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[精彩]外文翻译范文 本科毕业设计(论文) 外文参考文献译文及原文 学 院 信息工程学院 专 业 信息工程(电子信息工程方向) 年级班别 2004级(4)班 学 号 3104002975 学生姓名 陈英权 指导教师 刘喜英 2008 年 6 月 5 日 目 录 外文参考文献译文 1 锁相环 ..................................................................................................... 1 1.1 锁相特性 ....................................................................................... 1 1.2 历史与应用 ................................................................................... 2 1.3 其它应用 ....................................................................................... 4 2 光通信元件 ............................................................................................. 5 2.1 光纤 ............................................................................................... 5 2.2 调制器和检测器............................................................................ 7 外文参考文献原文 1 Phase Lock Loop .................................................................................... 9 1.1 Nature of Phaselock ....................................................................... 9 1.2 History and Application................................................................ 10 1.3 Other Applications ....................................................................... 13 2 Optical Communication Components ................................................. 14 2.1 The Optical Fiber ......................................................................... 14 2.2 Modulators and Detectors............................................................. 17 1 锁相环 1.1 锁相特性 锁相环包含三个组成部分: 1、相位检测器(PD)。 、环路滤波器。 2 3、压控振荡器(VCO)，其频率由外部电压控制。 相位检测器将一个周期输入信号的相位与压控振荡器的相位进行比较。相位检测器的输出是它两个输入信号之间相位差的度量。差值电压由环路滤波后，再加到压控振荡器上。压控振荡器的控制电压使频率朝着减小输入信号与本振之间相位差的方向改变。 当锁相环处于锁定状态时，控制电压使压控振荡器的频率正好等于输入信号频率的平均值。对于输入信号的每一周期，振荡器输出也变化一周，且仅仅变化一周。锁相环的一个显而易见的应用是自动频率控制(AFC)。用这种方法可以获得完美的频率控制，而传统的自动频率控制技术不可避免地存在某些频率误差。 为了保持锁定环路所需的控制电压，通常要求相位检测器有一个非零的输出，所以环路是在有一些相位误差条件下工作的。不过实际上对于一个设计良好的环路这种误差很小。 一个稍微不同的解释可提供理解环路工作原理的更好说明。让我们假定输入信号的相位或频率上携带了信息，并且此信号不可避免地受到加性噪声地干扰。锁相接收机的作用是重建原信号而尽可能地去除噪声。 为了重建原始信号，接收机使用一个输出频率与预计信号频率非常接近的本机振荡器。本机振荡和输入信号的波形由相位检测器比较，其误差输出示瞬时相位差。为了抑制噪声，误差在一定的时间间隔内被平均，将此平均值用于建立振荡器的频率。 如果原信号状态良好(频率稳定)，本机振荡器只需要极少信息就能实现跟踪，此信息可通过长时间的平均得到，从而消除可能很强的噪声。环路输入是含噪声的信号， 而压控振荡器输出却是一个纯净的输入信号(的复本)。所以，有理由认为环路是一种传输信号并抑制噪声的滤波器。 环路滤波器有两个重要的特性:其一是带宽可以非常窄，其二是滤波器能自动跟踪信号频率。自动跟踪和窄带的特点说明了锁相接收机的主要用途。窄带能够抑制大量的噪声，难怪锁相环路常用来恢复深深地淹没在噪声中的信号。 1.2 历史与应用 关于锁相的早期论述(思想)是Bellescize于1932 年提出的，并在处理无线电信号同步接收中得到应用。20世纪20年代开始使用超外差接收机，但人们一直努力寻求更简单的接收技术。一种方法就是同步接收机或零差接收机。这种接收机本质上只是由一个本机振荡器，一个混频器和一个音频放大器组成。为了正常工作，必须调节振荡器使其输出频率与输入的信号载波频率完全一致，于是载波被变换成0Hz的“中频”。混频器输出含有解调出来的，由信号边带携带的信息。干扰与本地振荡器不同步，因此由干扰信号引起的混频器输出是一个拍音，可用音频滤波器加以抑制。 对于同步接收，本振的正确调谐至关重要，任何一点频率误差都将严重损坏信号。此外，本振的相位必须与接收的载波相位一致，其间的误差限于周期的很小一部分。就是说，本振与输入信号之间必须实现相位锁定。 由于各种原因简单的同步接收机从未广泛应用过。现在锁相接收机几乎无例外地运用超外差原理，并趋于高度复杂化。锁相接收机最重要的应用之一是接收来自遥远的宇宙飞行器的极微弱信号。锁相技术的首次广泛使用是在电视接收机中的行和帧的同步扫描。与视频信号一起传送的脉冲发出电视图像每一行的开始信号和隔行扫描的半帧开始信号。作为一种非常粗糙的重建电视显象管扫描光栅的方法，这些脉冲可以剥离出来单独用于触发一对扫描发生器。 一个较为复杂的途径是利用一对自由振荡的张弛振荡器驱动扫描发生器。用这种方法，即使失去同步(消失)，扫描还是存在的。 将振荡器的自由振荡频率设置得略低于水平和垂直(扫描)脉冲频率，剥离出来的脉冲用于提前触发振荡器从而使振荡器与行频和半帧频同步(由于美国电视在交替的垂直扫描时进行隔行交织，所以是半帧频)。 在噪声不存在的情况下这种可提供良好的同步，这就完全可以了。不幸的是噪 声总是存在的，并且任何触发电路对噪声都是特别敏感的。在极端情况下触发扫描将完全失效，尽管在这样的信噪比条件下电视图像虽然较差却还能辩认。 在不是极端恶劣的条件下，噪声将造成起始时间抖动和偶尔的误触发。行抖动将降低行清晰度,并使得垂直线条呈现锯齿状。严重的水平误触发通常会造成画面出现狭窄的水平黑带。 帧扫描抖动会引起图像的垂直滚动。另外，相继半帧之间的隔行扫描行还会相对移动，使图像进一步恶化。 将两个振荡器与剥离出来的同步脉冲锁相可大大减小噪声起伏。锁相技术靠检查各振荡器和许多同步脉冲之间的相位关系来调节振荡频率，使得平均相位偏差很小，而不是仅用一个脉冲进行触发。由于锁相同步器检测许多脉冲，因此它不会被偶发的破坏同步器触发的大幅度脉冲噪声所干扰。目前电视接收机中使用的飞轮同步器实际上就是锁相环路。使用飞轮一词是因为此电路能够跟踪增加的噪声或微弱信号的周期。通过锁相可以获得同步性能的重大改进。 在彩色电视接收机中彩色副载波是由锁相环路同步的。 宇宙飞行的需要强烈地刺激了锁相技术的应用。