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.
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.
for more information on silicon photonics in communications: http://idstch.com/home5/international-defence-security-and-technology/technology/photonics/silicon-photonics-meet-needs-optical-fiber-space-communications-networking/
Recent advancements overcoming challenges
Recent advancements in Silicon Photonics enable tighter monolithic integration of many optical functions within a single device is leading to higher yield, high reliability, and low costs common in the electronics industry. Chips with onboard photonics together with novel packaging schemes to connect chips with optical fibers and standards to ensure interoperability is making accessible a large range of applications from fiber optics based communication systems to intra-chip communication or as optical interconnects.
Germanium on silicon processes are being developed has many advantages to enable higher performance designs that can be better incorporated into integrated photonic circuits (IPC). However, the growth of germanium for photodetection and III–V materials for light generation is technologically challenging on a silicon substrate due to mismatched lattice constants and thermal expansion coefficients.
The ultimate physical manifestation of the silicon photonic device would be as part of an optoelectronic integrated circuit (OEIC) formed monolithically in silicon, combining the photonic functionality and the electronic intelligence in seamless integration. This requires realization of integrated photonic circuits that combine various silicon photonic devices with excellent properties, such as laser sources, modulators, low-loss waveguides, wavelength filters, optical receivers and photonic switches.
One solution to this problem is to grow materials on a compatible substrate and then transfer them onto silicon. Two-dimensional (2D) materials are a class of crystals that naturally lend themselves to this
type of transfer process. Because these materials are covalently bonded in-plane and held together out-of-plane by van der Waals forces, individual atomic planes can be mechanically separated from the bulk crystal and placed onto arbitrary substrates. These materials have many unique electrical and optical properties and can be transferred to an arbitrary substrate without lattice matching requirements. Owing to strong quantum confinement out-of-plane, 2D materials have many unique properties that are uncommon in their 3D counterparts, which make them particularly attractive for optoelectronic applications. Some of the promising two-dimensional materials, graphene, black phosphorus and transition metal dichalcogenides.
MIT introduce nonlinearities in silicon photonics which will enable new class of complex devices
The telecom devices like modulators depend on second-order nonlinearities for complex signal modulations. Now MIT researchers present a practical way to introduce second-order nonlinearities into silicon photonics that would make optical signal processing more efficient and reliable .
They also report prototypes of two different silicon devices that exploit those nonlinearities: a modulator, which encodes data onto an optical beam, and a frequency doubler, a component vital to the development of lasers that can be precisely tuned to a range of different frequencies. Frequency doublers can be used to build extraordinarily precise on-chip optical clocks, optical amplifiers, and sources of terahertz radiation, which has promising security applications.
We now have the ability to have a second-order nonlinearity in silicon, and this is the first real demonstration of that,” says Michael Watts, an associate professor of electrical engineering and computer science at MIT and senior author on the new paper.
“Now you can build a phase modulator that is not dependent on the free-carrier effect in silicon. The benefit there is that the free-carrier effect in silicon always has a phase and amplitude coupling. So whenever you change the carrier concentration, you’re changing both the phase and the amplitude of the wave that’s passing through it. With second-order nonlinearity, you break that coupling, so you can have a pure phase modulator. That’s important for a lot of applications. Certainly in the communications realm that’s important.
Today, the design kits include standard cells for waveguides, couplers, 10G and 25G Mach Zehnder modulators, electro-absorption modulator, ring modulator, spot size converters, input output grating coupler, photodetectors and more.
Silicon photonics offers a potential breakthrough in optical interconnections that provide bandwidth and power performance, to remove communication bottlenecks within integrated circuit chips that incorporate over a billion transistors, and between integrated circuits in single boards that provide multi-teraflop (1012) computing capacity.
That trend is optical interconnection of components, now moving from systems to boards to chip packages to chips themselves, says Lionel Kimerling, the Thomas Lord Professor in Materials Science and Engineering and director of the MIT Microphotonics Center. “There are significant challenges for each one of those steps. Cost, bandwidth density, and power efficiency are the big three, and cost is the one that’s really controlling the entry of photonics into the system.
Waveguides are components used to route light signals on a chip. The high refractive index of silicon enables the high confinement of light in the waveguides and leads to the reduction of the footprint of the silicon-based photonic devices. Silicon-on-insulator processes, which are now widely available for CMOS, can easily create a low-loss waveguide by patterning the same semiconductor layer used for transistors and then covering it with insulator.
