Silicon photonics can be defined as the utilization of silicon-based materials for the generation (electrical-to-optical conversion), guidance, control, and detection (optical-to-electrical conversion) of light to communicate information over distance. The most advanced extension of this concept is to have a comprehensive set of optical and electronic functions available to the designer as monolithically integrated building blocks upon a single silicon substrate. 

Within the range of fibre optic telecommunication wavelength (1.3 ?m to 1.6 ?m), silicon is nearly transparent and generally does not interact with the light, making it an exceptional medium for guiding optical data streams between active components. But no practical modification to silicon has yet been conceived which gives efficient generation of light. Thus it

required the light source as an external component which was a drawback. 

There are two parallel approaches being pursued for achieving opto-electronic integration in silicon. The first is to look for specific cases where close integration of an optical component and an electronic circuit can improve overall system performance. One such case would AbstractIn its everlasting quest to deliver more data faster and on smaller components, the silicon industry is

moving full steam ahead towards its final frontiers of size, device integration and complexity. As the physical limitations of metallic interconnects begin to threaten the semiconductor industry's future, researches are concentrated heavily on advances in photonics that will lead to combining existing silicon infrastructure with optical communications technology, and a merger of electronics and photonics into one integrated dual functional device. Optical technology has always suffered from its reputation for being an expensive solution. This prompted research into using more common materials, such as silicon, for the fabrication of photonic components, hence the name silicon photonic.

IntroductionDuring the past few years, researchers at Intel have been actively exploring the use of silicon as the primary basis of photonic components. This research has

established Intel’s reputation in a specialized field called silicon photonics, which appears poised to provide solutions that break through longstanding limitations of silicon as a material for fiber optics.In a major advancement, Intel researchers have developed a silicon-based optical modulator operating at 50 GHz - an increase of over 50 times the previous research record of about 1GHz (initially 20MHz). This is a significant step towards building optical devices that move data around inside a computer at the speed of light. It is the kind of breakthrough that ripples across an industry over time, enabling other new devices and applications. It could help make the Internet run faster, build much faster high-performance computers and 

enable high-bandwidth applications like ultra-high-definition displays or vision recognition systems.Intel’s research into silicon photonics is an end-to-end program to extend Moore’s Law into new areas. In addition to this research, Intel’s expertise in fabricating processors from silicon could enable it to create inexpensive, high performance photonic devices that comprise numerous components integrated on one silicon die. “Siliconizing” photonics to develop and build optical devices in silicon has the potential to bring PC economics to high-bandwidth optical communications. Another advancement in silicon photonics is the demonstration of the first continuous silicon laser based on the Raman Effect. This research breakthrough paves the way for making optical amplifiers, lasers and wavelength converters to switch a signal’s color in low-cost silicon.Fiber optic communication is well established today due 

to the great capacity and reliability it provides. However, the technology has suffered from a reputation as an expensive solution. This view is based in largepart on the high cost of the hardware components. These components are typically fabricated using exotic materials that are expensive to manufacture. In addition, these components tend to be specialized and require complex steps to assemble and package. These limitations prompted Intel to research the construction of fiber-optic components from other materials, such as silicon. The vision of silicon photonics arose from the research performed in this area. Its overarching goal is to develop high-volume, low-cost optical components using standard CMOS processing – the same manufacturing process used for microprocessors and semiconductor devices

What Is Silicon Photonics?Photonics is the field of study that deals with light, especially the development of components for optical communications. It is the hardware aspect of fiber optics, and due to commercial demand for bandwidth, it has enjoyed considerable expansion and development during the past decade. Fiber-optic communication, as most people know, is the process of transporting data at high speeds using light, which travels to its destination on a glass fiber. Fiber optics is well established today due to the great capacity and reliability it provides. However, fiber optics has suffered from its reputation as an expensive solution. This view is based in large part on the high price of the hardware components. Optical devices typically have been made from exotic materials 

such as gallium arsenide, lithium niobate, and indium phosphide that are complicated to process. In addition, many photonic devices today are hand assembled and often require active or manual alignment to connect the components and fibers onto the devices. This non-automated process tends to contribute significantly to the cost of these optical devices. 