锁相的空间应用是随着早期美国人造卫星的发射而开始的。这些飞行体携带低功率(10毫瓦)的连续波发射机，相应的接收信号很微弱。由于多普勒频移和发射振荡器的频率漂移，接收信号的精确频率难以确定。在最初使用的108MHz频率上，多普勒频移可在,3kHz 范围内。 因此使用普通的固定调谐接收机时，带宽至少应为6kHz，然而信号本身却只占非常窄的频谱，大约在6Hz带宽内。 接收机中的噪声功率与带宽成正比，所以如果使用传统的技术，就不得不接受1000倍(30dB)噪声的代价。随着技术的进步这些数字变得更加惊人。发射频率上升到了S波段，使多普勒频移范围达到,75kHz，而接收机带宽则已减小到3Hz。这样一来常规技术的代价就将是47dB左右。这是无法接受的，也就是要使用窄带的锁相跟踪接收机的原因所在。 窄带滤波器能抑制噪声，但是如果滤波器被固定，则信号将几乎总是落在通带之外。一个可用的窄带滤波器必须有跟踪信号的能力。锁相环路既提供了窄带，又提供了所需的跟踪能力。而且，非常窄的带宽也能方便地获得(对于空间应用典型的是,到1000Hz)。如果需要的话，还能容易地改变带宽。 对于多普勒信号，用于确定飞船速度的信息是多普勒频移。锁相接收机很适合用于多普勒恢复，因为当锁相环路锁定时不存在频率误差。 1.3 其它应用 以下的应用阐述了目前锁相技术的一些应用，这些应用将在本书其他章节进一步讨论。 1、跟踪运动飞船的一种方法涉及到将相干信号发射到飞船上，将信号频率偏移并转发回地面。飞船上的相干应答器必须如此工作以使输入和输出频率严格地成m/n的比例关系，此处m和n 都是整数。锁相技术经常被用来建立相干性。 2、锁相环可用作频率解调器，锁相环在其中比传统的鉴频器具有更优越的性能。 3、带有噪声的振荡器可被包围在环路内，并使之锁定在一个纯净的信号上。如果环路带有的带宽，振荡器检测出自已的噪声，其输出被大大净化。 4、用锁相环路可构成频率倍乘器和分频器。 5、数字信号的发射通常应用锁相技术实现。 6、频率合成器可方便地用锁相环路构成。 2 光通信元件 2.1 光纤 正如先前所讨论的，大气不能被用来作为地面光通信的传输信道。最有前途的信道是光纤波导。光纤基本上由一个中心透明的称为纤芯的区域和一个环绕纤芯的称为包层的折射率较低的区域所组成。纤芯的折射率既可以是均匀的，也可以是从中心向外具有递减梯度的。前一种光 纤也称为匀芯光纤由于在纤芯包层的界面处的全内反射现象而形成光导。后一种光纤称为渐变率光纤是由光束朝纤芯中央连续折射而产生光导。 在光波导中，存在着不改变场结构并以固定的相位和群速传播的特殊的场分布。这些场结构称为光波导的模。这些模以不同的传播常数和不同的群速度为特征。在多模光波导中，存在着大量的这种传播模式，而在单模光波导中，只存在一种传播模式。每种模式的大部分能量都在纤芯内部，但由于纤芯外部存在的迅衰场(泄漏场)，一部分能量也在包层中传播。通过将包层做得足够厚，可使传播模式的场在包层,空气界面处很弱，使得光纤便于处置和支撑而不会严重地扰乱传播模式。 正如已经讨论过的那样，在光纤通信系统中，信息以离散脉冲的形式编码，通过光纤传输。系统的信息容量将由单位时间内可发送的脉冲数来确定。为了在输出端恢复信息，各个脉冲必须能在时间上被正确分辨。在光纤中由于不同模式之间的群速度不等，以及各模式的传输常数依赖于波长等因素，光脉冲会在光纤传输过程中变宽。 因此，即使两个脉冲在输入端可以很好地分辨，因脉冲展宽他们在输出端可能无法分辨。在这种情况下，就不能在输出端恢复信息。因此，对于某一给定的展宽，脉冲之间必须以一 个最小的时间间隔分开，这个时间间隔就确定了系统的最大信息容量。 当发射脉冲射入光纤时，就会激励光纤的各种模式。由于每一种模式一般都以不同的特征群速度传播，所以入射光脉冲随着传播而展宽，称为模间展宽。当光纤能只传播一种模式时，即在单模 光纤中，这种展宽不存在。但是，由于传播常数依赖于波长，仍然存在着某种展宽，称为模内展宽。模间扩散和模内扩散都是由于波导效应和材料效 应引起的。材料效应是由于光源为有限带宽，以及对不同波长的光有不同的折射率而产生的一种效应。这里要提一下，由于激光器与发光二极管(LED)相比具有更小的频谱宽度，因此使用激光器的系统与使用LED的系统相比其材料色散更小。例如，使用LED，由于材料色散脉冲展宽可能是每公里,毫微秒左右，而使用激光器，材料色散每公里小于0.2毫微秒。 可以利用几何光学概念形象地说明光脉冲在光纤中传播时的展宽现象。光脉冲注入匀芯光纤时会产生与轴线成不同夹角的光线。由于与轴线夹角较大的光线必须经过较长的光程，因此它们要用更长的时间到达输出端。所以，光脉冲在光纤中传播时就会展宽。与此相对应的是，在渐变率光纤中，尽管与轴线夹角较大的光线必须通过较长的光程，但它们是在折射率较低的媒介中传播。于是较长的光程被较高速的传播部分地补偿。所以与匀芯光纤比较，渐变光纤中的脉冲展宽必定更小。事实上情况确实如此，对于宽带应用渐变光纤比匀芯光纤更适用。 这里要提到，纤芯半径很小，同时纤芯与包层间折射率差也较小时，可以制成只存在一种传播模式的光纤。这类光纤称为单模光纤。因为只有一种模式存在，所以这些光纤中的色散很小，而且色散只是由模内展宽造成的。这种光纤确实可望用于将来的超带宽系统中。使用光波的系统除了具有极大的信息容量外，与同轴电缆等传统金属系统相比，通过光纤通信 或传输还有另一些优点。 1、由于实际可获得的光纤传输损耗极低，人们可以获得更大的中继距离，从而节约大量资金。 2、光纤的平均直径大约是100 微米，基本上由石英或玻璃制成。这使光纤的重量和所占空间都大大缩减，这一点对于敷设在已经布满电缆的管道中十分重要。重量和体积的这种节约 对于船用和航空使用光纤传送数据也十分重要。 3、光纤不受电磁干扰影响，而且没有串音。这对于国防上的保密通信十分重要。 4、由于不存在任何由短路等原因造成的危险，光纤可使用在易爆和高压环境中。 除了在电通信方面的主要应用，预期光纤也将在计算机网络，宇宙飞船，工业自动化和过程控制等领域中发挥重要作用。事实上，目前光纤已经被用来在Lawrence Livemore 实验室和Los Alamos 科学实验室的大型聚变激光器中传送数据和控制信息， 也使用在Nevada 试验场监控地下核爆炸。使用光纤的额外优点包括价格低廉和不受噪声的影响。 2.2 调制器和检测器 上面我们所讨论的只是光波通信系统各组成部分之一。除了光纤，人们还需要能把信息编码成 光波的调制器和能在接收端检测光脉冲并把光脉冲解译还原成信息的检测器。我们将简单讨论调制器和检测器的原理。 光源既可以通过改变其某个输入参数如输入电流来直接调制，也可以让输出的光通过称为调制器的器件实现外部调制。用在光纤通讯系统中最有前途的光源即半导体激光源很容易通过改变输入电流来调制。事实上，在数字系统中光源必须被键控调制。实际上半导体光源能以上升时间小于1毫微秒的高速来键控调制。键控调制是通过把激光二极管偏置在稍低于门限值上来实现的，门限值一般为100毫安左右。这里激光二极管起LED的作用，以较低的输出功率发射非相干光。由高速驱动器加入一个20mA左右的附加电流，将二极管激光器从非相干光发射状态转换成具有较大输出光功率的相干光发射状态。通过将“关闭”状态保持略低于阈值可使加在激光二极管上的电脉冲与所产生的光输出之间的延迟最小。这一延迟必须不大于比特之间的间隔从而使光脉冲能精确重建输入信号。 值得注意的一个重要因素是输出光功率的温度灵敏度。在上述方案中，这一点可通过用一个光 反馈回路改变直流偏置来保证，这样可以兼顾环境温度的缓慢变化和激光器本身逐渐老化两种因素。通常由收集激光器背面射出的光来实现输出功率监控，激光器前面发出的光则全部耦合到光纤中去。 对于非半导体激光源，使用外部调制器来实现调制。外部调制器利用不同材料所具有的不同特性。这样,某些晶体具有随外加电场变化的双折射，于是让光通过这样的晶体可以改变光线的偏振状态。如果把这个晶体放在正交的偏振镜之间，人们就可以进行光强调制。 类似地，声,光调制器是建立在声束与光波相互作用基础上的。传播的声波产生一个折射率光栅，反过来使光波发生衍射。 在接收终端或中继站，人们需要光检测器来接收输入光信号并把它变换成电信号。光波通信中应用的三种重要检测器是光电倍增管，PIN光二极管和雪崩光二极管。尽管 光电倍增管具有较大增益，后二种类型可望获得更广泛的应用，因为他们体积小，不需要高的偏置电压，而且更便宜。最简单的固态光检测器由一个具有开阔中心区域的反偏P-N结构成，为了接收入射光，该区域涂有抗反射的涂层。