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1、Optical Fiber Communication -introductionForewordThe use of light to send messages is not new .Fires were used for signaling in biblical times, smoke signals have been used for thousands of years and flashing lights have been used to communicate between warships at sea since the days of Lord Nelson.
2、The idea of using glass fiber to carry an optical communication signal originated with Alexander Graham Bell. However this idea had to wait some 80 years for better glasses and low-cost electronics for it to become useful in practical situations.The predominant use of optical technology is for trans
3、mission of data at high speed. Optical fibers replace electric wire in communications systems and nothing much else changes. Perhaps this is not quite fair. The very speed and quality of optical communications systems has itself predicated the development of a new type of electronic communications i
4、tself designed to be run on optical connections. ATM (Asynchronous Transfer Mode) and SDH (Synchronous Digital Hierarchy) technologies are good examples of the new type of systems.It is important to realize that optical communications is not like electronic communications. While it seems that light
5、travels in a fiber much like electricity does in a wire this is very misleading. Light is an electromagnetic wave and optical fiber is a waveguide. Everything to do with transport of the signal even to simple things like coupling (joining) two fibers into one is very different from what happens in t
6、he electronic world. The two fields (electronics and optics) while closely related employ different principles in different ways.Some people look ahead to “true” optical networks. These will be networks where routing is done optically from one end-user to another without the signal ever becoming ele
7、ctronic. Indeed some experimental local area (LAN) and metropolitan area (MAN) networks like this have been built. In 1998 optically routed nodal wide area networks are imminently feasible and the necessary components to build them are available. However, no such networks have been deployed operatio
8、nally yet.In 1998 the “happening” area in optical communications was Wavelength Division Multiplexing (WDM). This is the ability to send many (perhaps up to 1000) independent optical channels on a single fiber. The first fully commercial WDM products appeared on the market in 1996. WDM is a major st
9、ep toward fully optical networking. 1. Transmitting Light on a FiberAn optical fiber is a very thin strand of silica glass in geometry quite like a human hair. In reality it is a very narrow, very long glass cylinder with special characteristics. When light enters one end of the fiber, it travels (c
10、onfined within the fiber) until it leaves the fiber at the other end. Two critical factors stand out:Very little light is lost in its journey along the fiber.Fiber can bend around corners and the light will stay within it and be guided around the corners.An optical fiber consists of two parts: the c
11、ore and the cladding. The core is a narrow cylindrical strand of glass and the cladding is a tubular jacket surrounding it. The core has a (slightly) higher refractive index than the cladding. This means that the boundary (interface) between the core and the cladding acts as a perfect mirror. Light
12、traveling along the core is confined by the mirror to stay within it-even when the fiber bends around a corner.When light is transmitted on a fiber, the most important consideration is “what kind of light?” The electromagnetic radiation that we call light exists at many wavelengths. These wavelength
13、s go from invisible infrared through all the colours of the visible spectrum to invisible ultraviolet. Because of the attenuation characteristics of fiber, we are only interested in infrared “light” for communication applications. This light is usually invisible, since the wavelengths used are usual
14、ly longer than the visible limit of around 750 nanometers ( nm ) .If a short pulse of light from a source such as a laser or an LED is sent down a narrow fiber, it will be changed (degraded) by its passage down the fiber. It will emerge (depending on the distance) much weaker, lengthened in time (“s
15、meared out”), and distorted in other ways.2. Optical Transmission System ConceptsThe basic components of an optical communication system are optical transmitter and receiver,Fiber jumpers,Optical,fiber splice tray Optical fiber.A serial bit stream in electrical from is presented to a modulator, whic
16、h encodes the data appropriately for fiber transmission.A light source (laser or Light Emitting DiodeLED) is driven by the modulator and the light focused into the fiber. The light travels down the fiber (during which time it may experience dispersion and loss of strength).At the receiver end the li
17、ght is fed to a detector and converted to electrical form. The signal is then amplified and fed to another detector, which isolates the individual state changes and their timing. It then decodes the sequence of state changes and reconstructs the original bit stream.The timed bit stream so received m
18、ay then be fed to a using device.Optical communication has many well-known advantages.Weight and SizeFiber cable is significantly smaller and lighter than electrical cables to do the same job. In the wide area environment a large coaxial cable system can easily involve a cable of several inches in d
19、iameter and weighing many pounds per foot. A fiber cable to do the same job could be less than one half an inch in diameter and weigh a few ounces per foot. This means that the cost of laying the cable is dramatically reduced.Material Cost Fiber cable costs significantly less than copper cable for t
20、he same transmission capacity.Information CapacityThe idea rate of system in 1998 was generally 150 or 620Mbps on a single (unidirectional) fiber. This is because these systems were installed in past years. The usual rate for new systems is 2.4Gbps or even 10Gbps. This is very high in digital transm
21、ission terms.In telephone transmission terms the very best coaxial cable systems give about 2,000 analog voice circuits. A 150Mbps fiber connection gives just over 2,000 digital telephone (64kbps) connections. But the 150Mbps fiber is at a very early stage in the development of fiber optical systems
22、. The coaxial cable system with which it is being compared is much more costly and has been developed to its fullest extent.Fiber technology is still in its infancy. Using just a single channel per fiber, researchers have trial systems in operation that communicate at speeds of 100Gbps.By sending ma
23、ny (“wavelength division multiplexed ”) channels on a single fiber, we can increase this capacity a hundred and perhaps a thousand times. Recently researchers at NEC reported a successful experiment where 132 optical channels of 20Gbps each were carried over 120km. This is 2.64 terabits per second!
