A device that uses the principle of amplification of electromagnetic waves by stimulated emission of radiation and operates in the infrared， visible， or ultraviolet The term laser is an acronym for light amplification by stimulated emission of radiation， or a light However， just as an electronic amplifier can be made into an oscillator by feeding appropriately phased output back into the input， so the laser light amplifier can be made into a laser oscillator， which is really a light Laser oscillators are so much more common than laser amplifiers that the unmodified word “laser” has come to mean the oscillator， while the modifier “amplifier” is generally used when the oscillator is not See also Amplifier; Maser; O The process of stimulated emission can be described as follows: When atoms， ions， or molecules absorb energy， they can emit light spontaneously (as with an incandescent lamp) or they can be stimulated to emit by a light This stimulated emission is the opposite of (stimulated) absorption， where unexcited matter is stimulated into an excited state by a light If a collection of atoms is prepared (pumped) so that more are initially excited than unexcited (population inversion)， then an incident light wave will stimulate more emission than absorption， and there is net amplification of the incident light This is the way the laser amplifier A laser amplifier can be made into a laser oscillator by arranging suitable mirrors on either end of the These are called the Thus the essential parts of a laser oscillator are an amplifying medium， a source of pump power， and a Radiation that is directed straight along the axis bounces back and forth between the mirrors and can remain in the resonator long enough to build up a strong (Waves oriented in other directions soon pass off the edge of the mirrors and are lost before they are much ) Radiation may be coupled out by making one mirror partially transparent so that part of the amplified light can emerge through it (see illustration) The output wave， like most of the waves being amplified between the mirrors， travels along the axis and is thus very nearly a plane See also Optical Continuous-wave gas lasers Perhaps the best-known gas laser is the neutral-atom helium-neon (HeNe) laser， which is an electric-discharge-excited laser involving the noble gases helium and The lasing atom is The wavelength of the transition most used is 8 nanometers; however， many helium-neon lasers operate at longer and shorter wavelengths including 3390， 1152， 612， 594， and 543 Output powers are mostly around 1 A useful gas laser for the near-ultraviolet region is the helium-cadmium (HeCd) laser， wherelasing takes place from singly ionized Wavelengths are 325 and 442 nm， with powers up to 150 mW The argon ion laser provides continuous-wave (CW) powers up to about 50 W， with principal wavelengths of 5 and 488 nm， and a number of weaker transitions at nearby The argon laser is often used to pump other lasers， most importantly tunable dye lasers and titanium:sapphire For applications requiring continuous-wave power in the red， the krypton ion laser can provide continuous-wave lasing at 1 and 4 nm (as well as 521， 568， and other wavelengths)， with powers somewhat less than those of the argon ion The carbon dioxide (CO2) molecular laser has become the laser of choice for many industrial applications， such as cutting and Short-pulsed gas lasers Some lasers can be made to operate only in a pulsed Examples of self-terminating gas lasers are the nitrogen laser (337 nm) and excimer lasers (200–400 nm) The nitrogen laser pulse duration is limited because the lower level becomes populated because of stimulated transitions from the upper lasing level， thus introducing absorption at the lasing Peak powers as large as 1 MW are possible with pulse durations of 1–10 Excimer lasers are self-terminating because lasing transitions tear apart the excimer molecules and time is required for fresh molecules to replace Solid-state lasers The term solid-state laser should logically cover all lasers other than gaseous or Nevertheless， current terminology treats semiconductor (diode) lasers separately from solid-state lasers because the physical mechanisms are somewhat With that reservation， virtually all solid-state lasers are optically Historically， the first laser was a single crystal of synthetic ruby， which is aluminum oxide (Al2O3 or sapphire)， doped with about 05% (by weight) chromium oxide (Cr2O3) Three important rare-earth laser systems in current use are neodymium:YAG， that is， yttrium aluminum garnet (Y3Al5O12) doped with neodymium; neodymium:glass; and erbium: Other rare earths and other host materials also find Semiconductor (diode) lasers The semiconductor laser is the most important of all lasers， both by economic standards and by the degree of its Its main features include rugged structure， small size， high efficiency， direct pumping by low-power electric current， ability to modulate its output by direct modulation of the pumping current at rates exceeding 20 GHz， compatibility of its output beam dimensions with those of optical fibers， feasibility of integrating it monolithically with other semiconductor optoelectronic devices to form integrated circuits， and a manufacturing technology that lends itself to mass See also Integrated Most semiconductor lasers are based on III–V The laser can be a simple sandwich of p- and n-type material such as gallium arsenide (GaAs) The active region is at the junction of the p and n Electrons and holes are injected into the active