所吸收的光子把电子从价带激励到导带。由此产生的电子和空穴被外加电场分离，产生通过P-N结的光电流。 为了检测极低的光功率，人们使用雪崩光检测器。光子产生的电子-空穴对在这种器件中被加速，释放出更多的电子-空穴对，以此获得增益。 光纤通信系统中所需要的光检测器在工作波长上必须具有高响应度，为了适应系统的信息率，还要有足够的带宽。对于0.80微米波长范围的最有前景的光检测器似乎是硅晶体光二极管。它们具有极快的响应时间(<0.1 毫微秒)。量子效率即所产生的一次光电子与入射在检测器上的光子之比也很大。 1 Phase Lock Loop 1.1 Nature of Phaselock A phaselock loop contains three components: 1. A phase detector (PD). 2. A loop filter. 3. A voltage-controlled oscillator (VCO) whose frequency is controlled by an external voltage. The phase detector compares the phase of a periodic input signal against the phase of the VCO. Output of PD is a measure of the phase difference between its two inputs. The difference voltage is then filtered by the loop filter and applied to the VCO. Control voltage on the VCO changes the frequency in a direction that reduces the phase difference between the input signal and the local oscillator. When the loop is locked, the control voltage is such that the frequency of the VCO is exactly equal to the average frequency of the input signal. For each cycle of input there is one, and only one, cycle of oscillator output. One obvious application of phaselock is in automatic frequency control (AFC). Perfect frequency control can be achieved by this method, whereas conventional AFC techniques necessarily entail some frequency error . To maintain the control voltage needed for lock it is generally necessary to have a nonzero output from the phase detector. Consequently, the loop operates with some phase error present. As a practical matter, however, this error tends to be small in a well-designed loop . A slightly different explanation may provide a better understanding of loop operation. Let us suppose that the incoming signal carries information in its phase or frequency; this signal is inevitably corrupted by additive noise. The task of a phaselock receiver is to reproduce the original signal while removing as much of the noise as possible. To reproduce the signal the receiver makes use of a local oscillator whose frequency is very close to that expected in the signal. Local oscillator and incoming signal waveforms are compared with one another by a phase detector whose error output indicates instantaneous phase difference. To suppress noise the error is averaged over some length of time, and the average is used to establish frequency of the oscillator. If the original signal is well behaved (stable in frequency), the local oscillator will need very little information to be able to track, and that information can be obtained by averaging for a long period of time, thereby eliminating noise that could be very large. The input to the loop is a noisy signal, whereas the output of the VCO is a cleaned-up version of the input. It is reasonable, therefore, to consider the loop as a kind of filter that passes signals and rejects noise. Two important characteristics of the filter are that the bandwidth can be very small and that the filter automatically tracks the signal frequency. These features, automatic tracking and narrow bandwidth, account for the major uses of phase lock receivers. Narrow bandwidth is capable of rejecting large amounts of noise; it is not at all unusual for a PLL to recover a signal deeply embedded in noise. 