24、This is enough capacity to carry about 30 million uncompressed telephone calls (at 64kbps per channel). Thirty million calls is about the maximum number of calls in progress in the world at any particular moment in time. That is to say, we could carry the worlds peak telephone traffic over one pair
25、of fibers. Most practical fiber systems dont attempt to do this because it costs less to put multiple fibers in a cable than to use sophisticated multiplexing technology.No Electrical ConnectionThis is an obvious point but nevertheless a very important one . Electrical connections have problems. In
26、electrical systems there is always the possibility of “ground loops” causing a serious problem,especially in the LAN or computer channel environment . When you communicate electrically you often have to connect the grounds to one another or at least go to a lot of trouble to avoid making this connec
27、tion. One little known problem is that there is often a voltage potential difference between “ground” at different locations. The author has observed as much as 3 volts difference in ground potential between adjacent buildings (this was a freak situation). It is normal to observe 1or 2 volt differen
28、ces over distance of a kilometer or so.With shielded cable there can be a problem if you earth the shields at both ends of the connection.Optical connection is very safe. Electrical connections always have to be protected from high voltages because of the danger to people touching the wire . In some
29、 tropical regions of the world, lightning poses a severe hazard even to buried telephone cables! Of cause, optical fiber isnt subject to lightning problems but it must be remembered that sometimes optical cables carry wires within them for strengthening or to power repeaters . These wires can be a t
30、arget for lightning.No Electromagnetic InterferenceBecause the connection is not electrical, you can neither pick up nor create electrical interference (the major source of noise). This is one reason that optical communication has so few errors. There are very few source of things that can distort o
31、r interfere with the signal. In a building this means that fiber cables can be placed almost anywhere electrical cables would have problems, (foe example near a lift motor or in a cable duct with heavy power cables). In an industrial plant such as a steel mill, this gives much greater flexibility in
32、 cabling than previously available.In the wide area networking environment there is much greater flexibility in route selection. Cables may be located near water or power lines without risk to people or equipment.Distances between RegeneratorsAs a signal travels along a communication line it loses s
33、trength (is attenuated) and picks up noise. The traditional way to regenerate the signal, restoring its power and removing the noise, is to use either a repeater or an amplifier. Indeed it is the use of repeaters to remove noise that gives digital transmission its high quality.In long-line optical t
34、ransmission cables now in use by the telephone companies, the repeater spacing is typically 40 kilometers. This compares with 12 km for the previous coaxial cable electrical technology. The number of required repeaters and their spacing is a major factor in system cost.Open Ended Capacity The maximu
35、m theoretical capacity of installed fiber is very great (almost infinite). This means that additional capacity can be had on existing fibers as new technology becomes available. All that must be done is change the equipment at either end and change or upgrade the regenerators.Better SecurityIt is po
36、ssible to tap fiber optical cable. But it is very difficult to do and the additional loss caused by the tap is relatively easy to detect.There is an interruption to service while the tap is interested and this can alert operational staff to the situation. In addition, there are fewer access points w
