally, we discuss the network optimization perspective based on acquired data from channel probing and propose use cases for continuous channel probing in transparent optical networks.

QoT assessment of the optical spectrum as a service in disaggregated network scenarios

Pre-programming Layer-0 optical protection path restoration speeds is provided based on available path margin. Higher layer routers and switches can be made aware of the expected Layer-0 restoration time, and their switch time can be programmed accordingly. The proposed method can provide users an option to program a restoration speed for a specific photonic service on a per restoration path basis. The method can highlight which services will potentially be impacted by the selected restoration speed on that path. The user can proceed with the selected speed for restoring high priority layer-0 services even if that means the fast restoring event can potentially impact other low priority services already in-service on the restoration path.

Methods and apparatus for pre-programming layer-0 service turn up speeds for photonic service provisioning or restoration

This article will discuss the collaboratively formed cross-industry open cables concept for characterizing optical performance of undersea cables with the intent of assessing and understanding their capacity potential. The article proposes definitions of two critical nonlinear and linear performance metrics for open cables: GSNR (Gaussian or generalized signal to noise ratio) and SNRASE (Signal to noise ratio amplified spontaneous emission), including effects such as GAWBS (guided acoustic wave Brillouin scattering) and signal droop. Measurement methodologies for these metrics are proposed, with considerations for limitations and impact of the test conditions and characteristics of the transponders used. Expanded definitions are offered to enable variable symbol rate transponders to be used for measurement, with considerations for scaling of SNR values. Considerations for using these metrics for capacity assessment and applying these techniques to concatenated multi-segment systems are introduced. Recommendations on key parameters for system specification, system characterization, and proposals for SNR-based performance budgeting tables are also discussed as foundational elements to enabling accurate estimation of the capacity potential of a subsea open cable.

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Design, Acceptance and Capacity of Subsea Open Cables

Systems and methods of sharing optical spectrum between a plurality of users of a submarine optical system includes receiving one or more optical signals from the plurality of users of the submarine optical system, wherein each of the plurality of users are assigned a slice of optical spectrum on the submarine optical system; monitoring each of the one or more optical signals to determine compliance with one or more constraints; and adding the one or more optical signals to the submarine optical system if compliant with the one or more constraints.

Automated provisioning and control of shared optical spectrum in submarine optical networks

Submarine cable upgrades have changed markedly with the introduction of coherent transceiver technology. Adaptations taken from terrestrial networking created smart submarine line terminal equipment (SLTE), enabling efficient, remote, capacity additions. The combination of coherent impairment measurement and correction with the flexibility provided through the use of reconfigurable optical add-drop multiplexer (ROADM) devices provides real-time monitoring and correction of impairments and performance. In addition, the smart terminal enables flexible grid technology, which is compatible with existing and future transceiver technologies. These innovations required key developments in models, field trials, design methodologies, and power budget formulation to ensure successful implementation of coherent capacity upgrade installations. Submarine cable owners enjoy much larger capacities than originally envisioned using existing submarine cable assets. In addition, customers enjoy seamless connection to terrestrial networks as well as reduced equipment, software and operations costs. The economic life and competitiveness of submarine cables has been extended with coherent and smart SLTE technology.

Darwin Evans

Papers

Design, Acceptance and Capacity of Subsea Open Cables

Automated provisioning and control of shared optical spectrum in submarine optical networks

Submarine cable upgrades