Seminar Report On LIGHT PEAK

Light Peak is Intel's code-name for a new high-speed optical cable technology designed to connect electronic devices to each other in a peripheral bus. Optical networking technologies have been over the last two decades reshaping the entire telecom infrastructure networks around the world. As network bandwidth requirements increase, optical communication and networking technologies have been moving from their telecom origin into the enterprise. For example, today in data centers, all storage area networking is based on fiber interconnects with speeds ranging form 1 Gb/s to 10 Gb/s. As the transmission bandwidth requirements increase and the costs of the emerging optical technologies become more economical, the adoption and acceptance of these optical interconnects within enterprise networks will increase. This report provides the framework for the light peak optical interconnect technology. A brief overview of the light peak interconnects technology and its current application within the enterprise is presented.

II. LIGHT PEAK

  Light Peak is a new high-speed optical cable technology designed to connect electronic devices to each other in a peripheral bus. It has the capability to deliver high bandwidth, starting at 10 Gbit/s, with the potential ability to scale to 100 Gbit/s. It is intended as a single universal replacement for current buses such as SCSI, SATA, USB, FireWire, PCI Express and HDMI. In comparison to these buses, Light Peak is much faster, longer ranged, smaller, and more flexible in terms of protocol support.

Light Peak was developed as a way to reduce the proliferation of ports on modern computers. Bus systems like USB were intended to do the same, and successfully replaced a number of older technologies like RS232 and Centronics printer ports. However, increasing bandwidth demands have led to the introduction of a new series of high-performance systems like eSATA and Display Port that USB and similar systems can not address. Light Peak provides enough bandwidth to allow all of these systems to be driven over a single type of interface, and in many cases on a single cable using a daisy chain.The Light Peak cable contains a pair of optical fibers that are used for upstream and downstream traffic. This means that Light Peak offers a maximum of 10 Gbit/s in both directions at the same time. The prototype system featured two motherboard controllers that both supported two bidirectional buses at the same time, wired to four external connectors. Each pair of optical cables from the controllers is led to a connector, where power is added through separate wiring. The physical connector used on the prototype system looks similar to the existing USB or FireWire connectors.

Intel has stated that Light Peak is protocol independent, allowing it to support existing standards with a change of the physical medium. Few details on issues like protocol or timing contention have been released. Intel has stated that Light Peak has the performance to drive everything from storage to displays to networking, and it can maintain those speeds over 100 meter runs. As advantages over existing systems, they also note that a system using Light Peak will have fewer and smaller connectors, longer and thinner cables, higher bandwidth, and can run multiple protocols on a single cable.

One key piece of the device chain that has not been shown is a controller for the device-end of the bus. In the USB case, a single controller can contain the power circuitry, USB device logic, along with off-the-shelf, custom or programmable logic for running devices. A simple USB device can be built by adding a connector, one driver chip, and the hardware the system is meant to drive; a mouse is a good example of a system that is typically implemented using a single off-the-shelf chip. A similar single-chip solution will be in demand for Light Peak as well, but to date Intel has simply suggested it is working with industry partners to provide one. According to Intel, the companies that will produce Light Peak technology include Foxconn, Foxlink, IPtronics, SAE Magnetics FOCI Fiber Optics Communications Inc, Avago, Corning   Elaser, Oclaro, Ensphere Solutions, and Enablence.

In the next 5-10 years, people will have many reasons for higher bandwidth. Lots of attractive applications that significantly improve user experiences are depending on huge volume of data capturing, transfer, storage, and reconstruction. People will have more and more electrical devices, such as High Definition (HD) video camcorders, HD monitors, Mobile Internet Device (MID) s, laptops and other handheld devices,  and they want to be able to share data between these devices,   smoothly, quickly and easily. All these user requirements call for higher bandwidth. But existing electrical cable technology

in mainstream computing devices is approaching the practical limit for higher bandwidth and longer distance, due to the signal degradation caused by electro-magnetic interference (EMI) and signal integrity issues. Higher bandwidth can be achieved by sending the signals down with more wires, but apparently this approach increases cost, power and difficulty of PCB layout, which explains why serial links such as SATA,  SAS, USB are becoming the mainstream. However optical communications do not create EMI by using photonics rather than electrons, thus allowing higher bandwidth and longer distances. Besides, optical technology also allows for small form factors and longer, thinner cables.