1.2 History and Application An early description of phaselock was published by de Bellescize in 1932 and treated the synchronous reception of radio signals. Superheterodyne receivers had come into use during the 1920s, but there was a continual search for a simpler technique; one approach investigated was the synchronous, or homodyne, receiver. In essence, this receiver consists of nothing but a local oscillator, a mixer, and an audio amplifier. To operate, the oscillator must be adjusted to exactly the same frequency as the carrier of the incoming signal, which is then converted to an intermediate frequency of exactly 0 Hz. Output of the mixer contains demodulated information that is carried as sidebands by the signal. Interference will not be synchronous with the local oscillator, and therefore mixer output caused by an interfering signal is a beat-note that can be suppressed by audio filtering. Correct tuning of the local oscillator is essential to synchronous reception; any frequency error whatsoever will hopelessly garble the information. Furthermore, phase of the local oscillator must agree, within a fairly small fraction of a cycle, with the received carrier phase. In other words, the local oscillator must be phaselocked to the incoming signal. For various reasons the simple synchronous receiver has never been used extensively. Present-day phaselock receivers almost invariably use the superheterodyne principle and tend to be highly complex. One of their most important applications is in the reception of the very weak signals from distant spacecraft. The first widespread use of phaselock was in the synchronization of horizontal and vertical scan in television receivers. The start of each line and the start of each interlaced half-frame of a television picture are signaled by a pulse transmitted with the video information. As a very crude approach to reconstructing a scan raster on the TV tube, these pulses can be stripped off and individually utilized to trigger a pair of single sweep generators. A slightly more sophisticated approach uses a pair of free-running relaxation oscillators to drive the sweep generators. In this way sweep is present even if synchronization is absent. Free-running frequencies of the oscillators are set slightly below the horizontal and vertical pulse rates, and the stripped pulses are used to trigger the oscillators prematurely and thus to synchronize them to the line and half-frame rates (half-frame because United States television interlaces the lines on alternate vertical scans). In the absence of noise this scheme can provide good synchronization and is entirely adequate. Unfortunately, noise is rarely absent, and any triggering circuit is particularly susceptible to it. As an extreme, triggered scan will completely fail at a signal-to-noise ratio that still provides a recognizable, though inferior, picture. Under less extreme conditions noise causes starting-time jitter and occasional misfiring far out of phase. Horizontal jitter reduces horizontal resolution and causes vertical lines to have a ragged appearance. Severe horizontal misfiring usually causes a narrow horizontal black streak to appear. Vertical jitter causes an apparent vertical movement of the picture. Also, the interlaced lines of successive half-frames would so move with respect to one another that further picture degradation would result. Noise fluctuation can be vastly reduced by phaselocking the two oscillators to the stripped sync pulses. Instead of triggering on each pulse a phase-lock technique examines the relative phase between each oscillator and many of its sync pulses and adjusts oscillator frequency so that the average phase discrepancy is small. Because it looks at many pulses, a phaselock synchronizer is not confused by occasional large noise pulses that disrupt a triggered synchronizer. The flywheel synchronizers in present day TV receivers are really phase-locked loops. The name “flywheel” is used because the