光纤通信英文版Chapter 7
光纤通信系统Optical_Fiber_Communications_英文资料及中文翻译
光纤通信系统Optical Fiber Communications英文资料及中文翻译Communication may be broadly defined as the transfer of information from one point to another .When the information is to be conveyed over any distance a communication system is usually required .Within a communication system the information transfer is frequently achieved by superimposing or modulating the information on to an electromagnetic wave which acts as a carrier for the information signal .This modulated carrier is then transmitted to the required destination where it is received and the original information signal is obtained by demodulation .Sophisticated techniques have been developed for this process by using electromagnetic carrier waves operating at radio requites as well as microwave and millimeter wave frequencies.The carrier maybe modulated by using either optical an analog digital information signal.. Analog modulation involves the variation of the light emitted from the optical source in a continuous manner. With digital modulation, however, discrete changes in the length intensity are obtained (i.e. on-off pulses). Although often simpler to implement, analog modulation with an optical fiber communication system is less efficient, requiring a far higher signal to noise ratio at the receiver than digital modulation. Also, the linearity needed for analog modulation is mot always provided by semiconductor optical source, especially at high modulation frequencies .For these reasons ,analog optical fiber communications link are generally limited to shorter distances and lower bandwidths than digital links .Initially, the input digital signal from the information source is suitably encoded for optical transmission .The laser drive circuit directly modulates the intensity of the semiconductor last with the encoded digital signal. Hence a digital optical signal is launched into the optical fiber cable .The avalanche photodiode detector (APD) is followed by a front-end amplifier and equalizer or filter to provide gain as well as linear signal processing and noise bandwidth reduction. Finally ,the signal obtained isdecoded to give the original digital information .Generating a Serial SignalAlthough a parallel input-output scheme can provide fast data transfer and is simple in operation, it has the disadvantage of requiring a large number of interconnections. As an example typical 8 bit parallel data port uses 8 data lines, plus one or two handshake lines and one or more ground return lines. It is fairly common practice to provide a separate ground return line for each signal line, so an 8 bit port could typically use a 20 core interconnection cable. Whilst such a multi way cable is quite acceptable for short distance links, up to perhaps a few meters, it becomes too expensive for long distance links where, in addition to the cost of the multiword cable, separate driver and receiver circuits may be required on each of the 10 signal lines. Where part of the link is to be made via a radio link, perhaps through a space satellite, separate radio frequency channels would be required for each data bit and this becomes unacceptable.An alternative to the parallel transfer of data is a serial in which the states of the individual data bits are transmitted in sequence over a single wire link. Each bit is allocated a fixed time slot. At the receiving end the individual bit states are detected and stored in separate flip-flop stages, so that the data may be reassembled to produce a parallel data word. The advantage of this serial method of transmission is that it requires only one signal wire and a ground return, irrespective of the number of bits in the data word being transmitted. The main disadvantage is that the rate at which data can be transferred is reduced in comparison with a parallel data transfer, since the bits are dealt with in sequence and the larger the number of bits in the word, the slower the maximum transfer speed becomes. For most applications however, a serial data stream can provide a perfectly adequate data transfer rate . This type of communication system is well suited for radio or telephone line links, since only one communication channel is required to carry the data.We have seen that in the CPU system data is normally transferred in parallel across the main data bus, so if the input -output data is to be in serial form, then a parallel to serial data conversion process is required between the CPU data bus andthe external I/O line. The conversion from parallel data to the serial form could be achieved by simply using a multiplexed switch, which selects each data bit in turn and connects it to the output line for a fixed time period. A more practical technique makes use of a shift register to convert the parallel data into serial form.A shift register consists of a series of D type flip-flops connected in a chain, with the Q output of one flip-flop driving the D input of the next in the chain. All of the flip-flops ate clocked simultaneously by a common clock pulse, when the clock pulse occurs the data stored in each flip-flop is transferred to the next flip-flop to the right in the chain. Thus for each clock pulse the data word is effectively stepped along the shift register by one stage, At the end of the chain the state of the output flip-flop will sequence through the states of the data bits originally stored in the register. The result is a serial stream of data pulses from the end of the shift register.In a typical parallel to serial conversion arrangement the flip-flops making up the shift register have their D input switchable. Initially the D inputs are set up in a way so that data can be transferred in parallel from the CPU data bus into the register stages. Once the data word has been loaded into the register the D inputs are