37、here an intruder can gain the kind of access to a fiber cable necessary to insert a tap.3. Wavelength Division MultiplexingWavelength Division Multiplexing (WDM) is the basic technology of optical networking. It is a technique for using a fiber (or optical device) to carry many separate and independ
38、ent optical channels. The principle is identical to that used when we tune our television receiver to one of many TV channels. Each channel is transmitted at a different radio frequency and we select between them using a “tuner” which is just a resonant circuit within the TV set. Of course wavelengt
39、h in the optical world is just the way we choose to refer to frequency and optical WDM is quite identical to radio FDM.There are many varieties of WDM. A simple form can be constructed using 1310nm as one wavelength and 1550 as the other or 850 and 1310. This type of WDM can be built using relativel
40、y simple and inexpensive components and some applications have been in operation for a number of years using this principle.Wavelength selective couplers are used both to mix (multiplex) and to separate (de-multiplex) the signals. The distinguishing characteristic here is the very wide separation of
41、 wavelengths used (different bands rather than different wavelengths in the same band).There are many variations around on this very simple theme. Some systems use a signal fiber bidirectionally while others use separate fibers for each direction . Other systems use different wavelength bands from t
42、hose illustrated in the figure (1310and 1550 for example). The most common systems run at very low data rates. Common application areas are in video transport for security monitoring and in plant process control.Dense WDM however is another thing.Dense WDM refers to the close spacing of channels.Sad
43、ly，denseis a qualitative measure and just what dense means is largely in the mind of the description.Others use the term to distinguish systems where the wavelength spacing is 1nm per channel or less. Each optical channel is allocated its own wavelength or rather range of wavelengths.A typical optic
44、al channel might be 1nm wide. This channel is really a wavelength range within which the signal must stay. It is normally much wider than the signal itself. The width of a channel depends on many things such as the modulated line width of the transmitter,its stability and the tolerances of the other
45、 components in the system. In practical terms the transmitter is always a laser.It must have a line width which (after modulation) fits easily within its allocated band. It must not go outside the allocated band so it should have chirp and drift characteristics that ensure this. Depending on the wid
46、th of the allocated band,these characteristics dont need to be the most perfect obtainable.However they do have to be such that the signal stays where it is supposed to be. The receiver is relatively straightforward and is generally the same as a non-WDM receiver .This is because the signal has been
47、 de-multiplexed before it arrives at the detector. 光纤通信简介前言 使用光来传送信息并不新鲜。旧约时代就开始用火来传递信息，烟雾信号已使用千年。从纳尔逊勋爵时代开始，海上舰船间的通信就采用闪烁的灯光。 Alexander Graham Bell 最先提出用玻璃纤维来传送光通信信号。但直到80多年后，有了更好的玻璃纤维及低成本电子设备，该想法才真正切实可行。光技术主要用于高速数据传输。除了用光纤代替电缆外，与其他通信系统没有什么区别。不过，这样说也许并不太公平。光通信系统的高速及高质量的传输预示了以光纤作为传输媒介的电通信系统的新的发展方向。异
48、步传输模式（Asynchronous Transfer Mode,ATM ）和同步数字体系（Synchronous Digital Hierarchy, SDH）技术就是这种新系统的很好的例子。 认识到光通信不同于电通信很重要。人们很容易误解成光在光纤上传输就像电在电缆上传输一样。光是一种电磁波而光纤是一种波导。在光的世界里，任何的和信号的传输相关的方面（即使是诸如将两条纤维连在一起这样简单的事情）和电的世界中都完全不同。虽然电和光这两个领域紧密相关，但却在不同的方面采用不同的原理。有人预测未来将会出现“纯”光网络。这些网络中，完全通过光路由将信号从发射端传输到接收端，无须转换成电信号。实际上
49、，已经建立了一些这样的实验局域网和广域网。1998年，光路由节点广域网已经完全可行，且建造该网络的必要部件都是现成的。不过，这样的网络至今都没有部署运营。 1998年，光通信中的热门领域就是波分复用。该技术能够在一条单独的光纤上传输很多（也许超过1000个）独立的光信道。1996年，市场上出现了第一个完全商业化的波分复用设备。波分复用是迈向全光网络互联的重要的一步。1. 光在光纤上传输 光纤是非常细的硅玻璃线，很像人的头发。实际上，光线是具有特殊性质的又窄又长的玻璃柱体。光从光纤的一端进入后，将一直在光纤内传输，直到从另一端离开。光纤有两个关键因素： 1、光在光纤中传输时损失非常小。 2、光在拐弯处能够弯折，折弯后光仍旧在光纤中传输。 光纤由两部分组成：纤芯和纤包。纤芯是一个狭窄的圆柱形玻璃纤，纤包是围绕在其上的管状套。纤芯的折射系数逼纤包大一些，这说明纤芯和纤包间的边界（接口）就像一个平面镜。在纤芯