However, the high thermo-optic (TO) coefficient of silicon which is around 1.86 × 10−4/K and the high confinement of light in the waveguide core leads to high temperature sensitivity of the silicon photonic devices. Researchers from University of California, Santa Barbara (UCSB) and the California Institute of Technology (CalTech) came up with new microchip-scale, integrated waveguides for photonic delay employing silicon processing. Such photonic delays are useful in military applications ranging from small navigation sensors to wideband phased array radar and communication antennas, according to DARPA
Rajesh Menon and other Engineers at the University of Utah (Salt Lake City, UT) have developed an ultracompact beamsplitter—measuring only 2.4 by 2.4 microns or one-fiftieth the width of a human hair –for dividing light waves into two separate channels of information.
In 2015, researchers at the Harvard John A. Paulson School of Engineering and Applied Sciences (SEAS) developed the first on-chip metamaterial with a refractive index of zero, meaning that the phase of light could be stretched infinitely long. The metamaterial represented a new method to manipulate light and was an important step forward for integrated photonic circuits, which use light rather than electrons to perform a wide variety of functions.
Now, SEAS researchers have pushed that technology further – developing a zero-index waveguide compatible with current silicon photonic technologies. In doing so, the team observed a physical phenomenon that is usually unobservable — a standing wave of light.
When the refractive index is reduced to zero the light no longer behaves as a moving wave, traveling through space in a series of crests and troughs, otherwise known as phases. Instead, the wave is stretched infinitely long, creating a constant phase. The phase oscillates only as a variable of time, not space.
This is exciting for integrated photonics because most optical devices use interactions between two or more waves, which need to propagate in sync as they move through the circuit. If the wavelength is infinitely long, matching the phase of the wavelengths of light isn’t an issue, since the optical fields are the same everywhere.
“This adds an important tool to the silicon photonics toolbox,” said Camayd-Muñoz. “There’s exotic physics in the zero-index regime, and now we’re bringing that to integrated photonics. That’s an important step, because it means we can plug directly into conventional optical devices, and find real uses for zero-index phenomena. In the future, quantum computers may be based on networks of excited atoms that communicate via photons. The interaction range of the atoms is roughly equal to the wavelength of light. By making the wavelength large, we can enable long-range interactions to scale up quantum devices.”
The innovations in CMOS processing by which alloys of silicon and germanium can be integrated without any harm to transistors has led to integration of detectors on silicon. These alloys can absorb the light signal that passes freely through silicon, liberating electrons that can be detected electrically.
Ge photodetectors offer an enormous responsivity to laser wavelengths near 1.55μm at high frequencies to 40GHz, and they can be easily fabricated as part of a planar silicon processing schedule. At the same time, germanium has enormous potential for enabling 1.55 micron lasers on silicon and for enhancing the performance of silicon modulators.
Optical Modulators in Silicon
Optical modulators are devices that detect electrical signals and modulate a light beam that propagates either in free space or in an optical waveguide. This device can alter different beam parameters and therefore optical modulators can be categorized as amplitude, phase, or polarisation modulators. Modulators have been improved dramatically in recent years. Most notably the bandwidth has increased from the MHz to the multi GHz regime in little more than half a decade.
Ideally, optical modulators are required that have high modulation speed, large bandwidth, and small footprint, as well as low loss and ultra-low power consumption. They also need to be CMOS compatible. For any interconnect solution, optical modulation is one of the main functionalities required in a photonic circuit.
Existing silicon modulators utilize half p-type and half n-type doped silicon, meaning they have had impurities added to them through a standard process used in transistor manufacturing. The MIT researchers’ device is similar, except that the center of the modulator — including the waveguide that runs along its top — is undoped. When a voltage is applied, the free carriers don’t collect in the center of the device; instead, they build up at the boundary between the n-type silicon and the undoped silicon.
“Silicon has had a huge renaissance within the optical communication space for a variety of applications,” says Jason Orcutt, a researcher in the Physical Sciences Department at IBM’s Thomas J. Watson Research Center. “However, there are still remaining application spaces — from microwave photonics to quantum optics — where the lack of second-order nonlinear effects in silicon has prevented progress. This is an important step towards addressing a wider range of applications within the mature silicon-photonics platforms around the world.”
Russian scientists developed the world’s fastest nanoscale photonics switch
International team of researchers from Lomonosov Moscow State University and the Australian National University in Canberra created an ultrafast all-optical switch on silicon nanostructures.
Researchers developed a “device”: a disc 250 nm in diameter that is capable of switching optical pulses at femtosecond rates (femtosecond is a one millionth of one billionth of a second). Switching speeds that fast will allow creating data transmission and processing devices that will work at tens and hundreds terabits per second. This can make possible downloading thousands of HD-movies in less than a second.
The operation of the all-optical switch created by MSU researchers is based on the interaction between two femtosecond pulses. The interaction becomes possible due to the magnetic resonance of the silicon nanostructures. This type of resonance is characterized by strong localization of light waves on subwavelength scales, inside the nanoparticles.
