PAUL MOMTAHAN DWDM LINE SYSTEMS
DWDM: THE TOP FIVE LINE SYSTEM INNOVATIONS
While much of the industry’s attention has focused on coherent optical engine evolution, with embedded engines now able to deliver 800 Gbps and compact pluggables able to deliver 400 Gbps, the DWDM line systems that carry these wavelengths have also been evolving, writes Paul Momtahan , director, Product Solutions Marketing at Infinera.
K ey drivers for this evolution include maximising the capacity-reach of the coherent optical engines, enabling open optical networking, and minimising total cost of ownership (TCO). Line system innovation occurs at the component level: wavelength- selective switch (WSS), amplifier, multicast switch, optical channel monitor (OCM), optical supervisory channel (OSC), optical time-domain reflectometer (OTDR), etc. It also occurs at the systems level: shelf form factor, module form factor, link control software, management interfaces, etc. Considering both these types, here are my top five areas of line system innovation: 1: WSS INNOVATION ROADMs leverage WSS technology and are the primary type of line system for many applications, including metro, long-haul, and submarine line termination equipment (SLTE). While early ROADMs leveraged wavelength blocker and planar lightwave circuit technologies, the main WSS technologies today are microelectromechanical systems mirrors, including digital light processing (DLP), liquid crystal (LC), and liquid crystal on silicon (LCoS). Lower port counts typically leverage lower-cost DLP or LC technology, while higher port counts typically use the
Performance is improving with better cascadeability, enabling more WSSs in the wavelength path, as filter narrowing penalties have been reduced with a squarer passband shape. WSS footprint has shrunk dramatically, especially with the advent of edge-optimised (i.e., 1×4) WSSs. Twin WSSs have shrunk the required footprint for route-and-select ROADMs. Dynamic gain equalisers (DGE) leveraging WSS technology are also enabling extended reach when integrated with in- line amplifiers (ILAs) in long haul networks. In addition to the evolution of the WSS components, the form factor for ROADMs has evolved. The original form factor for ROADMs was individual modules for each component of a ROADM degree, shown in Figure 1. This has advantages in terms of
more expensive LCoS technology. WSSs have also evolved in terms of number of ports, from 1×2 to 1×30+, evolving to even higher port counts (48, 60, etc.) in the future. The amount of C-band spectrum has increased from 3,200 GHz to 4,800 GHz. Recent enhancements include 6,000 GHz in the C-band and 9,600 GHz with C+L-band, which itself is evolving from separate WSSs to a single C+L WSS. Contentioned and contentionless MxN WSSs have emerged to enable scalable colourless-directionless and colourless- directionless-contentionless add/drop. Channel spacing moved from 100 GHz to 50 GHz to flexible grid, first with 12.5 GHz and then 6.25 GHz granularity, with 3.125 GHz granularity becoming a requirement for SLTE applications.
Figure 1: Key components of a ROADM-based line system
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| ISSUE 30 | Q3 2022
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