switched so that the flip-flops from a shift register .Now for each successive clock pulse the data pattern is shifted through the register and comes out in serial form at the right hand end of the register.At the receiving end the serial data will usually have to be converted back into the parallel form before it can be used. The serial to parallel conversion process can also be achieved by using a shift register .In this case the serial signal is applied to the D input of the stage at the left hand end of the register. As each serial bit is clocked into the register the data word again moves step by step to the right, and after the last bit has been shifted in the complete data word will be assembled within the register .At this point the parallel data may be retrieved by simply reading out the data from individual register stages in parallel It is important that the number of stages in the shift register should match the number of bits in the data word, if the data is to be properly converted into parallel form.To achieve proper operation of the receiving end of a serial data link, it isimportant that the clock pulse is applied to the receive shift register at a time when the data level on the serial line is stable. It is possible to have the clock generated at either end of the link, but a convenient scheme is to generate the clock signal at the transmitting end (parallel-serial conversion )as the master timing signal. To allow for settling time and delays along the line, the active edge of the clock pulse at the receive end is delayed relative to that which operates the transmit register. If the clock is a square wave the simples approach might be to arrange that the transmit register operates on the rising edge of the clock wave, and the receive register on the falling edge, so that the receiver operates half a clock period behind the transmitter .If both registers operate on arising edge, the clock signal from the transmitter could be inverted before being used to drive the receive shifty register.For an 8 bit system a sequence of 8 clock pulses would be needed to send the serial data word .At the receiving end the clock pulses could be counted and when the eighth pulse is reached it might be assumed that the data in the receive register is correctly positioned, and may be read out as parallel data word .One problem here is that, if for some reason the receive register missed a clock pulse ,its data pattern would get out of step with the transmitted data and errors would result. To overcome this problem a further signal is required which defines the time at which the received word is correctly positioned in the receive shift register and ready for parallel transfer from the register .One possibility is to add a further signal wire along which a pulse is sent when the last data bit is being transmitted, so that the receiver knows when the data word is correctly set up in its shift register. Another scheme might be to send clock pulses only when data bits are being sent and to leave a timing gap between the groups of bits for successive data words. The lack of the clock signal could then be detected and used to reset the bit counter, so that it always starts at zero at the beginning of each new data word.Serial and Parallel Data lion is processed. Serial indicates that the information is handled sequentially, similar to a group of soldiers marching in single file. In parallel transmission the info The terms serial and parallel are often used in descriptions of data transmission techniques. Both refer to the method by which information isdivided in to characters, words, or blocks which are transmitted simultaneously. This could be compared to a platoon of soldiers marching in ranks.The output of a common type of business machine is on eight—level punched paper tape, or eight bits of data at a time on eight separate outputs. Each parallel set of eight bits comprises a character, and the output is referred to as parallel by bit, serial by character. The choice of cither serial or parallel data transmission speed requirements.Business machines with parallel outputs, how—ever, can use either parallel outputs, how—ever, can use either direct parallel data trans—mission or serial transmission, with the addition of a parallel—to—serial converter at the interface point of the business machine and the serial data transmitter. Similarly, another converter at the receiving terminal must change the serial data back to the parallel format.Both serial and parallel data transmission systems have inherent advantages which are some—what different. Parallel transmission requires that parts of the available bandwidth be used as guard bands for separating each of the parallel channels, whereas serial transmission systems can use the entire linear portion of the available band to transmit data, On the other hand, parallel systems are convenient to use because many business machines have parallel inputs and outputs. Though a serial data set has the added converters for parallel interface, the parallel transmitter re—quires several oscillators and filters to generate the frequencies for multiplexing each of the side—by—side channels and, hence, is more susceptible to frequency error.StandardsBecause of the wide variety of data communications and computer equipment available, industrial standards have been established to provide operating compatibility. These standards have evolved as a result of the coordination between manufacturers of communication equipment and the manufacturers of data processing equipment. Of course, it is to a manufacturer’s advantage to provide equipment that isuniversally acceptable. It is also certainly apparent that without standardization intersystem compatibility would be al—most impossible.Organizations currently involved in uniting the data communications