circuit is able to coast through periods of increased noise or weak signal. Substantial improvement in synchronizing performance is obtained by phase-lock. In a color television receiver, the color burst is synchronized by a phase-lock loop. Spaceflight requirements inspired intensive application of phaselock methods. Space use of phaselock began with the launching of the first American artificial satellites. These vehicles carried low-power (10 mw) CW transmitters; received signals were correspondingly weak. Because of Doppler shift and drift of the transmitting oscillator, there was considerable uncertainty about the exact frequency of the received signal. At the 108MHz frequency originally used, the Doppler shift could range over a ,3kHz interval. With an ordinary, fixed-tuned receiver, bandwidth would therefore have to be at least 6kHz, if not more. However, the signal itself occupies a very narrow spectrum and can be contained in something like a 6Hz bandwidth. Noise power in the receiver is directly proportional to bandwidth. Therefore, if conventional techniques were used, a noise penalty of 1000 times (30 dB) would have to be accepted. The numbers have become even more spectacular as technology has progressed; transmission frequencies have moved up to S-band, making the Doppler range some ,75kHz, whereas receiver bandwidths as small as 3 Hz have been achieved. The penalty for conventional techniques would thus be about 47 dB. Such penalties are intolerable and that is why narrowband, phase-locked, tracking receivers are used. Noise can be rejected by a narrowband filter, but if the filter is fixed the signal almost never will be within the pass-band. For a narrow filter to be usable it must be capable of tracking the signal. A phase-locked loop is capable of providing both the narrow bandwidth and the tracking that are needed. Moreover, extremely narrow bandwidths can be conveniently obtained (3 to 1000 Hz are typical for space applications); if necessary, bandwidth is easily changed. For a Doppler signal the information needed to determine vehicle velocity is the Doppler shift. A phase-lock receiver is well-adapted to Doppler recovery, for it has no frequency error when locked. 1.3 Other Applications The following applications, further discussed elsewhere in the book, represent some of the current uses of phase-lock. 1. One method of tracking moving vehicles involves transmitting a coherent signal to the vehicle, offsetting the signal frequency, and retransmitting back to the ground. The coherent transponder in the vehicle must operate so that the input and output frequencies are exactly related in the ratio m/n, where m and n are integers. Phase-lock techniques are often used to establish coherence. 2. A phase-locked loop can be used as a frequency demodulator, in which it has superior performance to a conventional discriminator. 3. Noisy oscillators can be enclosed in a loop and locked to a clean signal. If the loop has a wide bandwidth, the oscillator tracks out its own noise and its output is greatly cleaned up. 4. Frequency multipliers and dividers can be built by using PLLs. 5. Synchronization of digital transmission is typically obtained by phase-lock methods. 6. Frequency synthesizers are conveniently built by phase-lock loops. 