Optical Connections Magazine Autumn 2023

SERGE MELLE SUPER COHERENT OPTICS

SIXTH-GENERATION SUPER-COHERENT OPTICS: LOWERING NETWORK TCO Coherent optics provide the workhorse to enable optical transport networks to transmit data at ever-increasing speeds, powered by electronic digital signal processors (DSPs) that encode data streams into high-speed optical signals. New advances and capabilities are enabled by Moore’s Law and new silicon node geometries, allowing higher DSP speeds, lower power consumption, and advanced features. Serge Melle , Leader, Optical Networks Marketing, Nokia looks to the future. T he optical communications industry will soon see the introduction of sixth- generation coherent optics, operating at capacity with fewer coherent interfaces to continue reducing network TCO. Reducing OpEx involves various metrics, including deployment and operational costs, space needed, and power consumed. The math behind achieving this is simple: deploy fewer optics and implementation penalties and maximise performance. The DSP also implements advanced algorithms such as probabilistic constellation shaping (PCS) co-optimised with high-gain forward error correction (FEC) to eke out all the remaining performance just shy of the physical

speeds of 1.2Tbps or more, over a single wavelength. These will provide the tools needed by optical network operators to efficiently scale their networks to support unrelenting traffic growth, provide new benchmarks in optical performance to transport new 800 Gigabit Ethernet (800GE) services over long distances and enable sustainable network evolution by reducing network power consumption by up to 60%. Underlying the need for scale, performance and sustainability provided by sixth-generation coherent optics are the challenges faced by optical network operators today, and their need to lower the total cost of ownership (TCO) in the face of continued exponential growth of network bandwidth demands; typically ranging from 35-40% YoY. This begs the question of what metrics define TCO, and how these can be reduced with new technologies. Simply speaking, TCO is primarily defined by capital costs for new equipment (CapEx) and the ongoing operating cost of that equipment (OpEx). CapEx is mainly influenced by the number of coherent optics needed for a given amount of capacity deployed; often described as “cost per bit.” Previously, advances in coherent modem algorithms and modulation order helped increase the data-carrying capacity of coherent optics faster than the increases in the operating speed of their DSPs. However, further increases in total fibre capacity are limited by the Shannon Limit, and spectral efficiency (SE), or how many bits can be sent through a given amount of fibre spectrum, has stalled at approximately 8 bits per second per Hertz (bps/Hz) with little room for further meaningful increases. New innovations are therefore needed to allow network operators to add more

fewer technicians are subsequently needed for initial installation, ongoing maintenance and supervision. Likewise, higher-density systems enable more capacity to be deployed in a rack, reducing space costs. The most important is power consumption. Specifically, reducing the power per bit of coherent optics delivers bottom-line benefits by reducing ever-increasing electricity costs not just for powering the optics, but also for the building cooling systems where they are deployed. This has made network power efficiency a key part of network operators’ ESG and sustainability strategies. The need for ever-greater network bandwidth is also driving the evolution of IP networks to faster router interface speeds. With 400 Gigabit Ethernet interfaces now widely deployed, and network operators starting to migrate to 800GE router port speeds, network operators require efficient means to transport high-speed services across all applications including metro, long-haul and subsea links. HOW IS COHERENT TECHNOLOGY CONTINUING TO EVOLVE? The latest generation of coherent optics will help optical network operators address these challenges with multiple technological innovations. Optimised to enable the maximum capacity and the longest reach for challenging applications, they are also sometimes termed “super- coherent” optics due to their performance. The latest super-coherent optics leverage 5nm silicon ASIC technology to increase DSP operating speeds to 130Gbaud or more, while closely integrating the DSP with silicon photonics into a multi-chip module (MCM) to reduce

maximum of the Shannon Limit. Together, these enable super-coherent optics to operate at 1.2Tbps per wavelength in 150GHz WDM channel spacings, or at higher speeds using more spectrum. Beyond simply pushing the envelope on capacity-reach performance, the latest generation of coherent optics also adds interesting new features, such as continuous baud rate adjustment, which enables the DSP to transmit the greatest number of bits in the least amount of spectrum across any link, no matter the distance. This is especially valuable in subsea or long-haul cables operated in a “gridless” configuration and allows the greatest total capacity to be transmitted over scarce fibre resources. Another useful feature is the ability to obtain rapid telemetry information from the DSP to provide real-time data on the fibre’s transmission characteristics and on the signals carrying the data. This provides detailed performance and localisation information on developing network faults and reduces the need for separate test equipment such as OTDRs. Combined with intelligent network automation software, network operators can more quickly, easily and proactively identify network faults, and initiate needed corrective actions. ULTIMATE CAPACITY FOR METRO DCI One common application for coherent optics is for optical data center interconnection (DCI), enabling Terabits of data to be exchanged between large data centers, peering points and internet exchanges. A key challenge faced by data center operators is maximising the capacity of their DCI connections in the face of surging demand, which is expected

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| ISSUE 34 | Q3 2023

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