and computer fields are the CCITT, Electronic Industries Association (EIA), American Standards Association (ASA), and IEEE.A generally accepted standard issued by the EIA, RS—232—B, defines the characteristics of binary data signals, and provides a standard inter—face for control signals between data processing terminal equipment and data communications equipment. As more and more data communications systems are developed, and additional ways are found to use them, the importance ways are found to use them, the importance of standards will become even more significant.Of the most important considerations in transmitting data over communication systems is accuracy. Data signals consist of a train of pulses arranged in some sort of code. In a typical binary system, for example, digits 1 and 0 are represented by two different pulse amplitudes. If the amplitude of a pulse changes beyond certain limits during transmission, the detector at the receiving end may produce the wrong digit, thus causing an error.It is very difficult in most transmission systems to completely avoid. This is especially true when transmission system designed for speech signals. Many of the inherent electrical characteristics of telephone circuits have an adverse effect on digital signals.Making the circuits unsatisfactory for data transmission—especially treated before they can be used to handle data at speeds above 2000 bits per second.V oice channels on the switched (dial—up) telephone network exhibit certain characteristics which tend to distort typical data signal waveforms. Since there is random selection of a particular route for the data signal with each dialed connection, transmission parameters will generally change, sometimes upsetting the effect of built—in compensationNetworks. In addition, the switched network cannot be used of for large multipleaddress data systems using time sharing. Because of these considerations, specially treated voice bandwidth circuits are made available for data use. The characteristics and costs of these point—to—point private lines are published in document called tariffs, which are merely regulatory agreements reached by the FCC, state public utilities commissions, and operating telephone companies regarding charges for particular types of telephone circuits. The main advantage of private or dedicated facilities is that transmission characteristics are fixed and remain so for all data communications operations.Correlative TechniqueCorrelative data transmission techniques, particularly the Duobinary principle, have aroused considerable interest because of the method of converting a binary signal into three equidistant levels. This correlative scheme is accomplished in such a manner that the predetermined level depends on past signal history, forming the signal so that it never goes from one level extreme to another in one bit interval.The most significant property of the Duobinary process is that it affords a two—to—one bandwidth compression relative to binary signaling, or equivalently twice the speed capability in bits per second for a fixed bandwidth. The same speed capability for a multilevel code would normally require four levels, each of which would represent two binary digits.The FutureIt is universally recognized that communication is essential at every level of organization. The United States Government utilizes vast communications network for voice as well as data transmission. Likewise, business need communications to carry on their daily operations.The communications industry has been hard at work to develop systems that will transmit data economically and reliably over both private—line and dial up telephone circuits. The most ardent trend in data transmission today is toward higher speeds over voice—grade telephone channels. New transmission and equalization techniques now being investigated will soon permit transmitting digital data over telephone channels at speeds of 4800 bits per second or higher.To summarize: The major demand placed on telecommunications systems is for more information-carrying capacity because the volume of information produced increases rapidly. In addition, we have to use digital technology for the high reliability and high quality it provides in the signal transmission. However, this technology carries a price: the need for higher information-carrying capacity.The Need for Fiber-Optic Communications Systems The major characteristic of a telecommunications system is unquestionably its information-carrying capacity, but there are many other important characteristics. For instance, for a bank network, security is probably more important than capacity. For a brokerage house, speed of transmission is the most crucial feature of a network. In general, though, capacity is priority one for most system users. And there’s the rub. We cannot increase link capacity as much as we would like. The major limit is shown by the Shannon-Hartley theorem,Where C is the information-carrying capacity(bits/sec), BW is the link bandwidth (Hz=cycles/sec), and SNR is the signal-to-noise power ratio.Formula 1.1 reveals a limit to capacity C; thus, it is often referred to as the “ Shannon limit.” The formula, which comes from information theory, is true regardless of specific technology. It was first promulgated in 1948 by Claude Shannon, a scientist who worked at Bell Laboratories. R. V. L. Hartley, who also worked at Bell Laboratories, published a fundamental paper 20 years earlier, a paper that laid important groundwork in information theory, which is why his name is associated with Shannon’s formula.The Shannon-Hartley theorem states that information-carrying capacity is proportional to channel bandwidth, the range of frequencies within which the signals can be transmitted without substantial attenuation.What limits channel bandwidth? The