2 Optical Communication Components 2.1 The Optical Fiber As discussed earlier, the atmosphere cannot be used as a transmission channel for terrestrial communications using light beams. The most promising channel is the optical fiber waveguide. An optical fiber essentially consists of a central transparent region called the core which is surrounded by a region of lower refractive index called the cladding . The core could either be homogeneous or could have a gradient in refractive index with the refractive index decreasing away from the center of the core. In the former type of fiber, also referred to as homogeneous core fibers, the guidance of light occurs through the phenomenon of total internal reflection at the core-cladding interface. In the latter type of fibers, also referred to as graded-index fibers, the guidance of light occurs through continuous refraction of light rays towards the center of the core. In an optical waveguide, there exist specific field distributions which propagate without changing their form and with a definite phase and group velocity. These field configurations are referred to as the modes of the optical waveguide. These modes are characterized by different propagation constants and different group velocities. In a multimode waveguide, there exist a large number of these propagating modes while in a single-mode waveguide there exists only one mode. Each mode has most of the energy inside the core, but due to the evanescent fields outside the core, a part of the energy is also traveling in the cladding. By making the cladding sufficiently thick, the fields of the mode at the cladding-air boundary can be made small, thus making it easy to handle and support without causing much disturbance to the modes. As already discussed, in a fiber optic communication system the information is coded in the form of discrete pulses which are transmitted through the fiber. The information capacity of the system will be determined by the number of pulses that can be sent per unit time. For the information to be retrieved at the output end, the various pulses must be well resolved in time. In an optical fiber due to various factors like the differences in group-velocity between the different modes and the dependence of the propagation constant of a mode on wavelength, a pulse of light broadens as it propagates through the fiber. Hence, even though two pulses may be well resolved at the input end, because of broadening of the pulses they may not be so at the output. In such a case no information can be retrieved at the output. Thus for a given broadening, the pulses have to be separated by a minimum time interval which would determine the ultimate information-carrying capacity of the system. When a pulse of radiation is injected into a fiber, it excites various modes of the fiber. Since each mode propagates with, in general, a different characteristic group velocity, the incident pulse of light broadens as it propagates through the fiber. This is referred to as intermodal broadening. When the fiber can carry only one propagating mode, i.e., in a single-mode fiber, this broadening is absent, but due to the dependence of the propagation constant on wavelength, there is still some broadening; this is referred to as intramodal broadening. Both intermodal and intramodal dispersions arise as a result of (a) waveguide effects and (b) material effects; the latter due to the finite bandwidth of the source and the fact that at different wavelengths the refractive indices are different. It may be mentioned here that since lasers have much smaller spectral width as compared to light-emitting diodes, the material dispersion is much lower in a system employing lasers as compared to one using LEDs. For example, with an LED, the pulse broadening due to material dispersion may be ~4 nsec/km, whereas with a laser this would be less than 0.2 nsec/km. The broadening of a