region from the p and n regions Light is amplified by stimulating electron-hole The mirrors comprise the cleaved end facets of the chip (either uncoated or with enhanced reflective coatings) See also Electron-hole recombination; Semiconductor; Semiconductor Monochromaticity When lasers were first developed， they were widely noted for their extreme They provided far more optical power per spectral range (as well as per angular range) than was previously It has since proven useful to relate laser frequencies to the international time standard (defined by an energy-level difference in the cesium atom)， and this was done so precisely， through the use of optical heterodyne techniques， that the standard of length was redefined in such a way that the speed of light is In addition， extremely stable and monochromatic lasers have been developed， which can be used， for example， for optical communication between remote and moving frames， such as the Moon and the E See also Frequency measurement; Heterodyne principle; Laser spectroscopy; LTunable lasers Having achieved lasers whose frequencies can be monochromatic， stable， and absolute (traceable to the time standard)， the next goal is Most lasers allow modest tuning over the gain bandwidth of their amplifying However， the laser most widely used for wide tunability has been the (liquid) dye This laser must be optically pumped， either by a flash lamp or by another laser， such as the argon ion Considerable engineering has gone into the development of systems to rapidly flow the dye and to provide wavelength About 20 different dyes are required to cover the region from 270 to 1000 Free-election lasers The purpose of the free-electron laser is to convert the kinetic energy in an electron beam to electromagnetic Since it is relatively simple to generate electron beams with peak powers of 1010 W， the free-electron laser has the potential for providing high optical power， and since there are no prescribed energy levels， as in the conventional laser， the free-electron laser can operate over a broad spectral
基于WIN CE的ADSL线路参数研究ADSL line parameters research based on WIN CE _EMC&dq=ADSL&printsec=frontcover&source=web&ots=oJXbatzNWO&sig=fyomvlADYeB7NRS2gjTJAfpSapQ--------------Windows CE (also known officially as Windows Embedded CE since version 0[2][3]， and sometimes abbreviated WinCE) is a variation of Microsoft's Windows operating system for minimalistic computers and embedded Windows CE is a distinctly different kernel， rather than a trimmed-down version of desktop W It is not to be confused with Windows XP Embedded which is NT- It is supported on Intel x86 and compatibles， MIPS， ARM， and Hitachi SuperH FeaturesWindows CE is optimized for devices that have minimal storage—a Windows CE kernel may run in under a megabyte of Devices are often configured without disk storage， and may be configured as a “closed” system that does not allow for end-user extension (for instance， it can be burned into ROM) Windows CE conforms to the definition of a real-time operating system， with a deterministic interrupt It supports 256 priority levels and uses priority inheritance for dealing with priority The fundamental unit of execution is the This helps to simplify the interface and improve execution Microsoft has stated that the ‘CE’ is not an intentional initialism， but many people believe CE stands for ‘Consumer Electronics’ or ‘Compact Edition’; users often disparagingly called it “Wince”[4] Microsoft says it implies a number of Windows CE design precepts， including “Compact， Connectable， Compatible， Companion， and E”[5] The first version， known during development under the codename “Pegasus”， featured a Windows-like GUI and a number of Microsoft's popular applications， all trimmed down for smaller storage， memory， and speed of the palmtops of the Since then， Windows CE has evolved into a component-based， embedded， real-time operating It is no longer targeted solely at hand-held Many platforms have been based on the core Windows CE operating system， including Microsoft's AutoPC， Pocket PC 2000， Pocket PC 2002， Windows Mobile 2003， Windows Mobile 2003 SE， Windows Mobile 0， Windows Mobile 6， Smartphone 2002， Smartphone 2003 and many industrial devices and embedded Windows CE even powered select games for the Sega Dreamcast， was the operating system of the controversial Gizmondo handheld， and can partially run on modified Microsoft Xbox game A distinctive feature of Windows CE compared to other Microsoft operating systems is that large parts of it are offered in source code First， source code was offered to several vendors， so they could adjust it to their Then products like Platform Builder (an integrated environment for Windows CE OS image creation and integration， or customized operating system designs based on CE) offered several components in source code form to the general However， a number of core components that do not need adaptation to specific hardware environments (other than the CPU family) are still distributed in binary form Development toolsVisual StudioLate versions of Microsoft Visual Studio support projects for Windows CE / Windows Mobile， producing executable programs and platform images either as an emulator or attached by cable to an actual mobile A mobile device is not necessary to develop a CE The NET Compact Framework supports a subset of the NET Framework with projects in C# and VBNET， but not Managed C++Platform BuilderThis programming tool is used for building the platform (BSP + Kernel)， device drivers (shared source or custom made) and