frequency of the signal carrier. The higher the carrier’s frequency, the greater the channel bandwidth and the higher the information-carrying capacity of the system. The rule of thumb for estimating possible order of values is this: Bandwidth is approximately 10 percent of the carrier-signal frequency. Hence, if a microwave channel uses a 10-GHz carrier signal.Then its bandwidth is about 100 MHz.A copper wire can carry a signal up to 1 MHz over a short distance. A coaxial cable can propagate a signal up to 100 MHz. Radio frequencies are in the range of 500 KHz to 100 MHz. Microwaves, including satellite channels, operate up to 100 GHz. Fiber-optic communications systems use light as the signal carrier; light frequency is between 100 and 1000 THz; therefore, one can expect much more capacity from optical systems. Using the rule of thumb mentioned above, we can estimate the bandwidth of a single fiber-optic communication link as 50 THz.To illustrate this point, consider these transmission media in terms of their capacity to carry, simultaneously, a specific number of one-way voice channels. Keep in mind that the following precise value. A single coaxial cable can carry up to 13,000 channels, a microwave terrestrial link up to 20,000 channels, and a satellite link up to 100,000 channels. However, one fiber-optic communications link, such as the transatlantic cable TAT-13, can carry 300,000 two-way voice channels simultaneously. That’s impressive and explains why fiber-optic communications systems form the backbone of modern telecommunications and will most certainly shape its future.To summarize: The information-carrying capacity of a telecommunications system is proportional to its bandwidth, which in turn is proportional to the frequency of the carrier. Fiber-optic communications systems use light-a carrier with the highest frequency among all the practical signals. This is why fiber-optic communications systems have the highest information-carrying capacity and this is what makes these systems the linchpin of modern telecommunications.To put into perspective just how important a role fiber-optic communications will be playing in information delivery in the years ahead, consider the following statement from a leading telecommunications provider: “ The explosive growth of Internet traffic, deregulation and the increasing demand of users are putting pressure on our customers to increase the capacity of their network. Only optical networks can deliver the required capacity, and bandwidth-on-demand is now synonymous with wavelength-on-demand.” Th is statement is true not only for a specific telecommunications company. With a word change here and there perhaps, but withthe same exact meaning, you will find telecommunications companies throughout the world voicing the same refrain.A modern fiber-optic communications system consists of many components whose functions and technological implementations vary. This is overall topic of this book. In this section we introduce the main idea underlying a fiber-optic communications system.Basic Block DiagramA fiber-optic communications system is a particular type of telecommunications system. The features of a fiber-optic communications system can be seen in Figure 1.4, which displays its basic block diagram.Information to be conveyed enters an electronic transmitter, where it is prepared for transmission very much in the conventional manner-that is, it is converted into electrical form, modulated, and multiplexed. The signal then moves to the optical transmitter, where it is converted into optical detector converts the light back into an electrical signal, which is processed by the electronic receiver to extract the information and present it in a usable form (audio, video, or data output).Let’s take a simple example that involves Figures 1.1, 1.3, and 1.4 Suppose we need to transmit a voice signal. The acoustic signal (the information) is converted into electrical form by a microphone and the analog signal is converted into binary formby the PCM circuitry. This electrical digital signal modulates a light source and the latter transmits the signal as a series of light pulses over optical fiber. If we were able to look into an optical fiber, we would see light vary between off and on in accordance with the binary number to be transmitted. The optical detector converts the optical signal it receives into a set of electrical pulses that are processed by an electronic receiver. Finally, a speaker converts the analog electrical signal into acoustic waves and we can hear sound-delivered information.Figure 1.4 shows that this telecommunications system includes electronic components and optical devices. The electronic components deal with information in its original and electrical forms. The optical devices prepare and transmit the light signal. The optical devices constitute a fiber-optic communications system.TransmitterThe heart of the transmitter is a light source. The major function of a light source is to convert an information signal from its electrical form into light. Today’sfiber-optic communications systems use, as a light source, either light-emitting diodes (LEDs) or laser diodes (LDs). Both are miniature semiconductor devices that effectively convert electrical signals are usually fabricated in one integrated package. In Figure 1.4, this package is denoted as an optical transmitter. Figure 1.5 displays the physical make-up of an LED, an LD, and integrated packages.Optical fiberThe transmission medium in fiber-optic communications systems is an optical fiber. The optical fiber is the transparent flexible filament that guides light from a transmitter to a receiver. An optical information signal entered at the transmitter end of a fiber-optic communications system is delivered to the receiver end by the optical fiber. So, as with