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."