Trends in ROADM Line Card Development

Finisar is not only the leading manufacturer of LCoS-based Wavelength Selective Switches (WSS) but has also been a supplier of ROADM linecards, based on our range of WSS, for over 5 years. During this time, we have established the capability to supply both the components and sub-assemblies comprising line cards (including WSS, Optical Channel Monitors and Optical Amplifiers, both EDFA and Raman) as well as the line cards themselves1.

The evolution of ROADM line cards exemplifies the impact of the network operators’ needs for reductions in both Opex and Capex costs, size and power consumption whilst also providing greater flexibility and capability to support future network architectures.

These requirements are forcing a rethink of how line cards are designed and manufactured. Gone are the days of simply buying a bunch of optical subsystems from different manufacturers and assembling them on a circuit board with some additional control hardware. Instead, many designs require a complex disaggregation of optical modules to allow them to fit in the increasingly constrained space available2. Together with the need to optimize the relative performance of the different subsystems, this is driving changes in the way line cards are designed and manufactured.

To visualize what is happening, consider how an EDFA, which is a key subsystem on most line cards, can be transformed from an integrated module to a distributed system. In the latter, the pumps can be placed in the optimum location for thermal management, the Erbium fiber and fiber components can be arranged to fit around any other optical hardware, whilst the control electronics can be located wherever there’s room on the main line card circuit board.

Building on this idea of a distributed EDFA, the control electronics can also be designed to share common components (e.g. processors) amongst multiple subsystems which saves cost, circuit board real-estate and, critically, power the disaggregation and sharing of components allows the designer to optimize the packing density of optical functionality on the line card but that, in turn raises the problem that higher component density generally leads to greater thermal load (per unit area). Great care must therefore be taken to correctly model the thermal dissipation and ensure (as far as is possible) an effective layout of electronics and optics to optimize the thermal loading across the line card.

In addition to being able to pack more functionality into a given space by disaggregating the components, the fact that Finisar manufactures all the optical subsystems on a line card allows us to optimize the trade-offs between the different optical functions. We can therefore provide not only a smaller, lower power design, but also one that has improved functionality and hence provides more ‘bang for the buck’ for our customers.

To reduce costs, manufacturing practices must also change. For example, the ‘birds-nest’ of individual fibers, spliced together and placed in a fiber organizer is being replaced by ribbonized fibers where ever possible to simplify the assembly process and reduce costs. Ribbonized connectors like MTP assist in the process, as do subsystems such as multi-port Optical Channel Monitors. However, there is still some way to go before all the optical subsystems in a line card can be connected by fiber ribbon assemblies.

Looking ahead, the increased demand for denser packing is driving a requirement for ‘single-slot’ line cards. Whilst these can provide up to 2x the functional density of traditional double-height line cards, they introduce further pressure on component packing density and hence good thermal management engineering becomes even more important. For example, advanced cooling technologies such as heat-pipes3 may be required to prevent localized ‘hot-spots’ exceeding design temperatures.

The author would like to thank Ian Clarke and Ken Falta for useful discussions and inputs to this post.

References
1http://investor.finisar.com/releasedetail.cfm?ReleaseID=707170
2Clarke, I., “Building the next generation of linecards: the pleasure and pain of integration” Asia Communications and Photonics Conference (ACP), Beijing, November 2013
3See e.g. http://www.electronics-cooling.com/1996/09/heat-pipes-for-electronics-cooling-applications/

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