pulse of light as it propagates through an optical fiber can also be visualized by using the concept of geometrical optics. When a pulse of light is injected into a homogeneous core optical fiber, it excites rays traveling at different angles with the axis. As can be seen from Figure 16.2 since rays making larger angles with the axis have to traverse a longer optical path length, they take a longer time to reach the output end. Consequently the pulse of light broadens as it propagates through the fiber. In contrast, in a graded index fiber, even though rays making larger angles with the axis have to traverse longer path lengths they do so in a medium with a lower value of refractive index (see Figure 16.3). Thus the longer path length can be partially compensated by propagation at a higher velocity. Hence the broadening of a pulse must be much lower in a graded index fiber as compared to a homogeneous core fiber. In fact this is indeed the case, and for high-bandwidth applications, graded index fibers are more suitable than homogeneous core fibers. It may be mentioned here that in optical fibers having very small core radii and small index difference between the core and cladding, it can be so arranged that only one mode of propagation exists in the fiber. Such fibers are therefore referred to as single-mode fibers. Because of the presence of just one mode, the dispersion in these fibers is very small and is only due to intramodal broadening. Such fibers are indeed expected to be used in future super high bandwidth systems. In addition to the extremely large information-carrying capacity of a system using lightwaves, communication or transmission through optical fibers has several other additional advantages over the conventional metallic systems like the coaxial cable, etc. (i) Because of the extremely low transmission loss of practically available fibers, one can have much greater distance between repeater stations, resulting in substantial cost savings. (ii) Optical fibers are typically about 100 mm in diameter and are basically made of silica or glass. This results in a heavy reduction in weight and volume of space required, which is an important consideration for laying in already crowded available conduits. This saving in weight and volume is also important for shipboard applications and data handling using optical fibers in aircrafts. (iii) Optical fibers are immune to electromagnetic interference and there is no cross talk. This is an important consideration for secure communications in defense. (iv) Optical fibers can be used in explosive as well as high-voltage environments due to the absence of any hazard due to short circuits, etc. In addition to the main application in telecommunications, optical fibers are also expected to play an important role in computer links, space vehicles, industrial automation and process control, etc. In fact, recently optical fibers have been used to carry data and control information within big fusion lasers at Lawrence Livermore Laboratory and Los Alamos Scientific Laboratory and also for monitoring underground nuclear explosions at the Nevada test site. The additional advantages of using optical fibers include lower cost and immunity from noise. 