通信工程外文翻译

附录

一、英语原文

Optical fiber

Abstract:

An optical fiber (or optical fiber) is a flexible, transparent fiber made of a pure glass (silica) not much thicker than a human hair. It functions as a waveguide, or ―light pipe‖[1], to transmit light between the two ends of the fiber.[2] The field of applied science and engineering concerned with the design and application of optical fibers is known as fiber optics. Optical fibers are widely used in fiber-optic communications, which permits transmission over longer distances and at higher bandwidths (data rates) than other forms of communication. Fibers are used instead of metal wires because signals travel along them with less loss and are also immune to electromagnetic interference. Fibers are also used for illumination, and are wrapped in bundles so that they may be used to carry images, thus allowing viewing in confined spaces. Specially-designed fibers are used for a variety of other applications, including sensors and fiber lasers.

Optical fibers typically include a transparentcore surrounded by a transparent cladding material with a lower index of refraction. Light is kept in the core by total internal reflection. This causes the fiber to act as a waveguide. Fibers that support many propagation paths or transverse modes are called multi-mode fibers (MMF), while those that only support a single mode are called single-mode fibers (SMF). Multi-mode fibers generally have a wider core diameter, and are used for short-distance communication links and for applications where high power must be transmitted. Single-mode fibers are used for most communication links longer than 1,050 meters (3,440 ft).

Joining lengths of optical fiber is more complex than joining electrical wire or cable. The ends of the fibers must be carefully cleaved, and then spliced together, either mechanically or by fusing them with heat. Special optical fiber connectors for removable connections are also available.

【keywords】 optical Fiber Multi-mode fiber Index of refraction

Total internal reflection absorption

Fiber optics, though used extensively in the modern world, is a fairly simple, and relatively old, technology. Guiding of light by refraction, the principle that makes fiber optics possible, was first demonstrated by Daniel Colladon and Jacques

Babinetin Paris in the early 1840s. John Tyndall included a demonstration of it in his public lectures in London, 12 years later.[3] Tyndall also wrote about the property of total internal reflection in an introductory book about the nature of light in 1870: \the light passes from air into water, the refracted ray is bent towards the perpendicular... When the ray passes from water to air it is bent from the perpendicular... If the angle which the ray in water encloses with the perpendicular to the surface be greater than 48 degrees, the ray will not quit the water at all: it will be totally reflected at the surface.... The angle which marks the limit where total reflection begins is called the limiting angle of the medium. For water this angle is 48°27', for flint glass it is 38°41', while for diamond it is 23°42'.\[4][5] Unpigmented human hairs have also been shown to act as an optical fiber.[6

Practical applications, such as close internal illumination during dentistry, appeared early in the twentieth century. Image transmission through tubes was demonstrated independently by the radio experimenter Clarence Hansell and the television pioneer John Logie Baird in the 1920s. The principle was first used for internal medical examinations by Heinrich Lamm in the following decade. Modern optical fibers, where the glass fiber is coated with a transparent cladding to offer a more suitable refractive index, appeared later in the decade.[3] Development then focused on fiber bundles for image transmission. Harold Hopkins and Narinder Singh Kapany at Imperial College in London achieved low-loss light transmission through a 75 cm long bundle which combined several thousand fibers. Their article titled \flexible fibrescope, using static scanning\was published in the journal Nature in 1954.[7][8] The first fiber optic semi-flexible gastroscope was patented by Basil Hirschowitz, C. Wilbur Peters, and Lawrence E. Curtiss, researchers at the University of Michigan, in 1956. In the process of developing the gastroscope, Curtiss produced the first glass-clad fibers; previous optical fibers had relied on air or impractical oils and waxes as the low-index cladding material.

In 1880 Alexander Graham Bell and Sumner Tainter invented the 'Photophone' at the Volta Laboratory in Washington, D.C., to transmit voice signals over an optical beam.[9] It was an advanced form of telecommunications, but subject to atmospheric interferences and impractical until the secure transport of light that would be offered by fiber-optical systems. In the late 19th and early 20th centuries, light was guided through bent glass rods to illuminate body cavities.[10]Jun-ichi Nishizawa, a Japanese scientist at Tohoku University, also proposed the use of optical fibers for communications in 1963, as stated in his book published in 2004 in India.[11] Nishizawa invented other technologies that contributed to the development of optical fiber communications, such as the graded-index optical fiber as a channel for transmitting light from semiconductor lasers.[12][13] The first working fiber-optical data transmission system was demonstrated by German physicist Manfred B?rner at Telefunken Research Labs in Ulm in 1965, which was followed by the first patent application for this technology in 1966.[14][15]Charles K. Kao and George A. Hockham of the British company Standard Telephones and Cables (STC) were the first to promote the idea that the attenuation in optical fibers could be reduced below 20 decibels per kilometer (dB/km), making fibers a practical communication medium.[16]

