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1、东华理工大学长江学院毕业设计外 文 翻 译 学生姓名： 学 号： 09323119 专 业： 信息工程 系 别： 信息工程 指导教师： 职称： 讲师 二0一三年六月 五 日OriginalOptical Fiber CommunicationsThe General System Communication may be broadly dened as the transfer of information from one point to another. When the information is to be conveyed over any distance a communi
2、cation system is usually required. Within a communication system the information transfer is frequently achieved by superimposing or modulating the information onto an electromagnetic wave which acts as a carrier for the information signal. This modulated carrier is then transmitted to the required
3、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 frequencies as well as microwave and millimeter wave frequencies. However, communi
4、cation may also be achieved by using an electromagnetic carrier which is selected from the optical range of frequencies.An optical fiber communication system is similar in basic concept to any type of communication system.The communication system therefore consists of a transmitter or modulator link
5、ed to the information source, the transmission medium,and a receiver or demodulator at the destination point. In electrical communications the information source provides an electrical signal, usually derived from a message signal which is not electrical (e.g. sound), to a transmitter comprising ele
6、ctrical and electronic components which converts the signal into a suitable form for propagation over the trans-mission medium. This is often achieved by modulating a carrier, which, as mentioned previously, may be an electromagnetic wave. The transmission medium can consist of a pair of wires, a co
7、axial cable or a radio link through free space down which the signal is transmitted to the receiver, where it is transformed into the original electrical information signal (demodulated) before being passed to the destination. However, it must be noted that in any transmission medium the signal is a
8、ttenuated, or suffers loss, and is subject to degradations due to contamination by random signals and noise, as well as possible distortions imposed by mechanisms within the medium itself. Therefore, in any communication system there is a maximum permitted distance between the transmitter and the re
9、ceiver beyond which the system effectively ceases to give intelligible communication. For long-haul applications these factors necessitate the installation of repeaters or line amplifiers at intervals,both to remove signal distortion and to increase signal level before transmission is continued down
10、 the link. For optical fiber communications system shown in Figure (a) may be considered in slightly greater detail, as given in Figure (b). Fig(a) The general communication system (b)The optical fiber communication systemIn this case the information source provides an electrical signal to a transmi
11、tter comprising an electrical stage which drives an optical source to give modulation of the light wave carrier. The optical source which provides the electricaloptical conversion may be either a semiconductor laser or light-emitting diode (LED). The transmission medium consists of an optical fiber
12、cable and the receiver consists of an optical detector which drives a further electrical stage and hence provides demodulation of the optical carrier. Photodiodes (pn, pin or avalanche) and, in some instances, phototransistors and photoconductors are utilized for the detection of the optical signal
13、and the opticalelectrical conversion. Thus there is a requirement for electrical interfacing at either end of the optical link and at present the signal processing is usually performed electrically.The optical carrier may be modulated using either an analog or digital information signal. In the syst
14、em shown in Figure (b) 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 light intensity are obtained (i.e. onoff pulses). Although often simpler to implement, analog modulation with
15、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 not always provided by semiconductor optical sources, especially at high modulation frequencies. For thes
16、e reasons,analog optical fiber communication links are generally limited to shorter distances and lower bandwidth operation than digital links.Figure (c) shows a block schematic of a typical digital optical fiber link. Initially, the input digital signal from the information source is suitably encod
17、ed for optical transmission. The laser drive circuit directly modulates the intensity of the semiconductor laser with the encoded digital signal. Hence a digital optical signal is launched into the optical fiber cable. The avalanche photodiode (APD) detector is followed by a front-end amplifier equa
18、lizer or filter to provide gain as well as linear signal processing and noise bandwidth reduction. Fig(c)A digital optical fiber link using a semiconductor laser source and an avalanche photodiode(APD) detector待添加的隐藏文字内容2Finally, the signal obtained is decoded to give the original digital informatio
19、n. However, at this stage it is instructive to consider the advantages provided by light wave communication via optical fibers in com-parison with other forms of line and radio communication which have brought about the extensive use of such systems in many areas throughout the world.译文光纤通信一般来说把信息从一
20、点传送到另一点就称为通信。当信息跨越一段距离进行传送时，就需要一个通信系统。在通信系统中，信息传输是通过把信息叠加在电磁波上或对电磁波进行调制来实现的，电磁波起着载送信号的作用。这一经过调制的载频随后被传送到要求（到达）的目的地，在那里被接收，并且通过解调还原成原始信息。在运用电磁载波的领域，高新技术得到进一步的发展，比如射频、微波以及毫米波的频率都被用来作为载频使用。亦可选用光波范围内的频率作为载波来实现通信。从概念上讲，一个光纤通信系统与任何类型的通信系统是一样的。通信系统通常由这样几部分组成，即一个发射机或与信源相关联的调制器、传送媒介以及一个接收机或是放在目的地的解调器。在电通信系统中
21、，信源的作用是将原始的非电信号（如语音信号等）转变成电信号，然后通过电或电子器件将该信号转变成适合在传输媒介上传输的信号，这一过程通常是通过调制载频来完成的。如前所述，这个载频可以是电磁波，传输介质可以是一对导线、一条同轴电缆或通过自由空间的无线电，链路通过介质将调制好的信号送到接收端，信号在传送给终端之前，先复原成原是信号（即解调）。必须引起注意的是：经过任何传输介质，信号都会衰减或损耗，并且经常容易为随机信号和噪声所损害，以及可能有传输媒介自身引起的失真等，这些都会导致信号恶化。因此，在任何通信系统中，接收机和发射机之间都存在一个最大允许距离，否则，就不可能有可理解的通信。在长途干线中，上
22、述因素要求按一定间隔设置再生中继站或线路放大器，而再生中继站和线路放大器的设备都是为了消除失真，提高信号电平，以便传输得以继续。图（a）给出了光纤通信系统，较细致的图参见图（b）。在这个图中，信源提供电信号给发射机，发射机组成一个电子平台来驱动光源以完成对光载频的调制。光源是由发光二极管或半导体激光构成的，它完成电光变换。传输媒介由电光纤（缆）组成。光接收机包括一个含光检测器的电路驱动平台，用以完成对已调光载频的解调。用于检测光信号和进行光电变换的器件有光电二极管（p-n,p-i-n或雪崩管）、发光三极管以及光敏电阻等。因此，在光系统链路的两端都要求有电接口，并且在现阶段信号处理通常是通过电路
23、实现的。模拟或数字的信号均可用来调制光载频。在图（b）中，模拟调制是指从光源处发射的连续光强度的变化，而数字调制则不然，它是通过光强度离散的变化（如有无光脉冲）来实现的。虽然模拟调制实现起来常常比较简单，但使用光纤系统的模拟调制效率并不高，而且在接收端要求比数字调制高得多的信噪比。另外，对模拟调制线性度的要求也常常不能由导体光源来确定，在高频率调制区尤为如此。基于上述原因，一般地讲，与数字光系统相比较，模拟通信链路被限制在更短的通信距离和更窄的带宽使用上。图（c）是一个典型的数字光系统的示意图。一开始，信源的数字信号被适当的编码以用来进行光传输。激光器的驱动电路通过这些已编码的数字信号来直接调制激光器的发光强度，然后数字光纤信号被注入光纤。在接收端紧跟在雪崩发光二极管（APD）之后的是前置放大器和均衡器或滤波器，分别用来提供增益，对信号进行线性处理和减少噪声带宽。最后，信号通过解码得到原始信号。在本文中，对比一下光纤通信系统与其他形式的有线、无线通信系统，光纤通信的优点是很明显的，后者在许多地方已经做了介绍，这里就不详谈了。