any communication link, the optical fiber provides the connection between a transmitter and a receiver and, very much the way copper wire and coaxial cable conduct an electrical signal, optical fiber “ conducts” light.The optical fiber is generally made from a type of glass called silica or, less commonly nowadays, from plastic. It is about a human hair in thickness. To protect very fragile optical fiber from hostile environments and mechanical damage, it is usually enclosed in a specific structure. Bare optical fiber, shielded by its protective coating, is encapsulated use in a host of applications, many of which will be covered in subsequent chaptersReceiver The key component of an optical receiver is its photodetector. The major function of a photodetector is to convert an optical information signal back into an electrical signal (photocurrent). The photodetector in today's fiver-optic communications systems is a semiconductor photodiode (PD). This miniature device is usually fabricated together with its electrical circyitry to form an integrated package that provides power-supply connections and signal amplification. Such an integrated package is shown in Figure 1.4 as an optical receiver. Figure 1.7 shows samples of a photodiode and an integrated package.The basic diagram shown in Figure 1.4 gives us the first idea of what a fiber-optic communications system is and how it works. All the components of this point-to-point system are discussed in detail in this book. Particular attention is given to the study of networks based on fiber-optic communications systems.The role of Fiber-Optic Communications Technology has not only already changed the landscape of telecommunications but it is still doing so and at a mind-boggling pace. In fact, because of the telecommunications industry's insatiable appetite for capacity, in recent years the bandwidth of commercial systems has increased more than a hundredfold. The potential information-carrying capacity of a single fiber-optic channel is estimated at 50 terabits a second (Tbit/s) but, from apractical standpoint, commercial links have transmitted far fewer than 100 Gbps, an astoundingamount of data in itself that cannot be achieved with any other transmission medium. Researchers and engineers are working feverishly to develop new techniques that approach the potential capacity limit.Two recent major technological advances--wavelength-division multiplexing (WDM) anderbium-doped optical-fiber amplifiers (EDFA)--have boosted the capacity of existing system sand have brought about dramatic improvements in the capacity of systems now in development. In fact,' WDM is fast becoming the technology of choice in achieving smooth, manageable capacity expansion.The point to bear in mind is this: Telecommunications is growing at a furious pace, and fiber-optic communications is one of its most dynamically moving sectors. While this book refleets the current situation in fiber-optic communications technology, to keep yourself updated, you have to follow the latest news in this field by reading the industry's trade journals, attending technical conferences and expositions, and finding the time to evaluate the reams of literature that cross your desk every day from companies in the field.光纤通信系统一般的通信系统由下列部分组成：(1) 信息源。
Unit 7：Fiber Optic
TEXT
Optical fiber transmission has come of age as a major innovation in telecommunications. telecommunications：电讯, 长途通讯 无线电通讯 电信学 ：电讯 长途通讯, 无线电通讯, 光纤传输作为电信业一项重大变革的时代已经到来。 光纤传输作为电信业一项重大变革的时代已经到来。
Specialized English on Communication Engineering
One common example of refraction occurs when you stand at the edge of a pool and look at an object at the bottom of the pool. Unless you are directly over the object, it appears to be farther away than it really is. 当你站在湖边，看湖底的一个物体时， 当你站在湖边，看湖底的一个物体时，就会发生最普通的折 射的例子。如果你不是在物体的正上方， 射的例子。如果你不是在物体的正上方，这个物体就会看起 来比他真实的位置更远。 来比他真实的位置更远。
the angle of refraction constant refractive index of the first medium
Specialized English on Communication Engineering
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(a) Ray Escapes
光纤通信专业英语
光纤通信专业英语光通讯专业术语ADM Add Drop Multiplexer 分插复用器利用时隙交换实现宽带管理，即允许两个STM-N信号之间的不同VC实现互连，并且具有无需分接和终结整体信号，即可将各种G.703规定的接口信号(PDH)或STM-N信号(SDH)接入STM-M(M>N)内作任何支路。
AON Active Optical Network 有源光网络有源光网络属于一点对多点的光通信系统，由ONU、光远程终端OLT和光纤传输线路组成。
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一种结合ATM 多业务多比特率支持能力和无源光网络透明宽带传送能力的理想移动通信,通信工程师的家园,通信人才,求职招聘,网络优化,通信工程,出差住宿,通信企业黑名单)n+e*c,X)B0{6f3k长远解决方案，代表了面向21 世纪的宽带接入技术的最新发展方向。
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光通信英语作文
光通信英语作文Optical communication has revolutionized the way we transmit and receive information in the modern world. This technology, which utilizes light as the medium for data transmission, has become the backbone of global communication networks, enabling the seamless exchange of information across vast distances at unprecedented speeds. In this essay, we will explore the fundamental principles of optical communication, its evolution, and its pivotal role in shaping the digital landscape of the 21st century.At the core of optical communication is the use of light as the carrier of information. Light, a form of electromagnetic radiation, can be modulated to encode digital data, which can then be transmitted through various mediums such as optical fibers or free-space. The ability to harness the properties of light, including its high frequency, directionality, and low attenuation, has made optical communication a superior choice over traditional electrical communication systems.The history of optical communication can be traced back to the late 19th century, when the first experiments with light-basedcommunication were conducted. The invention of the laser in the 1960s, however, marked a significant turning point, as it provided a reliable and coherent light source that could be effectively