Adding an extra dimension to ROADM designs
U.K. start-up ROADMap Systems, a developer of wavelength-selective switch technology, has completed a second round of funding. The amount is undisclosed but the start-up is believed to have raised several million dollars to date.
Karl HeeksThe company will use the funding to develop a prototype of its two-dimensional (2D) optical beam-steering technique to integrate 24 wavelength-selective switches (WSSes) within a single platform.
The WSS is a key building block used within reconfigurable optical add-drop multiplexers (ROADMs).
The company’s WSS technology uses liquid crystal on silicon (LCOS) technology, the basis of existing WSS designs from the likes of Finisar and Lumentum. However, the start-up has developed a way to steer beams in 2D whereas current WSSes operate in a single dimension only.
The Cambridge-based company’s pre-production prototype will integrate 24,1x12 WSSes within a single package. The platform promises service providers ROADM designs that deliver space, power consumption and operational cost savings as well as systems advantages.
Wavelength-selective switch
A WSS takes wavelength-division multiplexed (WDM) channels from an input fibre and distributes them as required across an array of output fibres. Typical WSS configurations include a 1x9 - a one input fibre port and nine output ports - and a 1x20.
Current WSS designs comprise a diffraction grating, a cylindrical lens and an LCOS panel that is used to deflect the light channels to the required output fibres.
The diffraction grating separates the WDM channels while the cylindrical lens produces an elongated projection of the channels onto the LCOS device. The panel’s liquid crystals are oriented in such a way to direct the projected light channels to the appropriate output fibres. The orientation of the arrays of liquid crystals that perform the various steerings are holograms.
Commercial WSSes use the LCOS panel to steer in one dimension only: left or right. This means the output fibres are arranged in an array and the number of fibres is limited by the total deflection the LCOS can achieve. ROADMap Systems has developed a technique that produces holograms on the LCOS panel that steer light in two dimensions: left and right, up and down and diagonally.
Moreover, the holograms are confined to a small area of the panel, far fewer pixels than the elongated beams of a 1D WSS. Such confinement allows multiple light beams to be steered to the output fibre bundles.
“You use a much smaller area of the LCOS to bend things in 2D,” says Karl Heeks, CEO at ROADMap Systems.
Platform demonstrator
ROADMap System’s key intellectual property is its know-how to create the steering pattern - the hologram - programmed onto the LCOS panel.
The 2D WSS system requires calibration to create the precision holograms. The calibration data is generated during the device’s manufacture and forms the input to an algorithm that creates the holograms needed for the LCOS to steer accurately the traffic to the output fibres.
You use a much smaller area of the LCOS to bend things in 2D
ROADMap Systems has demonstrated its 2D steering technology to service providers, system vendors and optical subsystem players.
Now, the company is working to build the 24, 1x12 WSSs on an optical bench which it expects to complete by the year-end. The start-up is also creating the calibration software used for 2D beam steering as well as a user interface to allow networking staff to set up their required connections.
The first pre-production packaged systems – each one comprising a 4K LCOS panel and 312 fibres - are expected for delivery for trialling in 2019. The start-up is reluctant to give a firm date as it is still exploring design options. For example, ROADMap Systems has an improved lower-loss, more compact fibre coupling design but it has yet to decide whether to incorporate it or its existing design for its platform.
“We are not intending the prototype to go into a system within the network,” says Heeks. “It is more a vehicle to illustrate its capabilities.”
System benefits
The main benefit of ROADMap Systems’ 2D beam-steering WSS architecture is not so much its optical performance; the start-up expects its design to match the optical performance of existing 1D WSSes. Rather, there are architectural benefits besides the obvious integration and cost benefits of putting 24 WSSes in one platform.
The first system advantage is the ability to use the many WSSes to implement ROADMs of several degrees including the ROADM’s add-drop architecture. A two-degree ROADM handles east and west fibre pairs while a three-degree ROADM adds north-facing traffic as well.
A ROADM architecture using 1xN splitters as part of the multicast switch. Source: ROADMap Systems.
