CHRISTIAN UREMOVIC OPTICAL LINE SYSTEMS
using DWDM filters or colourless combiners before they are connected to the WSS to make more effective use of the WSS ports. Today, ROADMs support flexible-grid transmission and dynamic optimization of any coherent transceiver technology, regardless of channel width. The technology has progressed over the past decades from 1:2 to 1:9 and up to 1:20 port switches. The latest advancements support 1:32-port WSS modules and operation across the Super C-band and Super L-band fibre spectrum. There are also evolutions
will influence performance. Every span, every ROADM type, and every optical channel with its wavelength data rate and modulation would then have a unique power target for an optimal optical signal-to-noise ratio (OSNR). Today, GOSNR operation is fully automated in modern optical line systems. They utilize information on fibre type; measure span lengths dynamically; and utilize amplifier, ROADM, and wavelength information to optimize performance for every wavelength individually. MULTI-RAIL – PARALLELISM TO THE RESCUE Enhancing transmission capacity, especially between data centres, will inevitably require parallelism. Today, some of these connectivity demands exceed 100 Tbps, and we need to light up several fibre pairs along the same
faster in a variety of networks and can participate in comprehensive real-time monitoring and network automation. Amplifier data such as temperature, gain settings, gain tilt, input and output power, and other parameters can be utilized to observe trends, predict failures, improve performance, and qualify networks more precisely. With enhanced real-time monitoring and analysis, we can operate networks at smaller performance margins and improve economics. HOLLOW CORE – CHANGE THE MEDIUM However, as we approach the physical limits on current silica-based fibres, the need for disruptive technologies becomes increasingly important. One promising aspect for extending transmission capacity further is to explore different transmission mediums such as gas or vacuum instead of silica. This would provide benefits such as lower loss and would improve latency by about 30% as light can travel significantly faster in a gas compared to silica. It would also minimize nonlinear effects compared to a silica-based transmission medium, which would directly improve performance and reduce overall network costs. There are ongoing efforts to develop these so-called hollow-core fibres and improve their operations. The first promising deployments have happened, and there will be more to come as their benefits are tremendous.
to a higher-port-count WSS with combined C+L-band spectrum.
Together, Super C+L EDFAs, Super C+L WSSs, and Super C+L transceiver technology enable transmission of over 100 Tbps over a single fibre pair over 200 km and more.
GOSNR – OPTIMISE YOUR LINK CONTROL Network-wide optical layer optimization is crucial for maximizing performance. Maximizing performance enables wavelengths to travel longer distances without costly signal regeneration. To optimize optical layer performance, we need to take several aspects into account. Beyond optimized network design and effective traffic grooming, we also need effective optical power control and wavelength management. Generalized optical signal-to-noise ratio (GOSNR) is a powerful and automated methodology to optimize optical power control. Two primary effects are crucial for optical power control: linear noise and nonlinear noise. Nonlinear noise is fibre-type dependent and can only be roughly estimated. Several studies, including machine-learning techniques, are currently ongoing to derive nonlinear impairments more precisely. Linear noise mainly depends on amplified spontaneous emission (ASE) and is easier to calculate and estimate due to its linear nature. In order to address these effects, we need amplifier information such as gain and noise figures across the entire gain spectrum, and we need the fibre span length. The optimal optical launch power into a fibre is then calculated. Launch power that is too high would be penalized with ASE noise, and launch power that is too low would be penalized with nonlinear effects, which
route. Such transmission deployment is called multi-rail. Optical line systems today enable effective multi-rail deployments by supporting multiple amplifiers within the same chassis and system.
OPEN – MORE CHOICE, FASTER INNOVATION, IMPROVED ECONOMICS Optical line systems must also be designed to be open, or to support any wavelength, including those from third- party devices often referred to as alien wavelengths. Additionally, they should have the flexibility to accommodate both coherent pluggables and embedded technology. This will help operators accelerate innovation in their networks, enable more choice, and improve their economics. Lastly, the line system needs to support modern APIs and streaming telemetry so they can be deployed
BRINGING IT ALL TOGETHER Optical line systems play a critical role in meeting the ever-growing demand for network capacity. By exploring advancements in amplifiers, wavelength-selective switches, dynamic link control, open networking, and multi-rail parallelism, we can see the rich and evolving functionality in modern optical line systems. Expanded spectrum with the Super C- and Super
L-bands further enhances these solutions, and disruptive new
technologies like hollow-core fibre offer additional evolution to further enhance capacity and reduce network costs.
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ISSUE 40 | Q1 2025
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