Michael R. Bynum
Physics 464: Applied Optics
Copper interconnects will soon be the limiting factor of the performance of a computer. The aim of Silicon Photonics is to bring the high performance of optical interconnects to silicon, a relatively cheap and abundant resource. Researchers at Intel Corporation and in many universities are working toward a common goal Current research within this field has shown positive results with the development of a silicon modulator in 2003, and a silicon laser in 2005. These landmarks present a promising future for this field.
(Intel’s vision of an integrated silicon phonics chip)
Today optical data connections are most commonly found in long haul communications and in large server farms. These connections make up the backbone of our communications infrastructure because of their ability to transfer massive amounts of data at the fastest rate possible. Due to the high price of a single optical connection the optical interconnect market typically ends here. This high price is typically due to the exotic materials from which optical devices are constructed.
Current interconnect technology uses electrical signals sent along copper wire to transfer data between high speed fiber optic connections and the users to whom the data is being sent. Copper is also used as the core interconnect technology inside computers. Unfortunately copper is reaching its peak performance and the only way to transfer greater amounts of data using electrical signals is to have more connections. While this is currently not a major problem facing the majority of today’s technology this limitation of copper poses a serious threat to future of microprocessor manufacturers.
Over the last 30 years the speed of a microprocessor has doubled every eighteen months, this observation was made famous by Moore’s Law, which stated just that. With the exponential increase in performance of a microprocessor copper interconnects will soon become the limiting factor of the overall performance of a computer system. In order to utilize the peak performance of tomorrow’s computer system a new interconnect technology must be developed which can match both the scale and cost of a computer.
Silicon Photonics is a relatively new field with the goal of developing silicon based optical devices. This approach appears to be very enticing due to multiple reasons including the large established silicon fabrication infrastructure and the relatively low cost and high abundance of this material. Unfortunately silicon does not have inherently good optical properties, however many of these short comings are proving to be engineering problems and not absolute limitations.
Currently multiple university researches throughout the world and a team of researches at Intel are striving to fulfill a vision for silicon photonics. This vision is for the development of an integrated optical chip consisting of six significant components: waveguides, optical modulator, laser source, photo-detector, CMOS intelligence, and passive assembly.
(The building blocks of the integrated optical circuit)
Silicon waveguides are the first piece of the silicon photonics vision. The ability of silicon to guide light depends on many key properties of the material. The band gap of silicon determines the shortest possible wavelength of light able to propagate through the material without being absorbed. The band gap of silicon is approximately 1eV which correlates to a wavelength of 1.24μm; below this wavelength silicon becomes opaque. The wavelengths used in silicon photonics lie in the range of 1.3μm to 1.6μm which encompasses the 3 bandwidths commonly used in fiber optic communication systems: the C-band (1.53μm - 1.562μm), the S-band (1.485μm - 1.52μm), and the L-band (1.57μm - 1.61μm).
Waveguides and optical fibers operate based on the concept of total internal reflection (TIR), where light incident on a surface below a certain angle will be entirely reflected back into the waveguide rather than being transmitted in the surrounding medium. TIR is based on the difference between the index of refraction of the waveguide and its surrounding medium, the larger the difference in refractive indices, the smaller the angle required to maintain TIR. A major advantage of silicon is its relatively high refractive index of n=3.45. Silicon dioxide on the other hand, has an index of refraction of n=1.5 and is used as the surrounding medium around a silicon waveguide. The difference between these materials correlates to an index difference of nearly Δn ≈ 2. This property allows for very small devices and tight bends in waveguides.
The basic silicon waveguide is a rib waveguide where a silicon rib is raised above a silicon base resting on an insulating layer; these waveguides are commonly referred to as SOI rib waveguides. Multiple silicon waveguide devices such as Y-splitters, Couplers, Gratings, and Arrayed Waveguides have already been demonstrated.
A digital signal is imposed on light through one of two methods, direct and external modulation. In direct modulation the light source is repeatedly switched on and off, while in external modulation a continuous beam is blocked or forced to interfere deconstuctively.
The silicon modulator is an external modulator based on the design of a Mach-Zender interferometer, where an incoming beam of light is split in half and the two resulting beams travel through two different arms of the interferometer. Along these arms a voltage is applied which induces the Kerr effect, the Franz-Keldysh effect, provides a carrier injection. These three mechanisms provide the same result, a change in the refractive index. By controlling the refractive index the phase of the light can be changed so that when the two arms recombine they either add constructively or deconstructively, thus allowing for modulation of the light.
(Schematic of a Mach-Zender interferometer modulator with two phase shifter sections)
The Kerr Effect is a second-order electric field effect in which the refractive index is changed proportionally to the square of the applied electric field. The index change is calculated as:
where s33 is the Kerr coefficient and n0 is the unchanged refractive index of the medium.
The Franz-Keldysh Effect is due to distortion of the energy bands of the semiconductor upon application of an electric field. Unfortunately the Franz-Keldysh effect falls off greatly for 1.3μm light, and the resulting index shift of the Kerr effect at the application of 100V/μm is only about -10-4. The third method mentioned has a far greater impact on the index of refraction of the material and is given by the equation:
At a wavelength of 1.55μ this simplifies to:
At a wavelength of 1.3μm the equation becomes:
In the above equations ΔN refers to the number of carriers injected or depleted through this process. Δn refers to the change in refractive index. The resulting phase shift of through these processes can be determined by a much simpler formula corresponding to the effective refractive index, the wavelength, and the length of the phase shifter:
The silicon laser is one of the most challenging aspects in this field, this is due to silicon’s indirect band gap. The radiative recombination coefficient of silicon is very small (especially when compared to InP, GaAs, or other III-V materials), this implies that stimulated emission in silicon is very unlikely. Through another process known as Raman Effect or Raman Scattering, light can be amplified in silicon however the resulting additional photons generated through the amplification process are quickly absorbed due to long free carrier lifetimes.