modulated and transmitted over long distances. The development of low-loss optical fibers, which can guide light with minimal signal degradation, further propelled the growth of optical communication in the 1970s and 1980s.Today, optical communication systems are ubiquitous, underpinning a vast array of applications and technologies. In the telecommunications industry, optical fiber networks form the backbone of global communication infrastructure, enabling the transmission of voice, data, and video at unprecedented speeds. These fiber-optic networks have revolutionized the way we communicate, allowing for the seamless exchange of information across continents and oceans.Beyond telecommunications, optical communication has found widespread applications in various fields. In the field of data centers and cloud computing, optical interconnects are used to link servers and storage systems, providing the high-speed data transfer required to support the growing demand for computational resources. In the healthcare sector, optical communication techniques are employed in medical imaging and diagnostic equipment, enabling the capture and transmission of high-resolutionimages and data.The advent of fiber-optic sensors has also opened up new frontiers in fields such as structural health monitoring, environmental sensing, and industrial automation. These sensors, which utilize light-based detection mechanisms, can measure a wide range of physical, chemical, and environmental parameters with high precision and reliability.The advantages of optical communication extend beyond its raw speed and capacity. Optical signals are also inherently more secure than their electrical counterparts, as they are less susceptible to electromagnetic interference and eavesdropping. This makes optical communication an attractive choice for applications where data privacy and security are of paramount importance, such as in government, military, and financial sectors.Moreover, optical communication systems have a significantly lower energy footprint compared to traditional electrical communication systems. The use of light as the carrier of information, combined with the high efficiency of optical components, has led to a substantial reduction in power consumption and carbon emissions, making optical communication a more sustainable and environmentally friendly solution.As we look to the future, the potential of optical communication continues to expand. The development of advanced optical technologies, such as wavelength-division multiplexing, coherent detection, and all-optical signal processing, has enabled even greater bandwidth and capacity. The integration of optical communication with emerging technologies, such as 5G, the Internet of Things (IoT), and quantum computing, promises to unlock new frontiers of communication and information processing.Furthermore, the ongoing research and development in areas like free-space optical communication, where data is transmitted through the atmosphere using laser beams, hold the promise of revolutionizing communication in scenarios where traditional wired or wireless solutions are impractical or unavailable, such as in space exploration, disaster response, and remote areas.In conclusion, optical communication has transformed the way we transmit and receive information, becoming the foundation of modern global communication networks. Its ability to harness the properties of light has enabled unprecedented speeds, capacity, and energy efficiency, making it an indispensable technology in the digital age. As we continue to push the boundaries of optical communication, we can expect to see even more remarkable advancements that will shape the future of communication and information technology.。
光纤通信_第7章 光纤通信系统PPT课件
FOH FOH FOH FOH
123 … N 1 … N 1 … N 1 … 时隙
一帧
图7.11 数字信号的时分复用
PDH（Plesiochronous Digital Hierarchy）是指准同 步数字体系。根据国际电报电话咨询委员会CCITT （现改为国际电联标准化组织ITU-T）G.702建议， PDH的基群速率有两种， 即PCM30/32路系统和PCM24 路系统。 我国和欧洲各国采用PCM30/32路系统， 其 中每一帧的帧长是125μs，共有32个时隙（TS0～ TS31），其中30个为话路（TS1～TS15和TS17～ TS31），时隙TS0被用作帧同步信号的传输，而时隙 TS16用作信令及复帧同步信号的传输。
每个时隙包含8 bit， 所以每帧有8×32=256 bit， 码速 率为256 bit×（1/125 μs）=2.048 Mb/s。 日本和北美使 用的PCM24路系统， 基群速率为1.544 Mb/s。 几个基 群信号（一次群）又可以复用到二次群， 几个二次群 又可复用到三次群……。 表7.1是PDH各次群的标准比 特率。
模拟信号
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011 110 111 101 001 010 (3) (6) (7) (5) (1) (2)
图7.10 PCM编码和解码过程
PCM编码包括抽样、 量化、 编码三个步骤， 如 图7.10左半部分所示。 把连续的模拟信号以一定的抽 样频率f或时间间隔T抽出瞬时的幅度值， 再把这些幅 度值分成有限的等级， 四舍五入进行量化。 如图中把 幅度值分为8种， 所以每个范围内的幅度值对应一个量 化值， 这8个值可以用3位二进制数表示， 比如0对应 000， 1对应001， 2对应010， 3对应011， 4对应100， 5对应101， 6对应110， 7对应111。
中英对照：光纤通信
《光纤通信》光纤通信光纤常被电话公司用于传递电话、互联网，或是有线电视的信号，有时候利用一条光纤就可以同时传递上述的所有信号。
与传统的铜线相比，光纤的信号衰减（attenuation）与遭受干扰[来源请求]（interference）的情形都改善很多，特别是长距离以及大量传输的使用场合中，光纤的优势更为明显。
然而，在城市之间利用光纤的通信基础建设（infrastructure）通常施工难度以及材料成本难以控制，完工后的系统维运复杂度与成本也居高不下。
因此，早期光纤通信系统多半应用在长途的通信需求中，这样才能让光纤的优势彻底发挥，并且抑制住不断增加的成本。
从2000年光通信（optical communication）市场崩溃后，光纤通信的成本也不断下探，目前已经和铜缆为骨干的通信系统不相上下。
对于光纤通信产业而言，1990年光放大器（optical amplifier）正式进入商业市场的应用后，很多超长距离的光纤通信才得以真正实现，例如越洋的海底电缆。
到了2002年时，越洋海底电缆的总长已经超过25万公里，每秒能携带的数据量超过2.56Tb，而且根据电信业者的统计，这些数据从2002年后仍然不断的大幅成长中。
光纤通信的历史自古以来，人类对于长距离通信的需求就不曾稍减。
随着时间的前进，从烽火到电报，再到1940年第一条同轴电缆（coaxial cable）正式服役，这些通信系统的复杂度与精细度也不断的进步。
但是这些通信方式各有其极限，使用电气信号传递信息虽然快速，但是传输距离会因为电气信号容易衰减而需要大量的中继器（repeater）；微波（microwave）通信虽然可以使用空气做介质，可是也会受到载波频率（carrier frequency）的限制。
到了二十世纪中叶，人们才了解使用光来传递信息，能带来很多过去所没有的显著好处。
然而，当时并没有同调性高的发光源（coherent light source），也没有适合作为传递光信号的介质，也所以光通信一直只是概念。
光纤通信(双语)总复习课件 科大
scattering
geometric effects
total loss
18
fiber attenuation
2.5
Attenuation (dB/km)
Rayleigh scattering
Glass absorption In infrared
0.2 850 1300 glass absorption In ultraviolet 1550
3
1.2 The Basic Communications System
Transmitter
Information ChannelReceຫໍສະໝຸດ verBL product
4
1.2.10 computing power levels in decibels
decibel (dB)： the relative power level.