To add and drop light-paths, a multicast switch is used (shown in green in the diagram above). The multicast switch can be implemented using optical splitters, however, due to their loss, optical amplifiers are needed to boost the signals, adding to the overall cost and system complexity.
WSSes can be used instead of the splitters as part of the multicast switch architecture such that optical amplification is not needed; the optical loss the WSS stage adds being much lower than the splitters. Removing optical amplification impacts significantly the overall ROADM cost (see diagram below).
A ROADM architecture using 1xN WSSes as part of the multicast switch. Source: ROADMap Systems.
The integrated platform’s large number of WSSes will ease the implementation of the latest generation of ROADMs that are colourless, directionless and contentionless.
A colourless ROADM decouples the wavelength-dependency such that a light-path can be used on any of the network interface ports. Directionless refers to having full flexibility in the routeing of a light-path to any of the ports. Lastly, contentionless means non-blocking, where the same wavelength channel can be accommodated across all the degrees of the ROADM without contention.
And being LCOS-based, ROADMap’s WSSes also support a flexible grid enabling the ROADM to support channels such as coherent transmissions above 200 gigabit-per-second that do not conform to the rigid 50GHz-wide ITU grid spacings.
The second system advantage of the platform is that with its many WSSes, it can route and add-drop wavelengths across both the C and L-bands. However, the company is not planning to implement this feature in its preproduction prototype.
Next steps
ROADMap Systems says it is focussed on producing and testing its pre-production prototype. A further round of investment will be needed to turn the design into a commercial product.
“We believe that such a highly-integrated architecture will offer immediate performance and economic benefits to many teleccom applications,” says Heeks. “It is also well positioned for datacentre – DCI - applications where data needs to be routed between distributed datacentres linked by parallel fibres."
ECOC 2012 summary - Part 2: Finisar
Gazettabyte completes its summary of key optical announcements at the recent ECOC show held in Amsterdam. In Part 2, Finisar's announcements are detailed.
Part 2
"The general thought with system vendors is that the more they can shrink the in-line equipment into a fewer number of slots, the more slots they have open and available for revenue-generating transceiver and transponder cards"
Rafik Ward, Finisar
Finisar showed its board-mounted parallel optics module in use within a technology demonstrator from data storage firm Xyratex, showcased what it claims is the industry's first two-slot reconfigurable optical add/ drop multiplexer (ROADM) design, unveiled its first CFP2 pluggable transceiver and announced its latest WaveShaper products.
The data storage application uses Finisar's vertical-cavity surface-emitting laser (VCSEL)-based board mounted optical assembly. The optical assembly - or optical engine - comprises 24-channels, 12 transmitters and 12 receivers.
The optical engine sits on the board and is used for such applications as chip-to-chip interconnect, optical backplanes, and dense front panels, and supports a variety of protocols. These include PCI Express, Ethernet and Infiniband as well as proprietary schemes. Indeed the only limit is the VCSEL speed. The optical engine is designed to support traffic up to 28 Gigabit-per-second (Gbps) per channel, once 28 Gigabit VCSELs become available. Finisar have already demonstrated working 28Gbps VCSELs.
The ECOC demonstration showed the optical engine in use within Xyratex's demonstrator storage system. "They are carrying traffic between internal controller cards and the traffic being carried is 12-Gig SAS [serial attached SCSI]," says Rafik Ward, vice president of marketing at Finisar.
As well as the optical engine, the demonstration included polymer waveguides from Vario-optics which connect the optical engine to a backplane connector, built by Huber + Suhner, as well as SAS silicon from LSI.
Finisar first showed the waveguide and connector technologies in a demonstration at OFC 2012. "This is an early prototype but it's a very exciting one," says Ward. "It shows all elements of the ecosystem coming together and running in a live system."
Finisar also showcased what it claims is the industry's first two-slot ROADM line card. The line card was part of a Cisco Systems' platform, according to one analyst shown the demonstration.
The company-designed card uses a high port-count wavelength-selective switch (WSS) that enables both add and drop traffic. "We have built transmit and receive into the same line card using a high port-count device," says Ward. Finisar is not detailing the exact WSS used or how the system is implemented but describes it as a flexible spectrum, 2x1x17 port line card.