Recently, researchers at Intel have demonstrated a continuous wave silicon laser. The design of the laser is based on a Fabry-Perot resonator, in which a light propagates through a waveguide designed to be an integer multiple of the lasing wavelength.
(Top-down schematic of Intel’s silicon laser)
To reduce the life time of free carriers, a P-I-N structure is created across the waveguide by placing a P-type silicon rail along one side of the waveguide and an N-type silicon rail along the opposite side. When a voltage bias is place across the waveguide, free carriers are swept out of the waveguide and thus amplification can occur.
(Silicon Laser used in Intel’s experiments)
(SEM image of Intel’s Silicon Laser)
Through the Raman Effect, an optically pumped cavity will produce amplification in the light intensity at a wavelength (1.63μm) which is shifted from the wavelength of the pumping source (typically pumped at 1.55μm). The energy of a pump photon is split between a red-shifted photon and a phonon. Raman amplifiers and lasers are common in the telecom industry by using other materials, most commonly glass fiber. A major advantage of using silicon is the Raman gain coefficient, which is 10,000 times greater in silicon than in glass fibers. The amplification gained in a half kilometer of glass fiber can be accomplished in 5 centimeters of silicon.
At both ends of the waveguide, mirrors are added to trap this specific wavelength inside the waveguide. The constantly reflected beam builds an optical standing wave inside the resonator cavity. One of the end mirrors is only partially reflective so that when a critical intensity is reached the laser beam can escape the chamber in the form of monochromatic coherent light.
Once light is produced and a signal is impressed in the beam, the data travels to its destination and needs to be converted back into an electrical signal. This is the purpose of a photo-detector. Due to the band gap and for the same reason that silicon is transparent in the optical communication bandwidths, silicon is not a good photo-detection material for infra-red. While Silicon is not a good IR photo-detector germanium is much more responsive in this range. Germanium can be implanted into strained silicon to shift the band gap to make it capable of IR photo-detection.
(Absorption coefficient and penetration depth of various materials as a function of λ. Green lines mark important communications wavelengths of 1.31μm and 1.55μm)
The current design for a silicon based photo detector is to have a layer of SiGe between a P-type and an N-type layer. Because of the electric field across the SiGe layer (induced by the p-i-n structure) electrons raised to the conduction band by photon absorption will generate a current. A very desirable property of silicon is that it makes for great APDs (Avalanche Photo Detector) in which an electron pushed to the conduction band will set off a chain reaction in which many electrons will be affected causing a greater current spike to occur. Electrical probes can then be connected to the P and N regions of the photo-detector to transfer the data to the logic portion of the chip.
(SiGe waveguide-based photo-detector on a SOI wafer)
Technical concerns facing the silicon photo-detector include responsivity, speed, and dark current limitations. The speed of this device must match up with the speed of the modulator or else data imposed on the optical connection will be lost. The responsivity is an important factor in the design of the detector because determines whether or not an signal will produce enough of a voltage boost to be interpreted as a 1 as opposed to a 0. Dark current is a major aspect concerning the performance of the detector. Dark current is caused by lattice miss matches and creates a current which is present even when light is not. If the dark current of a photo-detector is too high the detector will read a 1 when it should be reading 0. Another major problem caused by dark current is that it degrades the quality of an incoming optical signal by making the voltage difference between a 1 and a 0 significantly smaller.
The biggest reason to use silicon in for its compatibility with CMOS standards. A large infrastructure already exists to support the development of silicon integrated devices. Silicon photonic devices require intelligence to drive the modulator and properly respond to signals from the photo-detector. While embedded intelligence is a critical component this is one which has already been researched at great length.
In order to bring these intricate pieces into products they must be able to be mass produced at a low cost to manufacturers. By using current photolithographic processes and designing these devices in silicon makes silicon photonics an ideally inexpensive solution. To fit the product cost model, the total cost should consist of three major production costs: the device itself, testing, and packaging. All three pieces of this model should have equal costs. Due to the small size of silicon waveguides and silicon’s high index of refraction, proper alignment of fibers is critical in order to couple in the maximum light possible.
Multiple assembly methods and practices are being researched which include facet preparation techniques, antireflective thin film coatings, passive alignment, and tapers. These considerations allow for faster and simpler assembly of these devices, which dramatically reduces assembly costs. Using both chemical etching and micromachining, groves (U-groves and V-groves) can be etched into the silicon to decrease fiber to silicon alignment times. These grooves allow fibers to be mechanically slid into place, rather than requiring three dimensional micro adjustments. Tapers allow for greater collecting surface area and direct the collected light into the smaller waveguide. Facet preparation improves the quality of the facet and reduces the number of anomalies in the coupling surface. Thin film coatings reduce the amount of light reflected by the silicon facet and increase the intensity of the beam inside the silicon waveguide, alternatively high reflective coatings are placed on the ends of waveguides intended to be used as lasers.
(SEM images of U-groves, two of which have optical fibers inserted and aligned to silicon waveguides)
(Schematic of a pseudo vertical tapering (a) and a 3d gray-scale taper into an SOI waveguide)
Silicon Photonics is just now becoming a reality and has the potential to be tomorrow’s method of data interconnects. Research is steadily developing this vision and is showing many positive results. Intel once made magic with silicon in the form of the integrated circuit, and appears to being doing it again. With this research more questions are being asked of the optical properties of silicon. With the development of the silicon modulator and silicon laser more people are shedding their skepticism of silicon as an optical material and beginning to find ways to implement this revolutionary technology. With every improvement come a new set of relationships and a new set of questions. The process of optimizing these optical circuits and refining their production will provide research opportunities for many years to come.
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