The technology industry is entering a new era of computing that requires IT systems and cloud computing services to process and analyze huge volumes of Big Data in real time, both within datacenters and particularly between cloud computing services. This requires that data be rapidly moved between system components without congestion.
The enormous growth in both data storage and the demand for high-performance computing is also leading to requirement of much higher bandwidth density for inter-chip communication than ever before (expected to surpass 40 Gbps per interconnect by 2020 ). Traditional electrical interconnects are not up to the challenge largely due to limited bandwidth, electrical cross-talk, and low input/output pin density. Silicon photonics has emerged as a disruptive technology to address data bottlenecks inside of systems and between computing components, enhancing power efficiency, improving response times and delivering faster insights from Big Data.
Silicon photonics uses photons to detect process and transmit information more efficiently than electrical signals, and yet have low manufacturing costs as a result of using conventional silicon-integrated-circuit processes. Silicon photonics refers to the application of photonic systems using silicon as an optical medium. The silicon material used in such photonic systems is designed with sub micrometer precision and is deployed into the microphotonic components. Silicon photonics combines technologies such as complementary metal oxide semiconductor (CMOS), micro-electro-mechanical systems (MEMS) and 3D Stacking.
Silicon photonics allows transfer of large volumes of data at very high speed between computer chips in servers, large datacenters, and supercomputers, overcoming the limitations of congested data traffic and high-cost traditional interconnects. In addition, features such as low environmental footprint, low heating of components, low operating cost, high optical functions integration, high density of interconnects, low error rate and spectral efficiency are adding value to the silicon photonics products.
Katharine Schmidtke, strategic sourcing manager, optical technology at Facebook says, “Silicon photonics can produce integrated designs, with all the required functions placed in one or two chips. Such designs will also be needed in volume, given that a large data centre uses hundred of thousands of optical transceivers, and that requires a high-yielding process. This is a manufacturing model the chip industry excels at, and one that silicon photonics, which uses a CMOS-compatible process, can exploit.”
Schmidtke is upbeat about silicon photonics’ prospects. “Why silicon photonics is attractive is integration; you are reducing the number of components and the bill of materials significantly, and that reduces cost,” she says. “Then there is all the alignment and assembly cost reductions; that is what makes this technology appealing.” The silicon photonics market is expected to reach $1.9 billion by 2022, at a CAGR of 22.1 percent between 2016 and 2022, according to a report by Reportlinker, a market research solution.
Silicon Photonics shall also benefit other application areas such as fibre-to-the-home (FTTH) or fibre-to-the-premises (FTTP), environmental monitoring, biological and chemical sensing, microwave photonic circuits, medical and military applications and astronomy. Eventually, the technology could reach home computers and mobile devices and improve applications from gaming to video streaming.
Military hopes integrated photonic circuits will eventually be the core of new communication devices that work faster, use less energy and can zip data across long distances, said Adele Ratcliff, director of the Defense Department’s manufacturing technology program. Mark Wright, spokesman for the Defense Department, noted how fiber optics are used to send digitized information between computers across the country, a technology that serves as the basis of the internet. Fiber optic cables help telecommunications companies sell high-speed internet to customers. The military could use that technology too, Mark Wright, said. “Despite the heavy interest of the commercial sector in this area, the DoD has planes, ships, and other platforms that need to move information around on them and adopting the solutions developed in this area are certainly of interest,” he said.
Silicon photonics to meet communication needs
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.
“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, said Dr Ségolène Olivier acting as project leader.
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.”
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.
Imec unveils 896 Gbit/s silicon-photonics transceiver
Imec (Leuven, Belgium) has unveiled a 896 Gbit/s silicon photonics transceiver of just a few square millimeters in area, targeting future Tbit/s optical links. The bidirectional transceiver combines dense arrays of 56 Gbit/s germanium-silicon (GeSi) electroabsorption modulators (EAMs) and GeSi waveguide photodetectors with a multicore fiber interface. It comprises arrays of 16 GeSi EAMs and 16 GeSi photodetectors (PDs), implemented with 100 µm channel pitch on a single silicon chip.
The chip cointegrates optical power splitters to feed a single laser source to the transmitter channels, and a dense array of fiber grating couplers to interface with a pitch reducing optical fiber array (PROFA), provided and packaged by Chiral Photonics .
“We obtained clear and wide open eye diagrams at 56 Gbit/s non-return-to-zero on-off keying (NRZ-OOK) data rate, for all EAM and PD channels tested in a loop-back transmission experiment,” says Joris Van Campenhout, director of the Optical I/O R&D program at Imec
100 Gb/sec Transceivers ready to Speed up Cloud and Big Data Applications
According to Intel veep Alexis Bjorlin: Intel Silicon Photonics is now in volume production and shipping in the form of 100G optical transceivers. The products, Intel Silicon Photonics 100G PSM4 (Parallel Single Mode fiber 4-lane) and Intel Silicon Photonics 100G CWDM4 (Coarse Wavelength Division Multiplexing 4-lane) are small form-factor, high speed, and low power consumption products, targeted for use in data communications applications, in particular for switch-to-switch optical interconnects in data centers.
“Today, these products are already being deployed to connect switches to switches in large data centers,” Bjorlin added. “In the future, as bandwidth to the server increases, the optical network will connect servers, displacing the copper interconnects that are increasingly limited in reach as bandwidth goes up.”
IBM has developed the first fully integrated wavelength multiplexed chip that uses four distinct colors of light travelling within an optical fiber, rather than traditional copper wiring, to transmit data in and around a computing system. The transceiver design, utilizes four channels of 25 Gb/sec that can be wavelength multiplexed on-chip to provide 100 Gb/s aggregate bandwidth over a duplex single-mode fiber, thus minimizing the cost of the installed fiber plant within the datacenter. IBM’s breakthrough enables the integration of different optical components side-by-side with electrical circuits on a single silicon chip using sub-100nm semiconductor technology.
“This integration scheme has the potential to massively reduce the cost of applying silicon photonics optical interconnects in computing systems,” Bert Offrein, manager of the photonics group at IBM Research, Zurich, said. “Such systems will be a key for future applications in the field of cloud-computing, big data, analytics and cognitive computing. In addition, it will enable novel architectures requiring high communication bandwidth, as for example in disaggregated systems,” Offrein said.
For datacenter applications with reach < 500m there are several wavelength protocols that have been standardized. The primary ones operate around 1310nm with performance requirements set by various standards such as PSM4, CLR4 and CWDM4. For PSM4 a single laser (typically at 1310nm) is coupled to the photonics IC and split into four light streams each of which is modulated at 25G inside the IC. The output is coupled to 4 optical fibers (total transmission is 4x25G = 100G) and transmitted typically over <500M where the received light per channel is detected and data recovered.
For CLR4 and CWDM4 the transceiver uses four different CWDM (coarse wavelength division multiplexing) lasers centered on 1310nm with each wavelength separated from other by 20nm. The four wavelengths are combined into one fiber, which carries 100G of information. Other implementations use the 1550nm wavelength band to implement the same functionality