It’s important to design flexible and effecient protocol to leverage the raw bandwidth enabled by optical fiber. Intel has announced its high speed optical cable technology, Light Peak (LPK) [10], which delivers high bandwidth starting at 10Gbps and has the ability to multiplex multiple protocols simultaneously over a single optical cable.

Combining the high bandwidth of optical fiber with Intel’s practice to mulplex multiple protocol over a single fiber, optical technology may change the landscape of IO system design in the future. It’s possible that most of the legacy IO protocols can be tunneled by optical-capable protocols, so some of the legacy IO interfaces can be converged to one single optical interface, significantly simplifying the form factor design of computers. This change in IO system will definitely affect the design of systems. The ultimate goal of system architects is to make a balanced and efficient system, on both power and cost grounds. It makes no sense to have a high throughput IO system with insufficient processing power or overloaded interconnections between IO system and the processor.

               

Mobile and handheld devices are two fast growing market segments which attract interests from processor vendors including Intel and they are our targets for application selection. For mobile and handheld devices, user interface and IO are two important factors besides computing power that affect end users’ purchase decision. Taking power into account, it’s possible that more carefully tuned IO workload offloading engines will be integrated into the IO controller, saving the power to move the data from IO a long way to the system memory.               

Because we think that the future computing devices, especially low-end and consumer electronics will be more IOcentric, our research philosophy is an IO-oriented one. We try to identify the future IO devices and their usage models first, then we investigate the requirements of the interconnection from the IO’s standpoint, after that we move up to the processor level, trying to figure out the needs of processing power and thus the best architecture for it.

With this philosophy in mind, an optical-enabled system model can be illustrated in Fig. 1. There are four main components in this figure, the IO devices, the IO controller which connects to the IO devices through optical fiber, the processing unit and the interconnection between the IO controller and the processing unit, whatever it can be implemented as. We are looking at the system from IO to processor as shown by the arrow.

2.1 Features and key benefit

·         Provide a standard low cost optical-based interconnect

·         Support for key existing protocols (USB, HDMI, DP, PCIe, etc)

·         Scalable bandwidth, cost, power to support broad base for 10+ years

·         Support wide range of devices (handhelds, laptops, PCs, CE, and more)

·         Common optical I/O architecture for the next decade and more

·         Single, flexible cable that can carry any platform I/O

·         Economies of scale from a single optical solution

·         1.Higher bandwidth –10Gbs to 100Gbs over the next decade

·         Enables I/O performance for the next generation

·         Allows for balanced platform, with external I/O keeping up with most platform interconnects

·         2.Longer, thinner cables and smaller connectors

·         Up to 100 meters on an optical-only cable

·         Each fiber is only 125 microns wide, the width of a human hair

·         .Supports multiple existing I/O protocols over a single cable

·         Smooth transition for today’s existing electrical I/O protocols

·         Can connect to more devices with the same cable, or to combo devices such as docking stations.   

3COMPONENTOVERVIEW

 Light Peak consists of a controller chip and an optical module that would be included in platforms supporting this technology. The optical module performs the conversion from electricity to light and vice versa, using miniature lasers and photo detectors. Intel is planning to supply the controller chip, and is working with other component manufacturers to deliver all the Light Peak components.The main components are fibre optics,optical module,control chip.

4.THE FUNDAMENTALS OF OPTICAL COMPONENTS

 A basic optical communication link consists of three key building blocks: optical fiber, light sources, and light detectors. We discuss each one in turn.

4.1 Optical Fibers

In 1966, Charles Kao and George Hockmam predicted that purified glass loss could be  reduced to below 20 dB per kilometer, and they set up a world-wide race to beat  this prediction. In September 1970, Robert Maurer, Donald Keck, and Peter Schultz of Corning succeeded in developing a glass fiber with attenuation less than 20  dB/km: this was the necessary threshold to make fiber  optics a viable transmission technology. The silica-based optical fiber structure consists of a cladding layer with a lower refractive index than the fiber core it surrounds. This refractive index difference causes a total internal reflection, which guides the propagating light through the  fiber core. There are many types of optical fibers with different size cores and cladding. Some optical fibers are not even glass-based such as Plastic Optical Fibers (POFs), which are made for short-distance  communication. For telecommunications, the fiber is glass based with two main categories: SMF and MMF.