If the pulses arrive at the nanostructure simultaneously, one of them interacts with the other and dampers it due to the effect of two-photon absorption. If there is a 100-fs delay between the two pulses, the interaction does not occur, and the second pulse goes through the nanostructure without changing.
“We were able to develop a structure with the undesirable free-carrier effects are suppressed, — says Maxim Shcherbakov. — Free carriers (electrons and electron holes) place serious restrictions on the speed of signal conversion in the traditional integrated photonics. Our work represents an important step towards novel and efficient active photonic devices– transistors, logic units, and others. Features of the technology implemented in our work will allow its use in silicon photonics. In the nearest future, we are going to test such nanoparticles in integrated circuits”.
Integrated laser Sources
Wide-spread adoption of silicon photonics has been hampered in part by the lack of monolithically integrated laser sources. The integration on silicon of efficient indium phosphide based light sources, currently driving long-range telecommunication networks, is known to be very challenging, owing to the large mismatch in crystal lattice constants between both materials.
Whilst there has been a range of microminiature lasers incorporated directly into silicon over the years, including melding germanium-tin lasers with a silicon substrate and using gallium-arsenide (GaAs) to grow laser nanowires, these methods have involved compromise.
Laser integration Roadmap: External versus direct heterogeneously integrated lasers
The most common laser used in photonics links is an InP based chip laser that is separate (external) from the silicon photonic IC. The laser is packaged either in traditional single mode packages like TO-cans or in silicon based micropackage.
Imec and Ghent University have for the first time monolithically integrated arrays of indium phosphide lasers on 300mm silicon substrates in a CMOS pilot line. Lasing operation was demonstrated for all tested devices consisting of an array of ten indium phosphide lasers. Typical lasing threshold powers of around 20mW were observed at room temperature under optical pumping.
One of the important goals of silicon photonics is the realization of practical silicon lasers. An optically-pumped Raman silicon laser is a bulk silicon laser capable of room-temperature continuous-wave (cw) operation. Japanese researchers from Osaka Prefecture University and Kyoto University developed Microwatt-Threshold continuous-wave Raman silicon laser using a photonic-crystal high-Q nanocavity.
A team of collaborating scientists from Hong Kong University of Science and Technology, the University of California, Santa Barbara, Sandia National Laboratories and Harvard University have found a way to create microscopically-small lasers directly from silicon, unlocking the possibilities of direct integration of photonics on silicon and taking a significant step towards light-based computers.
“Our lasers have a very low threshold and match the sizes needed to integrate them onto a microprocessor, and these tiny high-performance lasers can be grown directly on silicon wafers, which is what most integrated circuits (semiconductor chips) are fabricated with,” said professor Kei May Lau, Department of Electronic and Computer Engineering, Hong Kong University of Science and Technology.
Researchers first had to find a way to refine silicon crystal lattices so that their inherent defects were reduced significantly enough to match the smooth properties found in GaAs substrate lasers. They did this by etching nano-patterns directly onto the silicon to confine the defects and ensure the necessary quantum confinement of electrons within quantum dots grown on this template.
The researchers were then able to use optical pumping, which is a process in which light is used to raise or “pump” electrons from a lower energy level to a higher one, to demonstrate that the devices they created were able to operate as lasers.
The next step for this research, according to Lau, is to see if it is possible to create electrically-pumped lasers using standard microelectronics technology
Researchers from the University of California, Santa Barbara have managed to place billions of light-emitting dots, or “quantum dots,” directly onto silicon, allowing lasers and other components to be easily integrated into silicon, paving the way for advanced photonic integrated circuits with far more functionality than can be achieved today.
The work was sponsored by Defense Advanced Research Projects Agency under their Electronic-Photonic Heterogeneous Integration. E-PHI program was set out in 2011 to develop technologies and architectures to enable chip-scale electronic-photonic / mixed-signal integrated circuits on a common silicon substrate.
Research project integrates laser directly on silicon with a modulator
IRT Nanoelec, an R&D consortium have announced the first co-integration of a III-V/silicon laser and silicon Mach Zehnder modulator demonstrating 25 Gbps transmission on a single channel. This transmission rate usually is achieved using an external source, over a 10 km single-mode fiber.
”To achieve these recent results, silicon photonics circuits integrating the modulator were processed first on a 200mm SOI wafer, although 300mm wafers also could be used in the near future. Then, a two-inch wafer of III-V material was directly bonded on the wafer. In the third step, the hybrid wafer was processed using conventional semiconductor and/or MEMS process steps to produce an integrated modulator-and-laser transmitter.
“Jointly obtained by STMicroelectronics and Leti in the frame of the IRT Nanoelec cooperation, these results, especially fabricating the laser directly on silicon, demonstrate IRT Nanoelec’s worldwide leadership in III/V-on-silicon integration to achieve high-data-rate fiber-optic modules,” said Stéphane Bernabé, project manager.
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