Wavelength (nm)
19
attenuation coefficient
Pin
L
Pout
⎛ Pout 10 α (dB / km) = − log10 ⎜ ⎜P L ⎝ in
⎞ ⎟ ⎟ ⎠
20
Q4
A fiber system operates at the wavelength 1300nm, where the fiber loss is 0.5dB/km. The LED light source emits 1.59mW and couples into the fiber with a 16-dB loss. Connectors and splices in the system contribute a total loss of 6dB. The receiver sensitivity (the power required for the receiver to detect the message with a specified error rate or signal-to-noise ratio) is given as -30dBm. A 4-dB margin is specified to account for system degradations (such as aging of the LED). What is the maximum length of fiber that can be used?
光纤通信(第五版)(英文版)[(美)帕勒里斯著][样章]
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does not change the average power or the modulation frequencies,but it does lower the signal variation.The transmitted information is contained in this variation,so its attenua-We may think of this result as broadening the signal peak (lowering its amplitude) and filling in the valley (raising its level).Excessive broadening will cause Distortion caused by material (or waveguide) dispersion can be reduced by usingby using more coherent emitters.A laser diode has the advantage over an LED in this respect.In principle,dispersive distortion could be reduced by filtering the optic beam at the transmitter or receiver,allowing only a very narrow band of wavelengths to reach the photodetector.This technique hasA wave incident on a plane boundary between two dielectrics (refrac-) is partially transmitted and partially reflected.(3.30)Although somewhat formidable in appearance,these equations are easily evalu-ated when the two indices of refraction,the incident angle,and the polarization are (3.29) and (3.30) cannot be understated,because they predict the phenomenon by which dielectric fibers guide light.The reflectance is found by squaring the magnitudes of the reflection coeffi-Results are shown in Fig.3.22for an air-to-glass interface and for a glass-to-air interface.The general characteristics shown on the figures appear when there are reflections between any two dielectrics.Some interesting,and features can be noted:The reflectance does not vary a great deal for incident angles near zero.For thethe reflectance value calculated for normal incidence,4%,is a good approximation for angles as large as 20°.meaning full transmission,for certain incident angles andindicating total reflection,for a range of incident angles.-21n 22-n 12sin 2 u i2+21n 22-n 12 sin 2 u i 2The evanescent electric field decays exponentially according to the expression where the attenuation factor and is the free-space propagation factor.the attenuation coefficient discussed in the first section of this chapter.The attenuation coefficient is attributed to actual power losses,critical angle,decay.The decay rate merely indicates how far the field extends into the second medi-um before returning to the incident region.er and the fields decay faster.Rays incident at angles greater than,waves that decay slowly and penetrate deeply into the second medium,dent far above the critical angle produce waves that disappear after only a short pene-tration into the second medium.The reflection coefficient,tity,having a magnitude and an angle when is unity under the condition of total reflection.the reflected wave relative to the incident wave.SUMMARY AND DISCUSSIONThis chapter concentrated on developing fundamental ideas about light waves that apply directly to fiber optics.and polarization —should now be clear.was studied extensively because of its impact on the information-handling capacity of fibers.Other causes of pulse distortion will be considered in Chapter 5.The dependence of information rate on the spectral width of the optic source indicated the importance of this light-emitter property.longitudinal mode structure appearing in the output spectrum of a laser diode.shall see in Chapter 4,resonance also explains the mode structure in a dielectric wave-guide.Reflections at dielectric boundaries play a major role in fiber optics.nal reflection makes it possible for dielectrics to form waveguides for light rays.sin u i =n 2/k 0。
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7.3 Semiconductor Photodiode 半导体光电二极管
hf Reverse Bias i v RL +
VB
hf
p
n
上海师范大学电气信息系
p hf E
n
hf Electron Energy + + + + + + + + + 电子能
----------
+ Junction Region 结区
上海师范大学电气信息系
90%
Output
7.2 Photomultiplier 光电倍增管
First, Vacuum Photodiode 首先，真空光电二极管 Cathode 阴极
-
hf
Anode 阳极 +
外加偏置电压使得光电二极管阳极 为正，阴极为负。在无光照情况下， 通过负载电阻的电流和输出电压均 为零。当阴极受到光照射，入射光 子被吸收，阴极金属材料中的电子 获得能量。其中一些电子获得的能 量足够大以至从阴极逃逸，这些自 由电子受阳极正电荷吸引并向其移 动。同时，正电荷被不断靠近的带 负电荷电子吸引，通过外电路向阳 极运动，外电路产生了电流。
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Calculation of the responsivity of the Vacuum
Photodiode 真空光电二极管的响应的计算
P Optical Power Detector i Electrical Current
Let η be the quantum efficiency量子效率, defined by
input power(输入功率) is 1 W. Let RL = 50 .