SMFs typically have a core diameterof about 9 μm while  MMFs typically have a core diameter ranging from 50 to 62.5 μm. Optical fibers have two primary types of impairment, optical attenuation and dispersion. The fiber optical attenuation, which is mainly caused by absorption and the intrinsic Rayleigh scattering, is a wavelengthdependent  loss with optical losses as low as 0.2 dB/km around 1550  nm for conventional SMF (i.e., SMF-28∗) [6].

The optical fiber is a dispersive waveguide. The dispersion results in Inter Symbol Interference (ISI) at the receiver. There are three primary types of fiber dispersions: modal dispersion, chromatic dispersion, and polarization-mode dispersion. The fiber modal dispersion depends on both the fiber core diameter and transmitted wavelengths. For a single-mode transmission, the stepindex  fiber core diameter (D) must satisfy the following

where λ is the transmitted wavelength and n1 and n2 are  the refractive indices of fiber core and cladding layer, respectively. Consequently, for a single-mode operation at 850 nm wavelength, the fiber must have a core diameter of 5 μm. Since a conventional SMF has typically a core diameter of 9 μm, single-mode operation can be only supported for wavelengths in the 1310 nm wavelength band or longer. The fiber chromatic dispersion is due to the wavelengthdependent refractive index with a zero-dispersion wavelength occurring at 1310 nm in conventional SMF [6]. At 1550 nm, the fiber dispersion is about 17 ps/nm/km for SMF-28. When short duration optical pulses are launched into the fiber, they tend to broaden  since different wavelengths propagate at different group velocities, due to the spectral width of the emitter. Optical transmission systems operating at rates of 10 Gb/s or higher and distances above 40 km are sensitive to this phenomenon. There are other types of SMFs such as Dispersion Shifted Fibers (DSFs) where the zero dispersion occurs at 1550 nm.

  Polarization-Mode Dispersion (PMD) is caused by small amounts of asymmetry and stress in the fiber core due to the manufacturing process and environmental changes such as  temperature and strains. This fiber core asymmetry and stress leads to a polarization-dependent index of refraction and propagation constant, thus limiting the transmission distance of high speed (≥ 10 Gb/s) over SMF in optical communication systems. Standard SMF has a PMD value of less than 0.1 ps/√km [6]. Special SMFs were developed to address this issue. Optical fiber is never bare. The fiber is coated with a thin primary coating by the fiber manufacturer; then a cable manufacturer, not necessarily the fiber manufacturer, cables the fiber. There is a wide variety of cable construction. Simplex cable has a single fiber in the center while duplex cables contain two fibers. Composite cable incorporates both single-mode and multimode fiber. Hybrid cables incorporate mixed optical fiber and copper cable. In the enterprise, the MMF is housed in a cable with an orange colored jacket, and the SMF is housed in a yellow jacket cable.

