中山复试专业英语

1、专 业 英 语 Specialized English Unit 7：第二部分 通信技术 Fiber Optic光纤Specialized English on Communication Engineering Snells Law(斯涅尔定律)相关背景知识n1n2A1 (angle of incidence)A2(angle of refraction)Specialized English on Communication Engineering NEW WORDS AND PHRASES interference n.干涉;干扰propagate v. 传播; propagation

2、n. refraction n. 折射；refractive index v. 折射率core n. （光纤）纤芯cladding n. 包层 dispersion n. 色散profile n. 曲线，分布图 multimode fiber 多模光纤single-mode fiber 单模光纤 graded-index fiber 渐变（折射率）光纤stepped-index fiber 阶跃（折射率）光纤 Specialized English on Communication Engineering Optical fiber transmission has come of age a

3、s a major innovation in munications. munications：电讯, 长途通讯, 无线电通讯, 电信学光纤传输作为电信业一项重大变革的时代已经到来。TEXTSpecialized English on Communication Engineering Such systems offer extremely high bandwidth, freedom from external interference, immunity from interception by external means, and cheap raw material (sili

4、con, the most abundant material on the Earth). bandwidth：带宽； external interference：外界干扰interception ：中途夺取, 拦截,侦听；raw material：原料 光纤通信系统提供了极宽的带宽，它不受外界干扰，不能通过外部手段进行信号截取，它的原材料是廉价的（硅，地球上最丰富的物质）。Specialized English on Communication Engineering Optical fibers guide light rays within the fiber material. Th

5、ey can do this because light rays bend or change direction when they pass from one medium to another. light rays ：光纤medium ：介质，媒介Fundamentals of Fiber Optic SystemSpecialized English on Communication Engineering They bend because the speed of propagation of light in each medium is different. This ph

6、enomenon is called refraction. speed of propagation：传播速度phenomenon：现象refraction：折射光线发生弯曲是因为光的传播速度在各种介质中是不同的。这种现象就称作折射。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

7、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，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，这个物体就会看起来比他真实的位置更远。Specialized English on Communication Engineering This effect occurs because the speed of the light rays from the object increases as the light rays pass f

8、rom the water to the air. light rays：光线发生这种效应是因为，来自物体的光线从水中进入空气时，其速度会增加。 Specialized English on Communication Engineering This causes them to bend, changing the angle at which you perceive the object. You can obtain an appreciation for the manner by which light flows by focusing upon Snells Law. per

9、ceive ：察觉，感知, 感到这使得光线发生弯曲，改变了观察物体的角度。通过Snell定律可以理解光线遵循的传播方式。Specialized English on Communication Engineering How optical fibers work can be explained by Snells Law, which states that the ratio of the sine of the angle of incidence to the sine of the angle of refraction is equal to the ratio of the p

10、ropagation velocities of the wave in the two respective media.the ratio of A to B：表示A与B之比the angle of refraction：折射角 the angle of incidence：入射角propagation velocities：传播速度光纤的工作原理可通过斯涅尔折射定律来解释，该定理阐明，入射角正弦值与折射角正弦值之比等于光波在两种媒介中的传输速度之比。 Specialized English on Communication Engineering the angle of refract

11、ionthe angle of incidenceconstant refractive index of the second medium refractive index of the first medium Snells Law Specialized English on Communication Engineering (a) Ray Escapes(b) Ray is Absorbed(c) Total Internal ReflectionSpecialized English on Communication Engineering Fig.2-1(b) demonstr

12、ates the condition that exists when A1 is at the critical angle and angle A2 is at 90dg.demonstrate:表示，表明critical angle：临界角（定义为折射角为90度时的入射角）dg表示角度单位，dg=degree度。 图2-1(b)表示当A1为临界角， A2为90度时的情况。Specialized English on Communication Engineering As shown in Fig.2-1 (c), any light rays that are incident at

13、angles greater than A1 of Fig.2-1 (b) will be reflected back into material 1 with angle A2 equal to A1. incident：入射的reflect：反射如图2-1 (c)所示，入射角大于图2-1 (b)中的入射角A1的任何入射光线，都会被以反射角A2 = A1反射回材料1。Specialized English on Communication Engineering An optical fiber is a dielectric (nonconductor of electricity) w

14、aveguide made of glass or plastic.dielectric：电介质, 绝缘体waveguide ：波导plastic ：塑料光纤是由玻璃或塑料制成的介质（电绝缘体）波导。Fiber Composition (光纤结构) Specialized English on Communication Engineering It consists of three distinct regions: a core, the cladding, and a sheath or jacket. The sheath or jacket protects the fiber b

15、ut does not govern the transmission capability of the fiber. cladding：包层 sheath：涂敷层，外壳jacket ：护套govern ：控制，统治, 支配transmission capability ：传输性能Specialized English on Communication Engineering The index of refraction of the assembly varies across the radius of the cable, and the core has a constant or