Solution: From (7.7)
Change RL to 50,000 and recompute the voltage (重新计算电压).
上海师范大学电气信息系
7.2 Photomultiplier tube 光电倍增管(PMT) Cathode 阴极 -
~100V
~400V
Anode 阳极 + ~1000 V
~200V V -
~300V
Dynodes 倍增电极 Secondary Electrons 二次发射电子 i + RL v
( V ~ 1000 volts )
Fast-moving electrons hit the metal dynodes倍增电极 causing the release of additional (secondary emission二次电子发射) electrons. 快速移动的电子撞击倍增电极引起额外(二次电子发射)电子的释放。
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Important Detector Properties 重要的检测器特性
1. Responsivity 响应度:
P
i Electrical Current 电流 Photodetect
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2. Spectral Response谱响应: It is displayed as a curve of responsivity versus wavelength. (它展示了响应曲线与波长之间的关系。) Example: 0.5 Silicon Photodiode Response 硅光电二极管响应
i
hf
Example: Solution
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Properties: 特性
Fast: tr=0.1ns High cost 成本高 Large size 体积大
High weight 重量大
Hundreds of volts for bias 数百伏偏置电压
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上海师范大学电气信息系
External Photoelectric Effect外部光电效应 Generation of free electrons when photons strike the surface of a metal. Electrons are emitted from the surface. 当光子入射到金属表面时，产生自由电子 。电子从金属表面被发射出来。 Devices: Vacuum Photodiode 真空光电二极管 Photomultiplier tube 光电倍增管
上海师范大学电气信息系
The current(电流) is thus: (7.6) And the responsivity(响应) is: (7.7) This result is valid for all photodetectors. 这个结果对所有光电检测器都有效。
上海师范大学电气信息系
上海师范大学电气信息系
Example: Consider Cesium铯, where = 1.9 eV.
Find the cutoff wavelength(找到截止频率).
This wavelength is shorter than the wavelengths commonly used for glass fiber systems. Thus, this detector will not suitable for fiber systems. 这 个波长比通常使用的玻璃光纤系统的波长短。因此，这 个检测器将不适合用于光纤系统。
Chapter 7
Light detectors 光检测器
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Contents 目录
Principles of photodetection 光检测原理 Photomultiplier 光电倍增器 Semiconductor photodiode 半导体光电二极管 PIN photodiode PIN型光电二极管 Avalanche photodiode 雪崩光电二极管
上海师范大学电气信息系
7.1 Principle of photodetection 光检测器
Detectors convert light signals into electrical signals. 检测 器将光信号转换成电信号。 Internal Photoelectric Effect内部光电效应 Generation of free mobile charge carriers in semiconductors by absorption of photons. 在半导体中通 过吸收光子产生自由电荷载流子(电子和空穴) Devices: pn photodiode pn光电二极管 PIN photodiode PIN型光电二极管 Avalanche photodiode (APD) 雪崩光电二极管
In general then, we have shown that
i P
From the photodiode circuit, the output voltage is 从光电二极管电路来说，输出电压是
or
(7.8)
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Conclusions 结论
1. The detected current is proportional to the optical power (i∝p), which itself is proportional to the message signal current. Thus, the receiver current is proportional to the input message current as required. 检测电流正比于光功率，而光功率 正比于信息信号电流。因此，如要求的，接收机电流正比于输入信 息电流。
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Gains of 2 to 6 per dynode are typical. 每个倍增电极的 增益为2到6是典型的。 Let = gain per dynode, 每个倍增电极的增益 and N = number of dynodes 倍增电极数 The total gain(总增益): M = N (7.9) The current(电流): (7.10) ePM
0 0.5 0.7 0.9 (m) 1.1
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3. Speed of Response响应速度: As before, if tr is the rise time, the bandwidth is (approximately) 之间，如果tr是上升时间，带宽(大约)是 (7.2) Recall P Input Output 10% tr Other Important Properties(其他重要特性): Size, temperature sensitivity, gain, lifetime, circuit complexity, and cost.体积，温度敏感性，增益， 寿命，电路复杂性，成本。
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As before Condition for detection: Photon energy be greater than the bandgap energy: 光子能量比带隙大。
At cutoff(截止波长处), then
or
1.24 c Wg
(7.11)
where Wg is in eV and c is in m. Only wavelengths where c will be detected.仅仅 波长在 c 处将被检测到。
Electrons 电子 i + RL V v Vacuum Photodiode 真空 光电二极管
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Define work function 定义功函数: Energy required to liberate an electron from the metal cathode. 从金属的阴极释放一个电子所需要的能量 Units : Joules 焦耳(energy) In order to free an electron: the photon energy must equal to, or exceed, the work function. 目的是为了释放 一个电子：光子的能量必须相等或超过功函数