4.2 Light Sources

The light source is often the most costly element of an optical communication system. It has the following key characteristics: (a) peak wavelength, at which the source emits most of its optical power, (b) spectral width, (c) output power, (d) threshold current, (e) light vs. Current linearity, (f) and a spectral emission pattern. These characteristics are key to system performance. There are two types of light sources in widespread use: the Laser Diode (LD) and the Light Emitting Diode (LEDs). All light emitters that convert electrical current into light are semiconductor based. They operate with theprinciple of the p-n semiconductor junction found intransistors. Historically, the first achievement of laser action in GaAs p-n junction was reported in 1962 by three groups [1-4]. Both LEDs and LDs use the same key materials: Gallium Aluminum Arsenide (GaAIAs) for short-wavelength devices and Indium Gallium Arsenide Phosphide (InGaAsP) for long-wavelength devices. Semiconductor laser diode structures can be divided into the so-called edge-emitters, such as Fabry Perot (FP) and Distributed Feedback (DFB) lasers and vertical-emitters, such as Vertical Surface Emitting Lasers (VCSELs). When edge-emitters are used in optical fibercommunication systems, they incorporate a rear facet photodiode to provide a means to monitor the laser output, as this output varies with temperature. In today’s optical networks, binary digital modulation is typically used, namely on (i.e., light on) and off (no light) to transmit data. These semiconductor laser devicesgenerate output light intensity which is proportional to the current applied to them, therefore making them suitable for modulation to transmit data. Speed and linearity are therefore two important characteristics.Modulation schemes can be divided into two main categories, namely, a direct and an external modulation. In a direct modulation scheme, modulation of the input current to the semiconductor laser directly modulates its output optical signal since the output optical power is proportional to the drive current. In an external modulation scheme, the semiconductor laser is operating  in a Continuous-Wave (CW) mode at a fixed operating point. An electrical drive signal is applied to an optical modulator, which is external to the laser. Consequently, the applied drive signal modulates the laser output light on and off without affecting the laser operation.One important feature of the laser diode is its frequency chirp. The frequency of the output laser light changes dynamically in response to the changes in the modulation  current. A typical DFB has a frequency chirp of about 100-MHz/mA. This spread of the wavelength interacts with the fiber dispersion. As previously mentioned, as the data rate is increased, this interaction limits the transmission distance of optical transmission systems due to the additional ISI generated at the receiver [1−4]. Optical back-reflection is one of key issues when coupling the output light from a laser source to a fiber.The optical back-reflection disturbs the standing wave in the laser cavity, increasing its noise floor, and thus making the laser unstable. One practical way to reduce the phenomenon of back-reflection is to place an isolator between the laser cavity and the fiber, which adds a significant additional cost to the laser [1, 4]. Temperature also affects the peak wavelength of the laser; threshold current also increases with temperature as slope efficiency decreases. For DWDM applications, which require very precise operating wavelengths, most of the current laser diode designs need to be cooled to within ± 0.3 °C. As previously explained, the direct modulation of a laser diode has several limitations, including limited propagation distance due to the interaction between the laser frequency chirp and fiber dispersion. This is not an issue for enterprise networks which are short distance, but could be a serious limiting factor for telecommunications applications. To overcome this limitation, the laser diode is operated in a CW mode, and output light is externally modulated by an optical modulator. Intensity modulators can be divided into two main groups: Mach-Zehnder Interferometer (MZI) and Electro-Absorption (EA) modulators. In an MZI modulator, a single input waveguide is split into two optical waveguides by a 3 dB  Y junction and then recombined by a second 3 dB Y junction into a single output. A Radio Frequency ( RF) signal, which is applied to a pair of electrodes constructed along the waveguides, modulates the propagating optical beam. The modulator key parameters are its modulation bandwidth, linearity, and the required drive signal voltage for π phase shift. MZI modulators based on LiNbO3 are high-performance modulators with a large form-factor (about 2.5 inches) that are not suitable for optical integration [4, 7]. EA modulators are based on a voltageinduced shift of the semiconductor bandgap so that the modulator becomes absorbing for the lasing wavelength. The advantages of an EA modulator is its low driving  voltage, high-speed operation, and suitability for optical integration with InP-based laser diodes [8]. A tunable laser is a new type of laser where its main lasing longitudinal mode can be tuned over a wide range of wavelengths such as the C band (1510−1540 nm) of an Erbium-Doped Fiber Amplifier (EDFA), which is commonly used for DWDM systems [1−4]. The use of tunable lasers is driven by the potential cost savings in DWDM transport networks since a significantly reduced inventory of fixed-wavelength lasers could be maintained for a robust network operation. The technical challenges are to provide both broad wavelength tunability and excellent wavelength accuracy over the laser life. A broadly tunable External Cavity Laser (ECL) employing micromachined, thermally tuned silicon etalons has been designed to achieve these goals.

4.3 Light Detectors

Light detectors convert an optical signal to an electrical signal. The most common light detector is a photodiode. It operates on the principle of the p-n junction. There are two main categories of photodetectors: a p-i-n (positive,  intrinsic, negative) photodiode and an Avalanche Photodiode (APD), which are typically made of InGaAs or germanium. The key parameters for photodiodes are (a) capacitance, (b) response time, (c) linearity, (d) noise, and (e) responsivity. The theoretical responsivity is 1.05 A/W at a wavelength of 1310 nm. Commercial photodiodes have responsivity around 0.8 to 0.9 A/W at the same wavelength [1-4]. The dark photo-current is a small current that flows through the photo-detector even though no light is present because of the intrinsic resistance of the photo-detector and the applied reverse voltage. It is temperature sensitive and contributes to  noise. Since the output electrical current of a photodiodeis typically in the range of μA, a Transimpedance

Amplifier (TIA) is needed to amplify the electric current to a few mA [2−4].APDs provide much more gain than the pin photodiodes, but they are much more expensive and require a high voltage power to supply their operation [2]. APDs are also more temperature sensitive than pin photodiodes.