16、 smoothly varying index of refraction called nc. The cladding region has another constant index of refraction called n. assembly ：集合，组装radius ：半径constant ：常数光纤的折射率沿着半径变化,纤芯的折射率nc是常数或者是平滑变化的。包层的折射率n是另一个常数。Specialized English on Communication Engineering The core possesses a high refractive index, whe

17、reas the cladding is constructed to have a lower refractive index. The result of the difference in the refractive indices keeps light flowing through the core after it gets into the core, even if the fiber is bent or tied into a knot. assembly ：集合，组装radius ：半径constant ：常数纤芯具有高的折射率，而构建的包层具有较低的折射率。折射率

18、差异这一结果使得光进入光纤以后能够通过纤芯传播，即使光纤发生弯曲或者打结。Specialized English on Communication Engineering For a fiber designed to carry light in several modes of propagation at the same time (called a multimode fiber), the diameter of the core is several times the wavelength of the light to be carried. mode：模式 multimod

19、e fiber：多模光纤diameter：直径 wavelength：波长能够同时携载多个传输模式的光纤（称作多模光纤），其纤芯直径是所携载波长的几倍。Specialized English on Communication Engineering Wavelength is a measure of the distance between two cycles of the same wave measured in nanometers (nm), or billionths of a meter, and the cladding thickness will greater than

20、 the radius of the core. nanometer： 纳米，简写为nm；billionth：十亿分之一radius：半径nana- (前缀)纳（诺），10-9波长是同一个光波2周之间距离的量度标准，以纳米即一米的十亿分之一来表示，光纤包层厚度要大于光纤纤芯的半径。Specialized English on Communication Engineering A light source emits light at many angles relative to the center of the fiber. In Fig.2-2, light ray A enters

21、the fiber perpendicular to the face of the core and parallel to the axis. light source：光源relative to：相对于perpendicular to ：垂直于，正交于parallel to：平行于Specialized English on Communication Engineering For optical fibers in which the diameter of the core is many times the wavelength of the light transmitted,

22、 the light beam travels along the fiber by bouncing back and forth at the interface between the core and the cladding. bounce：反射back and forth：来来往往地, 来回地interface：界面，边界对于纤芯直径是光波长几倍的光纤，光束通过在纤芯和包层界面处来回反射而沿着传播。Modal Delay（模式延迟）Specialized English on Communication Engineering Rays entering the fiber at

23、differing angles are refracted varying numbers of times as they move from one end to the other and consequently do not arrive at the distant end with the same phase relationship as when they started.phase relationship ：相位关系当光线从光纤的一端传到另一端时，以不同角度进入光纤的光线，产生的折射的次数不同，因此到达远端时，其相位关系也与开始时不同。 Specialized Eng

24、lish on Communication Engineering The differing angles of entry are called modes of propagation, and a fiber carrying several modes is called a multimode fiber.modes of propagation：传播模式 multimode fiber ：多模光纤以不同角度进入的光线称作传播模式，携带多种模式的光纤称作多模光纤。Specialized English on Communication Engineering Multimode p

25、ropagation causes the rays leaving the fiber to interfere both constructively and destructively as they leave the end of the fiber. This effect is called modal delay spreading. constructively：相长地 destructively ：相消地modal delay spreading ：模式延迟扩散当光线从光纤末端传出时，多模传输引起相长干涉和相消干涉。这种效应称作模式延迟扩散。Specialized Engl

26、ish on Communication Engineering Because most optical communications systems transmit information in digital form consisting of pulse of light, the effect of modal delay spreading limits the capability of the fiber to transport recognizable pulses. pulse of light：光脉冲recognizable：可识别的因为大多数光纤通信系统都是以光脉

27、冲这种数字方式传输信息的，所以模式延迟扩散限制了光纤传输可识别脉冲的容量。 Specialized English on Communication Engineering TIMETIMETIMETIMEPOWERPOWERSpecialized English on Communication Engineering The effect of pulse spreading is to make it difficult or impossible for an optical receiver to differentiate one pulse from another after

28、a given transmission distance. optical receiver ：光接收机recognizable：可识别的经过一段特定的传输距离后，脉冲扩散使得光接收机很难或不可能将相邻脉冲彼此区分开来。 Specialized English on Communication Engineering Thus, after a predefined transmission distance, a multimode fiber either causes a very high error rate or precludes the capability of the p

29、ulse to be recognized and terminates the capability of the cable to be used for communications. error rate ：误码率preclude：阻止, 妨阻碍因此，在经历了预定的传输距离之后，多模光纤要么会导致很高的误码率，要么无法识别光脉冲，这样光纤就无法用于通信中了。 Specialized English on Communication Engineering If the diameter of the fiber core is only a few times the wavelength of the transmitted light (say, a factor of 3), only one ray or mode will be propagated, and no destructive interference between rays will occur.factor：表示“因数、倍”如果光纤纤芯的直径仅仅是传输光波波长的几倍（比如说3倍），那么只有一种光或传输模式可以传输，并且在光线之间不会产生干涉相消。Specialized English o