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Silicon photonics to meet communication needs

Schematic for a 2.4Tbt/s on board optical transceiver with 24 fibres, each carrying four coarse WDM wavelengths

Data centres and supercomputing installations are challenging data communications requirements. To meet these challenges, the industry is increasingly looking to photonics based solutions.

A recently launched project, funded by the European Union’s Horizon 2020 programme, hopes to develop silicon photonics based transceivers that will meet these requirements. Called COSMICC, the project intends to combine CMOS electronics and silicon photonics with innovative high-throughput fibre attachment techniques. If all goes to plan, its solutions will bring an order of magnitude improvement over current technology, while meeting the cost and data rate requirements of future systems.

The COSMICC project – whose name stands for ‘CMOS solutions for mid-board integrated transceivers with breakthrough connectivity and ultra low cost – brings together 11 partners from five countries, with contributions from the likes of STMicroelectronics, University of St Andrews and the University of Southampton.

Project coordination is in the hands of Grenoble based CEA-Leti, with Dr Ségolène Olivier acting as project leader. “By enhancing an R&D photonic integration platform from STMicroelectronics, the partners in COSMICC aim to demonstrate the transceivers by 2019. We also plan to establish a new value chain that will facilitate rapid adoption of the technologies developed by our members.”

According to Dr Olivier, the use of optical technology in data centres and supercomputing isn’t new. “They are already using this approach, based mainly on VCSEL transceivers running at 10Gbit/s. While there are already some commercial products that support 100Gbit/s communications, we need to prepare for the next generation – for example, devices that can support 400Gbit/s and 1Tbit/s, as well as developing technology that can aggregate data rates beyond 1Tbit/s.” In particular, the COSMICC project is looking to develop technology with a cost/bit that cannot be achieved using the current wavelength division multiplexing approach.

In its roadmap, the COSMICC project is looking to meet a cost target of €0.15/Gbit/s, while consuming just 2pJ/bit. By contrast, today’s technology costs something like €20/Gbit/s, while consuming 35pJ/bit.

Despite anticipated developments in photonics technology, the COSMICC project is planning to use four wavelengths to achieve its target data rates. “We’ll be targeting four wavelengths using coarse wavelength division multiplexing (WDM),” Dr Olivier explained, “and each wavelength will be 20nm apart.”

The project is also looking to use what she called a ‘large number of fibres’. “Until now,” she continued, “we have been limited to using four fibres. We want to increase this to 12 for transmission and a further 12 for reception. However, this will need new packaging techniques to be developed in order to allow the fibres to be attached to the photonic IC.”

But data rates is only one of the challenges which COSMICC is addressing. “There is also the need to reduce power consumption,” Dr Olivier pointed out. “That will be a big challenge because the power requirements of next generation data centres will be stringent.”

One of the major consumers of power is the optical modulator. “We will need to make them more power efficient,” she continued, “and smaller, so they take up less space.”

A further complication is the need for robust components, particularly when it comes to temperature. “We don’t want to have to integrate a temperature controller into the devices because it would be too expensive,” Dr Olivier noted. “So we need to enhance existing technology by introducing silicon nitride – something which will be new to photonics.”

The benefit of silicon nitride is that it is ten times less sensitive to temperature than silicon. “But we will face a challenge when it comes to integrating lasers in order to reduce the global energy consumption of photonics ICs.”

Dr Olivier said the need to integrate more wavelengths also needs to be explored. “We’ll need other materials for that,” she claimed, “so we will have to develop a III-V hybrid on silicon.”

For the initial stages of the project, the partners will need four lasers to generate the discrete wavelengths needed for the coarse WDM design. “We will start the development by integrating two lasers, but if we want to integrate four lasers, we will need to create a different kind of III-V epitaxial stack.”

The COSMICC project's roadmap is targeting high data rates, but also low cost and low energy consumption

Looking to address the various issues, the COSMICC project has brought together a number of key industrial and research partners, with expertise in such aspects as silicon photonics, CMOS electronics, packaging, optical transceivers and data centres, with the ambition to commercialise its work in 2019.

As a first step, the partners will enhance STMicroelectronics’ photonic integration platform; ST will also be providing foundry services to the project, as well as designing the electronics needed to drive photonic ICs. Varioptics will be developing a high throughput package, whilst Finisar will be involved with packaging, as well as development of the initial transceiver module. Once developed, Seagate will test the optical transceiver modules in a data centre environment.

Academic partners will also make significant contributions to the project. The two UK partners – St Andrews and Southampton – will be developing photonic components, including multiplexers and modulators, as well as ultra compact optical modulators based on SiGe.

Southampton is leading Work Package 2, while developing a novel SiGe deposition technique. It is also collaborating on silicon based low power capacitive modulators.

Dr Frederic Gardes, senior lecturer in Southampton’s Optical Research Centre (ORC), said: “We’re developing a multilayer architecture with SiN on the top. But, while SiN has been around for a while, its use on top of active and photonic layers is new and a lot of groups around the world are pushing in this direction.”

ORC is also working with Universite Paris Sud and ST to develop capacitive modulators, which transform electrical signals into the optical domain. “Today, modulators have relatively high power consumption and are large,” Dr Gardes noted. “Capacitive modulators are smaller and use less power, but are harder to make. If we can make them from SiGe, we gain so much more in terms of performance and size. They can be 100 times smaller and 100 times more power efficient.

“Where a standard modulator is probably about 1mm, a silicon based capacitive modulator is about 500µm, but a SiGe version measures 40µm.”

By combining CMOS electronics and silicon photonics with high throughput fibre attachment techniques, the COSMICC project believes it will be able to develop solutions that can scale to meet the future requirements of data centres and supercomputers.

According to Dr Olivier, the goals are ambitious. “We’re targeting our first demonstration in October 2017 and will show that photonics components are capable of carrying data at 50Gbit/s,” she said. Through the use of multiple fibres, this demonstrator will have a capacity of 400Gbit/s, with a target cost of €2.50/Gbit/s and an energy consumption of 8pJ/bit. In parallel, the project will be developing the packaging techniques needed to increase the number of photonic ICs from two to four. A further demonstrator, planned for 2018, will be aiming to show that data rates of 1Tbit/s are possible.

Author
Graham Pitcher

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