12-10-2012, 04:34 PM
SILICON PHOTONICS
[attachment=36230]
INTRODUCTION
During 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 a 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 fibre optics.
In a major advancement, researchers have developed a silicon-based optical modulator operating at50 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.
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.
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.
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.
PHYSICAL PROPERTIES
Optical Guiding and Dispersion Tailoring
Silicon 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 nano meters. 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.
Kerr Nonlinearity
Silicon 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 side bands and the eventual breakup of the waveform into a train of pulses.
Two-Photon Absorption
Silicon 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.
Free Charge Carrier Interactions
The 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 enhance carrier recombination. A suitable choice of geometry can also be used to reduce the carrier lifetime. Rib waveguides (in which the waveguides consist of thicker regions in a wider layer of silicon) enhance both the carrier recombination at the silica silicon interface and the diffusion of carriers from the waveguide core.
[attachment=36230]
INTRODUCTION
During 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 a 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 fibre optics.
In a major advancement, researchers have developed a silicon-based optical modulator operating at50 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.
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.
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.
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.
PHYSICAL PROPERTIES
Optical Guiding and Dispersion Tailoring
Silicon 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 nano meters. 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.
Kerr Nonlinearity
Silicon 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 side bands and the eventual breakup of the waveform into a train of pulses.
Two-Photon Absorption
Silicon 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.
Free Charge Carrier Interactions
The 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 enhance carrier recombination. A suitable choice of geometry can also be used to reduce the carrier lifetime. Rib waveguides (in which the waveguides consist of thicker regions in a wider layer of silicon) enhance both the carrier recombination at the silica silicon interface and the diffusion of carriers from the waveguide core.