WDM-PON

System vendors will be held increasingly responsible for the power consumption of their telecom and datacom platforms. That’s because for each watt the equipment generates, up to six watts is required for cooling. It is a burden that will only get heavier given the relentless growth in network traffic.

"Enterprises are looking for huge capacity at low cost and are increasingly concerned about the overall impact on power consumption"

David Smith, CIP Technologies

No surprise, then, that the European 7th Framework Programme has kicked-off a research project to tackle power consumption. The Colorless and Coolerless Components for Low-Power Optical Networks (C-3PO) project involves six partners that include component specialist CIP Technologies and system vendors ADVA Optical Networking.

CIP is the project’s sole opto-electronics provider while ADVA Optical Networking's role is as system integrator.

“It’s not the power consumption of the optics alone,” says David Smith, CTO of CIP Technologies. “The project is looking at component technology and architectural issues which can reduce overall power consumption.”

The data centre is an obvious culprit, requiring up to 5 megawatts. Power is consumed by IT and networking equipment within the data centre – not a C-3PO project focus – and by optical networking equipment that links the data centre to other sites. “Large enterprises have to transport huge amounts of capacity between data centres, and requirements are growing exponentially,” says Smith. “They [enterprises] are looking for huge capacity at low cost and are increasingly concerned about the overall impact on power consumption.”

One C-3PO goal is to explore how to scale traffic without impacting the data centre’s overall power consumption. Conventional dense wavelength division multiplexing (DWDM) equipment isn’t necessarily the most power-efficient given that DWDM tunable lasers requires their own cooling. “There is the power that goes into cooling the transponder, and to get the heat away you need to multiply again by the power needed for air conditioning,” says Smith.

Another idea gaining attention is operating data centres at higher ambient temperatures to reduce the air conditioning needed. This idea works with chips that have a wide operating temperature but the performance of optics - indium phosphide-based actives - degrade with temperature such that extra cooling is required. As such, power consumption could even be worse, says Smith

A more controversial optical transport idea is changing how line-side transport is done. Adding transceivers directly to IP core routers saves on the overall DWDM equipment deployed. This is not a new idea, says Smith, and an argument against this is it places tunable lasers and their cooling on an IP router which operates at a relatively high ambient temperature. The power reduction sought may not be achieved.

But by adopting a new transceiver design, using coolerless and colourless (reflective) components, operating at a wider temperature range without needing significant cooling is possible. “It is speculative but there is a good commercial argument that this could be effective,” says Smith.

C-3PO will also exploit material systems to extend devices’ temperature range - 75^oC to 85^oC - to eliminate as much cooling as possible. Such material systems expertise is the result of CIP’s involvement in other collaborative projects.

"If the [WDM-PON] technology is deployed on a broad scale - that is millions of user lines – every single watt counts"

Klaus Grobe, ADVA Optical Networking

Indeed a companion project, to be announced soon, will run alongside C-3PO based on what Smith describes as ‘revolutionary new material systems’. These systems will greatly improve the temperature performance of opto-electronics. “C-3PO is not dependent on this [project] but may benefit from it,” he says.

Colourless and coolerless

CIP’s role in the project will be to integrate modulators and arrays of lasers and detectors to make coolerless and colourless optical transmission technology.  CIP has its own hybrid optical integration technology called HyBoard.

“Coolerless is something that will always be aspirational,” says Smith. C-3PO will develop technology to reduce and even eliminate cooling where possible to reduce overall power consumption. “Whether you can get all parts coolerless, that is something to be strived for,” he says.

Colourless implies wavelength independence. For light sources, one way to achieve colourless operation is by using tunable lasers, another is to use reflective optics.

CIP Technologies has been working on reflective optics as part of its work on wavelength division multiplexing, passive optical networks (WDM-PON). Given such reflective optics work for distances up to 100km for optical access, CIP has considered using the technology for metro and enterprise networking applications.

Smith expects the technology to work over 200-300km, at data rates from 10 to 28 Gigabit-per-second (Gbps) per channel. Four 28Gbps channels would enable low-cost 100Gbps DWDM interfaces.

Reflective transmission

CIP’s building-block components used for colourless transmission include a multi-wavelength laser, an arrayed waveguide grating (AWG), reflective modulators and receivers (see diagram).

Reflective DWDM architecture. Source: CIP Technologies

Smith describes the multi-wavelength laser as an integrated component, effectively an array of sources. This is more efficient for longer distances than using a broadband source that is sliced to create particular wavelengths. “Each line is very narrow, pure and controlled,” says Smith.

The laser source is passed through the AWG which feds individual wavelengths to the reflective modulators where they are modulated and passed back through the AWG.  The benefit of using a reflective modulator rather than a pass-through one is a simpler system. If the light source is passed through the modulator, a second AWG is needed to combine all the sources, as well as a second fibre. Single-ended fibre is also simpler to package. 

For data rates of 1 or 2Gbps, the reflective modulator used can be a reflective semiconductor optical amplifier (RSOA). At speeds of 10Gbps and above, the complementary SOA-REAM (reflective electro-absorption modulator) is used; the REAM offers a broader bandwidth while the SOA offers gain.

The benefit of a reflective scheme is that the laser source, made athermal and coolerless, consumes far less power than tunable lasers. “It has to be at least half the cost and we think that is achievable,” says Smith.

Using the example of the IP router, the colourless SFP transceiver – made up of a modulator and detector - would be placed on each line card.  And the multi-wavelength laser source would be fed to each card’s module.

Another part of the project is looking at using arrays of REAMs for WDM-PON. Such an modulator array would be used at the central office optical line terminal (OLT). “Here there are real space and cost savings using arrays of reflective electro-absorption modulators given their low power requirements,” says Smith. “If we can do this with little or no cooling required there will be significant savings compared to a tunable laser solution.”

ADVA Optical Networking points out that with an 80-channel WDM-PON system, there will be a total of 160 wavelengths (see the business case for WDM-PON). “If you consider 80 clients at the OLT being terminated with 80 SFPs, there will be a cost, energy consumption and form-factor overkill,” says Klaus Grobe, senior principal engineer at ADVA Optical Networking. “The only known solution for this is high integration of the transceiver arrays and that is exactly what C-3PO is about.”

The low-power aspect of C-3PO for WDM-PON is also key. “In next-gen access, it is absolutely vital,” says Grobe. “If the technology is deployed on a broad scale - that is millions of user lines – every single watt counts, otherwise you end up with differences in the approaches that go into the megawatts and even gigawatts.”

There is also a benchmarking issue: the WDM-PON OLT will be compared to the XG-PON standard, the next-generation 10Gbps Gigabit passive optical network (GPON) scheme. Since XG-PON will use time-division multiplexing, there will be only one transceiver at the OLT. But this is what a 40- or 80-channel WDM-PON OLT will be compared with.

CIP will also be working closely with 3-CPO partner, IMEC, as part of the design of the low-power ICs to drive the modulators.

Project timescales

The C-3PO project started in June 2010 and will last three years. The total funding of the project is €2.6 million with the European Union contributing €1.99 million.

The project will start by defining system requirements for the WDM-PON and optical transmission designs.

At CIP the project will employ the equivalent of two full-time staff for the project’s duration though Smith estimates that 15 CIP staff will be involved overall.

ADVA Optical Networking plans to use the results of the project – the WDM-PON and possibly the high-speed transmission interfaces - as part of its FSP 3000 WDM platform.

CIP expects that the technology developed as part of 3-CPO will be part of its advanced product offerings.