2.2 Modulators and Detectors What we have discussed above is just one of the components of a lightwave communication system. In addition to this, one requires modulators, which would code the information into the lightwave, and detectors, which could detect the pulses of light at the receiver and decode the information. We will discuss briefly the principle behind modulators and detectors. Light sources can either be modulated directly by varying some source input parameter like input current, or the light output can be modulated externally by passing it through devices known as modulators. The most promising source to be used in optical fiber communication systems, namely, the semiconductor laser sources, can be modulated easily by varying the input current. In fact, in digital systems, the source has to be on-off modulated, and practically, the semiconductor sources can be on-off modulated at high speeds with rise times of less than a nanosecond. The on-off modulation is done by biasing the laser diode slightly below the threshold value, which is typically ~100 mA. At this stage, the laser diode operates as an LED and emits incoherent light at a low optical output power. An additional current (~20 mA) is added by a high-speed driver, which switches the laser diode from incoherent emission state to a coherent emission with large output optical power. By keeping the “off state” slightly below threshold, the delay between the applied electrical pulse and the resulting optical output pulse is minimized; this delay must indeed not be more than the bit interval so that the optical pulses accurately reproduce the input signal. An important factor to be taken care of is the temperature sensitivity of the output optical power. In the above-mentioned scheme of operation, this fact can be taken care of by varying the DC bias through an optical feedback circuit so as to take care of both slow changes in ambient temperature and the gradual aging of the laser itself. The monitoring of the output power is usually done by collecting the light emanating from the backside of the laser, the light from the front surface being coupled into the fiber itself. For nonsemiconductor laser sources, an external modulator is used for modulation. The external modulators make use of various properties possessed by different materials. Thus, certain crystals have a birefringence which changes in the presence of an applied electric field. Thus, the state of polarization of a beam can be changed by passing it through such a crystal. If the crystal is placed between crossed polarizers, one would have an intensity modulation. Similarly, acousto-optic modulators are based on the interaction of an acoustic beam with the light wave. The propagating acoustic wave creates a refractive index grating which in turn diffracts the optical wave. At the receiving terminals or at repeater stations, one requires optical detectors which receive the input optical signal and convert it into electrical signals. The three important detector types that find use in lightwave communication are the photomultiplier, the PIN photodiode, and the avalanche photodiode. Even though photomultipliers possess large gains, the latter two are expected to find more widespread application because they are less bulky, do not need high bias voltages, and are much cheaper. The simplest solid-state photodetector consists of a reverse-biased p-n junction with an open center area that is anti-reflection coated to receive the incident light. The absorbed photons excite electrons from the valence band into the conduction band. The electrons and holes so generated are separated by an applied electric field to induce a photocurrent across the junction. In order to detect very low optical powers one uses the avalanche photodetector. In this device, the electron-hole pair produced by a photon of light is accelerated in the device and is made to release more electron-hole pairs, thus leading to a gain.