Silicon photonics research at Intel hopes to establish that manufacturing processes using silicon can overcome some of these limitations. Intel’s goal is to manufacture and sell optical devices that are made out of easy-to-manufacture silicon. Silicon has numerous qualities that make it a desirable material for constructing small, low-cost optical components: it is a relatively inexpensive, 

plentiful, and well understood material for producing electronic devices. In addition, due to the longstanding use of silicon in the semiconductor industry, the fabrication tools by which it can be processed into small components are commonly available today. Because Intel has more than 35 years of experience in silicon and device fabrication, it finds a natural fit in exploring the design and development of silicon photonics.Silicon photonics is the study and application of photonic systems which use silicon as an optical medium. It can be simply defined as the photonic technology based on silicon chips. Silicon photonics can be defined as the utilization of silicon-based materials for the generation (electrical-to-optical conversion), guidance, control, and detection (optical-to-electrical conversion) of light to communicate information

over distance. The most advanced extension of this concept is to have a comprehensive set of optical and electronic functions available to the designer as monolithically integrated building blocks upon a single silicon substrate.The goal is to siliconize photonics-specifically to build in silicon all the functions necessary for optical transmission and reception of data. The goal is then to integrate the resulting devices onto a single chip. An analogy can be made that such optical chips hold the same relationship to the individual components as integrated circuits do to the transistors that constitute them: they provide a complete unit that can be manufactured easily and inexpensively using standard silicon fabrication techniques. Intel has recently been able to demonstrate basic feasibility to siliconize

many of the components needed for optical communication. The most recent advance involves encoding high-speed data on an optical beam.There are two parallel approaches being pursued for achieving optoelectronic integration in silicon. The first is to look for specific cases where close integration of an optical component and an electronic circuit can improve overall system performance. One such case would be to integrate a Si-Ge photo-detector with a Complementary Metal-Oxide-Semiconductor (CMOS) trans-impedance amplifier. The second is to achieve a high level of photonic integration with the goal of maximizing the level of optical functionality andoptical performance. This is possible by increasing light emitting efficiency if silicon.

Why Silicon Photonics?Fiber-optic communication is the process of transporting data at high speeds on a glass fiber using light. Fiber optic communication is well established today due to the great capacity and reliability it provides. However, the technology has suffered from a reputation as an expensive solution. This view is based in large part on the high cost of the hardware components. These components are typically fabricated using exotic materials that are expensive to manufacture. In addition, these components tend to be specialized and require complex steps to assemble and package. These limitations prompted Intel to research the construction of fiber-optic components from other materials, such as silicon. The vision of silicon photonics arose from the research performed in this area. Its overarching goal is to develop high-volume, 

low-cost optical components using standard CMOS processing – the same manufacturing process used for microprocessors and semiconductor devices. Silicon presents a unique material for this research because the techniques for processing it are well understood and it demonstrates certain desirable behaviors. For example, while silicon is opaque in the visible spectrum, it is transparent at the Infra-red wavelengths used in optical transmission, hence it can guide light. Moreover, manufacturing silicon components in high volume to the specifications needed by optical communication is comparatively inexpensive. Silicon’s key drawback is that it cannot emit laser light, and so the lasers that drive optical communications have been made of more exotic materials such as indium phosphide and gallium arsenide. However, silicon can be used to manipulate the light emitted by

inexpensive lasers so as to provide light that has characteristics similar to more-expensive devices. This is just one way in which silicon can lower the cost of photonics.Silicon photonic devices can be made using existing semiconductor fabrication techniques, and because silicon is already used as the substrate for most integrated circuits, it is possible to create hybrid devices in which the optical and electronic components are integrated onto a single microchip. The propagation of light through silicon devices is governed by a range of nonlinear optical phenomena including the Kerr effect, the Raman effect, Two Photon Absorption and interactions between photons and free charge carriers. The presence of nonlinearity is of fundamental importance, as it enables light to interact with light, thus permitting applications such as wavelength conversion and all-optical signal

routing, in addition to the passive transmission of light. Within the range of fiber optic telecommunication wavelength (1.3 µm to 1.6 µm), silicon is nearly transparent and generally does not interact with the light, making it an exceptional medium for guiding optical data streams between active components. Also optical data transmission allows for much higher data rates and would at the same time eliminate problems resulting from electromagnetic interference. The technology may also be useful for other areas of optical communications, such as fiber to the home.