4.3 Packaging: Optical Sub-assembly (OSA) and Optical Transceivers

As previously described, laser diodes and photodiodes are semiconductor devices. To enable the reliable operation of these devices, an optical package is required. In general, there are many discrete optical and electronic components, which are based on different technologies that must be optically aligned and integrated within the optical package. Optical packaging of laser diodes and photodiodes is the primary cost driver. These packages are sometimes called Optical Sub-Assemblies (OSAs). The Transmitter OSA package is called a TOSA and the Receiver OSA package is called a ROSA. Figure 1 shows, for example, a three-dimensional schematic view of a DFB laser diode mounted on a Thermo-Electric Cooler (TEC) inside a hermetically sealed 14-pin butterfly package with an SMF pigtail [9]. Most of the telecom-grade laser diodes are available in the so-called TO can or butterfly packages. The standard butterfly package is a stable and high-performance package, but it has a relatively large form-factor and it is costly to manufacture. These packages are typically used for applications where cooling is required using a TEC

The TEC requires a large amount of power to regulate the temperature of a laser inside the package. This type of optical packaging was used for the early 10 Gb/s modules. More recently, tunable 10 Gb/s lasers are using a similar butterfly optical package. The butterfly package design uses a coaxial interface for passing broadband data into the package, which requires the use of a coaxial interface to the host Printed Circuit Board (PCB). Although coaxial cables and connectors have been reduced in size, they still consume valuable real estate in

the optical transceiver. The evolution of optical module packages is toward smaller footprint packages. If relatively easy for receivers, the trend toward smaller packages is particularly challenging for laser transmitter modules due to the power and thermal dissipation constraints. Figure 2 shows the evolution of 10 Gb/s optical module packaging technology. To operate with high- performance, uncooled designs must be implemented with more advanced control systems that can adjust the laser and driver parameters over temperature. The smaller packages utilize a coplanar approach to the broadband interface, which more closely resembles a surface-mount component and enables much smaller RF interfaces. TO-can-based designs, which have been used extensively

in lower data rate telecom and datacom systems up to 2 Gb/s as well as CD players and other high-volume consumer applications, are now maturing to support highperformance 10 Gb/s optical links. Leveraging the fact that these packages are already produced in high volume will further reduce the cost of the 10 Gb/s optical modules in optical transceiver designs.

4.4 Optical Transceivers

                For telecommunication applications, the optical transmitter and receiver modules are usually packaged into a single package called an optical transceiver. Figure 3 shows an example of different transceivers and Figure 4 shows an example of the printed circuit board of a transceiver. There are several form factors for this optical transceiver depending on their operating speed and applications. The industry worked on a Multi-Source Agreement (MSA) document to define the properties of the optical transceivers in terms of their mechanical, optical, and electrical specifications. Optical transponders operating at 10 Gb/s, based on MSA, have been in the market since circa 2000, beginning with the 300-pin MSA, followed by XENPAK, XPAK, X2, and XFP.

9 CONCLUSION

 Intel is working with the optical component manufacturers to make Light Peak components ready to ship in 2010, and will work with the industry to determine the best way to make this new technology a standard to accelerate its adoption on a plethora of devices including PCs, handheld devices, workstations, consumer electronic devices and more. Light Peak is complementary to existing I/O technologies, as it enables them to run together on a single cable at higher speeds.

At the present time, Intel has conducted three successful public demonstrations of the Light Peak technology and confirmed that the first Light Peak-enabled PCs should begin shipping next year. To say the company is bullish on the technology is an understatement. In his keynote address at the Consumer Electronics Show earlier this year, Intel CEO Paul Otellini called Light Peak “the I/O performance and connection for the next generation,” and confirmed that both Nokia and Sony have publicly announced their support.

Victor Krutul, director of Intel’s optical development team and founder of the Light Peak program, is even more effusive, calling Light Peak “the biggest thing to happen to the optical industry ever, or at least since the creation of the laser.”