The advantage of a denser ROADM line card is that it frees up slots in a system vendor's chassis. A slot can be used for either in-line equipment - WSSes and amplifiers - or terminal equipment that host the transceivers and transponders.
"It is like valuable real-estate," says Ward. "The general thought with system vendors is that the more they can shrink the in-line equipment into a fewer number of slots, the more slots they have open and available for revenue-generating transceiver and transponder cards."
The company also detailed its first CFP2 100 Gigabit optical transceiver. The CFP2 uses a single TOSA comprising four distributed feedback (DFB) lasers, a shared thermo-electric cooler and the multiplexer. The CFP2 consumes under 8W by using the DFBs and an integrated transceiver optical sub-assembly (TOSA).
ROADMs: core role, modest return for component players
Next-generation reconfigurable optical add/ drop multiplexers (ROADMs) will perform an important role in simplifying network operation but optical component vendors making the core component - the wavelength-selective switch (WSS) - on which such ROADMs will be based should expect a limited return for their efforts.
"[Component suppliers] are going to be under extreme constraints on pricing and cost"
Sterling Perrin, Heavy Reading
That is one finding from an upcoming report by market research firm, Heavy Reading, entitled: "The Next-Gen ROADM Opportunity: Forecast & Analysis".
"We do see a growth opportunity [for optical component vendors]," says Sterling Perrin, senior analyst and author of the report. “But in terms of massive pools of money becoming available, it's not going to happen; it is a modest growth in spend that will go to next-generation ROADMs."
That is because operators’ capex spending on optical will grow only in single digits annually while system vendors that supply the next-generation ROADMs will compete fiercely, including using discounting, to win this business. "All of this comes crashing down on the component suppliers, such that they are going to be under extreme constraints on pricing and cost," says Perrin. The report will quantify the market opportunity but Heavy Reading will not discuss numbers until the report is published.
Next-generation ROADMs incorporate such features as colourless (wavelength-independence on an input port), directionless (wavelength routing to any port), contentionless (more than one same-wavelength light path accommodated at a port) and flexible spectrum (variable channel width for signal rates above 100 Gigabit-per-second (Gbps)).
Networks using such ROADMs promise to reduce service providers' operational costs. And coupled with the wide deployment of coherent optical transmission technology, next-generation ROADMs are set to finally deliver agile optical networks.
Other of the report’s findings include the fact that operators have been deploying colourless and directionless ROADMs since 2010, even though implementing such features using current 1x9 WSSs are cumbersome and expensive. However, operators wanting these features in their networks have built such systems with existing components. "Probably about 10% of the market was using colourless and directionless functions in 2010," says Perrin.
Service providers are requiring ROADMs to support flexible spectrum even though networks will likely adopt light paths faster than 100Gbps (400Gbps and beyond) in several years' time.
The need to implement a flexible spectrum scheme will force optical component vendors with microelectromechanical system (MEMS) technology to adopt liquid crystal technology – and liquid-crystal-on-silicon (LCoS) in particular - for their WSSs (see Comments). "MEMS WSS technology is great for all the stuff we do today - colourless, directionless and contentionless - but when you move to flexible spectrum it is not capable of doing that function," says Perrin. "The technology they (vendors with MEMS technology) have set their sights on - and which there is pretty much agreement as the right technology for flexible spectrum - is the liquid crystal on silicon." A shift from MEMS to LCoS for next-generation ROADM technology is thus to be expected, he says.
Perrin also highlights how coherent detection technology, now being installed for 100 Gbps optical transmission, can also implement a colourless ROADM by making use of the tunable nature of the coherent receiver. "It knocks out a bunch of WSSs added to the add/ drop," says Perrin. "It is giving a colourless function for free, which is a huge advantage."
Perrin views next-gen ROADMs as a money-saving exercise for the operators, not a money-making one. "This is hitting on the capex as well as the opex piece which is absolutely critical," he says. "You see the charts of the hockey stick of bandwidth growth and flat venue growth; that is what ROADMS hit at."
The Heavy Reading report will be published later this month.
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