Physical PropertiesA. Optical Guiding and Dispersion TailoringSilicon is transparent to infrared light with wavelengths above about 1.1 microns. Silicon also has a very high refractive index, of about 3.5. The tight optical confinement provided by this high index allows for microscopic optical waveguides, which may have cross-

sectional dimensions of only a few hundred nanometers. This is substantially less than the wavelength of the light itself, and is analogous to a sub wavelength-diameter optical fiber. Single mode propagation can be achieved, thus (like single-mode optical fiber) eliminating the problem of modal dispersion. The strong dielectric boundary effects that result from this tight confinement substantially alter the optical dispersion relation. By selecting the waveguide geometry, it is possible to tailor the dispersion to have desired properties, which is of crucial importance to applications requiring ultra-short pulses. In particular, the group velocity dispersion (that is, the extent to which group velocity varies with wavelength) can be closely controlled. In bulk silicon at 1.55 microns, the group velocity dispersion (GVD) is normal in that pulses with longer wavelengths travel with higher group 

velocity than those with shorter wavelength. By selecting suitable waveguide geometry, however, it is possible to reverse this, and achieve anomalous GVD, in which pulses with shorter wavelengths travel faster. Anomalous dispersion is significant, as it is a prerequisite for modulation instability. In order for the silicon photonic components to remain optically independent from the bulk silicon of the wafer on which they are fabricated, it is necessary to have a layer of intervening material. This is usually silica, which has a much lower refractive index (of about 1.44 in the wavelength region of interest), and thus light at the silicon-silica interface will (like light at the silicon-air interface) undergo total internal reflection, and remain in the silicon. This construct is known as silicon on insulator. It is named after the technology of silicon on insulator in electronics, whereby components are built upon a layer 

of insulator in order to reduce parasitic capacitance and so improve performance.

B. Kerr NonlinearitySilicon has a focusing Kerr nonlinearity, in that the refractive index increases with optical intensity. This effect is not especially strong in bulk silicon, but it can be greatly enhanced by using a silicon waveguide to concentrate light into a very small cross-sectional area. This allows nonlinear optical effects to be seen at low powers. The nonlinearity can be enhanced further by using a slot waveguide, in which the high refractive index of the silicon is used to confine light into a central region filled with a strongly nonlinear polymer. Kerr nonlinearity underlies a wide variety of optical phenomena. One example is four-wave mixing, which has been applied in silicon to realize both optical parametric amplification and parametric

wavelength conversion. Kerr nonlinearity can also cause modulation instability, in which it reinforces deviations from an optical waveform, leading to the generation of spectral-sidebands and the eventual breakup of the waveform into a train of pulses.

C. Two-Photon AbsorptionSilicon exhibits Two Photon Absorption (TPA), in which a pair of photons can act to excite an electron-hole pair. This process is related to the Kerr effect, and by analogy with complex refractive index, can be thought of as the imaginary-part of a complex Kerr nonlinearity. At the 1.55 micron telecommunication wavelength, this imaginary part is approximately 10% of the real part. The influence of TPA is highly disruptive, as it both wastes light, and generates unwanted heat. It can be mitigated, however, either by switching to longer wavelengths (at which the TPA to Kerr ratio drops),

or by using slot waveguides (in which the internal nonlinear material has a lower TPA to Kerr ratio). Alternatively, the energy lost through TPA can be partially recovered by extracting it from the generated charge carriers. 

D. Free Charge Carrier InteractionsThe free charge carriers within silicon can both absorb photons and change its refractive index. This is particularly significant at high intensities and for long durations, due to the carrier concentration being built up by TPA. The influence of free charge carriers is often (but not always) unwanted, and various means have been proposed to remove them. One such scheme is to implant the silicon with helium in order to

Conclusion

It is clear that an enormous amount of work,corresponding to huge capital investments, is stillrequired before silicon photonics can be established as akey technology. However, the potential merits motivatebig players such as Intel to pursue this developmentseriously. If it is successful, it can lead to a verypowerful technology with huge benefits for photonicsand microelectronics and their applications.Although research in the area of planar optics in siliconhas been underway for several decades, recent efforts atIntel Corporation have provided better understanding ofthe capabilities of such devices as silicon modulators,ECLs and SiGe detectors. Silicon modulators operatingat 50 GHz have demonstrated several orders ofmagnitude improvement over other known Si-basedmodulators, with theoretical modeling indicatingperformance capabilities beyond 1 THz. Through

further research and demonstration of novel siliconphotonics devices, integrated silicon photonics has aviable future in commercial optoelectronics.