also the This is a one step environment to get the system up and One can also use Platform Builder to export an SDK (standard development kit) for the target microprocessor (SuperH， x86， MIPS， ARM ) to be used with another associated tool set named Embedded Visual C++ (eVC)The Embedded Visual C++ tool is for development of embedded application for Windows CE based This tool can be used standalone using the SDK exported from Platform Builder or using the Platform Builder using the Platform Manager connectivity Relationship to Windows Mobile， Pocket PC， and SmartPhoneOften Windows CE， Windows Mobile， and Pocket PC are used This practice is not entirely Windows CE is a modular/componentized operating system that serves as the foundation of several classes of Some of these modules provide subsets of other components' features ( varying levels of windowing support; DCOM vs COM)， others which are mutually exclusive (Bitmap or TrueType font support)， and others which add additional features to another One can buy a kit (the Platform Builder) which contains all these components and the tools with which to develop a custom Applications such as Excel Mobile/Pocket Excel are not part of this The older Handheld PC version of Pocket Word and several other older applications are included as samples， Windows Mobile is best described as a subset of platforms based on a Windows CE Currently， Pocket PC (now called Windows Mobile Classic)， SmartPhone (Windows Mobile Standard)， and PocketPC Phone Edition (Windows Mobile Professional) are the three main platforms under the Windows Mobile Each platform utilizes different components of Windows CE， as well as supplemental features and applications suited for their respective Pocket PC and Windows Mobile is a Microsoft-defined custom platform for general PDA use， and consists of a Microsoft-defined set of minimum profiles (Professional Edition， Premium Edition) of software and hardware that is The rules for manufacturing a Pocket PC device are stricter than those for producing a custom Windows CE-based The defining characteristics of the Pocket PC are the digitizer as the primary Human Interface Device and its extremely portable The SmartPhone platform is a feature rich OS and interface for cellular phone SmartPhone offers productivity features to business users， such as email， as well as multimedia capabilities for The SmartPhone interface relies heavily on joystick navigation and PhonePad Devices running SmartPhone do not include a touchscreen SmartPhone devices generally resemble other cellular handset form factors， whereas most Phone Edition devices use a PDA form factor with a larger Windows Mobile 5 supports USB 0 and new devices running this OS will also conform to the USB Mass Storage Class， meaning the storage on PPC can be accessed from any USB-equipped PC， without requiring any extra software， except requiring a compliant In other words， you can use it as a flash Competing productsCompetitors to consumer CE based PDA platforms like Pocket PC – the main application of Windows CE – are Java， Symbian OS， Palm OS， iPhone OS and Linux based packages like Qtopia Embedded Linux environment from Trolltech， Convergent Linux Platform from a La Mobile， and Access Linux Platform from Orange and AThe secondary usage of CE is in devices in need of graphical user interfaces， (point of sale terminals， media centers， web tablets， thin clients) as the main selling point CE is the look and feel being similar to desktop W The competition is Windows XP， Linux and graphical packages for simpler embedded operating Being an RTOS， Windows CE is also theoretically a competitor to any realtime operating system in the embedded space， like VxWorks， ITRON or eC The dominating method， however， of mixing Windows look and feel with realtime on the same hardware， is to run double operating systems using some virtualization technology， like TRANGO Hypervisor from TRANGO Virtual Processors or Intime from TenAsys in the case of Windows， and OS Ware from VirtualLogix， Padded Cell from Green Hills Software， OKL4 from Open Kernel Labs， TRANGO Hypervisor from TRANGO Virtual Processors， RTS Hypervisor from Real-Time Systems or PikeOS from Sysgo， in case of the ---------Asymmetric Digital Subscriber Line (ADSL) is a form of DSL， a data communications technology that enables faster data transmission over copper telephone lines than a conventional voiceband modem can It does this by utilizing frequencies that are not used by a voice telephone A splitter - or microfilter - allows a single telephone connection to be used for both ADSL service and voice calls at the same Because phone lines vary in quality and were not originally engineered with DSL in mind， it can generally only be used over short distances， typically less than 3mi (5 km) [William Stallings' book]At the telephone exchange the line generally terminates at a DSLAM where another frequency splitter separates the voice band signal for the conventional phone Data carried by the ADSL is typically routed over the telephone company's data network and eventually reaches a conventional internet In the UK under British Telecom the data network in question is its ATM network which in turn sends it to its IP network IP CThe distinguishing characteristic of ADSL over other forms of DSL is that the volume of data flow is greater in one direction than the other， it is Providers usually market ADSL as a service for consumers to connect to the Internet in a relatively passive mode: able to use the higher speed direction for the "download" from the Internet but not needing to run servers that would require high speed in the other There are both technical and marketing reasons why ADSL is in many places the most common type offered to home On the technical side， there is likely to be more crosstalk from other circuits at the DSLAM end (where the wires from many local loops are close to each other) than at the customer Thus the upload signal is weakest at the noisiest part of the local loop， while the download signal is strongest at the noisiest part of the local It therefore makes technical sense to have the DSLAM transmit at a higher bit rate than does the modem on the customer Since the typical home user in fact does prefer a higher download speed， the telephone companies chose to make a virtue out of necessity， hence ADSL On the marketing side， limiting upload speeds limits the attractiveness of this service to business customers， often causing them to purchase higher cost Digital Signal 1 services In this fashion， it segments the digital communications market between business and home usersHow ADSL worksOn the wireCurrently， most ADSL communication is full Full duplex ADSL communication is usually achieved on a wire pair by either frequency division duplex (FDD)， echo canceling duplex (ECD)， or time division duplexing (TDD) FDM uses two separate frequency bands， referred to as the upstream and downstream The upstream band is used for communication from the end user to the telephone central The downstream band is used for communicating from the central office to the end With standard ADSL (annex A)， the band from 875 kHz to 138 kHz is used for upstream communication， while 138 kHz – 1104 kHz is used for downstream Each of these is further divided into smaller frequency channels of 3125 kH During initial training， the ADSL modem tests which of the available channels have an acceptable signal-to-noise The distance from the telephone exchange， noise on the copper wire， or interference from AM radio stations may introduce errors on some By keeping the channels small， a high error rate on one frequency thus need not render the line unusable: the channel will not be used， merely resulting in reduced throughput on an otherwise functional ADSL Vendors may support usage of higher frequencies as a proprietary extension to the However， this requires matching vendor-supplied equipment on both ends of the line， and will likely result in crosstalk issues that affect other lines in the same There is a direct relationship between the number of channels available and the throughput capacity of the ADSL The exact data capacity per channel depends on the modulation method [edit] ModulationADSL initially existed in two flavours (similar to VDSL)， namely CAP and DMT CAP was the de facto standard for ADSL deployments up until 1996， deployed in 90 percent of ADSL installs at the However， DMT was chosen for the first ITU-T ADSL standards， G1 and G2 (also called Gdmt and Glite respectively) Therefore all modern installations of ADSL are based on the DMT modulation Annexes J and M shift the upstream/downstream frequency split up to 276 kHz (from 138 kHz used in the commonly deployed annex A) in order to boost upstream Additionally， the "all-digital-loop" variants of ADSL2 and ADSL2+ (annexes I and J) support an extra 256 kbit/s of upstream if the bandwidth normally used for POTS voice calls is allocated for ADSL While the ADSL access utilizes the 1 MHz band， ADSL2+ utilizes the 2 MHz The downstream and upstream rates displayed are theoretical Note also that because Digital subscriber line access multiplexers and ADSL modems may have been implemented based on differing or incomplete standards some manufacturers may advertise different For example， Ericsson has several devices that support non-standard upstream speeds of up to 2 Mbit/s in ADSL2 and ADSL2+[edit] Installation issuesDue to the way it uses the frequency spectrum， ADSL deployment presents some It is necessary to install appropriate frequency filters at the customer's premises， to avoid interferences with the voice service， while at the same time taking care to keep a clean signal level for the ADSL In the early days of DSL， installation required a technician to visit the A splitter was installed near the demarcation point， from which a dedicated data line was This way， the DSL signal is separated earlier and is not attenuated inside the customer However， this procedure is costly， and also caused problems with customers complaining about having to wait for the technician to perform the As a result， many DSL vendors started offering a self-install option， in which they ship equipment and instructions to the Instead of separating the DSL signal at the demarcation point， the opposite is done: the DSL signal is "filtered off" at each phone outlet by use of a low pass filter， also known as This method does not require any rewiring inside the customer A side effect of the move to the self-install model is that the DSL signal can be degraded， especially if more than 5 voiceband devices are connected to the The DSL signal is now present on all telephone wiring in the building， causing attenuation and A way to circumvent this is to go back to the original model， and install one filter upstream from all telephone jacks in the building， except for the jack to which the DSL modem will be Since this requires wiring changes by the customer and may not work on some household telephone wiring， it is rarely It is usually much easier to install filters at each telephone jack that is in