They proposed that the attenuation in fibers available at the time was caused by impurities that could be removed, rather than by fundamental physical effects such as scattering. They correctly and systematically theorized the light-loss properties for optical fiber, and pointed out the right material to use for such fibers — silica glass with high purity. This discovery earned Kao the Nobel Prize in Physics in 2009.[17

Principle of operation

An optical fiber is a cylindrical dielectric waveguide (nonconducting waveguide) that transmits light along its axis, by the process of total internal reflection. The fiber consists of a core surrounded by a cladding layer, both of which are made of dielectric materials. To confine the optical signal in the core, the refractive index of the core must be greater than that of the cladding. The boundary between the core and cladding may either be abrupt, in step-index fiber, or gradual, in graded-index fiber. Index of refraction

The index of refraction is a way of measuring the speed of light in a material. Light travels fastest in a vacuum, such as outer space. The speed of light in a vacuum is about 300,000 kilometers (186,000 miles) per second. Index of refraction is calculated by dividing the speed of light in a vacuum by the speed of light in some other medium. The index of refraction of a vacuum is therefore 1, by definition. The typical value for the cladding of an optical fiber is 1.52.[34] The core value is typically 1.62.[34] The larger the index of refraction, the slower light travels in that medium. From this information, a good rule of thumb is that signal using optical fiber for communication will travel at around 200 million meters per second. Or to put it another way, to travel 1000 kilometers in fiber, the signal will take 5 milliseconds to propagate. Thus a phone call carried by fiber between Sydney and New York, a 12000 kilometer distance, means that there is an absolute minimum delay of 60 milliseconds (or around 1/16 of a second) between when one caller speaks to when the other hears. (Of course the fiber in this case will probably travel a longer route, and there will be additional delays due to communication equipment switching and the process of encoding and decoding the voice onto the fiber). Total internal reflection

When light traveling in an optically dense medium hits a boundary at a steep angle (larger than the \reflected. This is called total internal reflection. This effect is used in optical fibers to confine light in the core. Light travels through the fiber core, bouncing back and forth off the boundary between the core and cladding. Because the light must strike the boundary with an angle greater than the critical angle, only light that enters the fiber within a certain range of angles can travel down the fiber without leaking out. This range of angles is called the acceptance cone of the fiber. The size of this acceptance cone is a function of the refractive index difference between the fiber's core and cladding.In simpler terms, there is a maximum angle from the fiber axis at which light may enter the fiber so that it will propagate, or travel, in the core of the fiber. The sine

of this maximum angle is the numerical aperture (NA) of the fiber. Fiber with a larger NA requires less precision to splice and work with than fiber with a smaller NA. Single-mode fiber has a small NA. Multi-mode fiber

Fiber with large core diameter (greater than 10 micrometers) may be analyzed by geometrical optics. Such fiber is called multi-mode fiber, from the electromagnetic analysis (see below). In a step-index multi-mode fiber, rays of light are guided along the fiber core by total internal reflection. Rays that meet the core-cladding boundary at a high angle (measured relative to a line normal to the boundary), greater than the critical angle for this boundary, are completely reflected. The critical angle (minimum angle for total internal reflection) is determined by the difference in index of refraction between the core and cladding materials. Rays that meet the boundary at a low angle are refracted from the core into the cladding, and do not convey light and hence information along the fiber. The critical angle determines the acceptance angle of the fiber, often reported as a numerical aperture. A high numerical aperture allows light to propagate down the fiber in rays both close to the axis and at various angles, allowing efficient coupling of light into the fiber. However, this high numerical aperture increases the amount of dispersion as rays at different angles have different path lengths and therefore take different times to traverse the fiber.

In graded-index fiber, the index of refraction in the core decreases continuously between the axis and the cladding. This causes light rays to bend smoothly as they approach the cladding, rather than reflecting abruptly from the core-cladding boundary. The resulting curved paths reduce multi-path dispersion because high angle rays pass more through the lower-index periphery of the core, rather than the high-index center. The index profile is chosen to minimize the difference in axial propagation speeds of the various rays in the fiber. This ideal index profile is very close to a parabolic relationship between the index and the distance from the axis. Single-mode fiber

Fiber with a core diameter less than about ten times the wavelength of the propagating light cannot be modeled using geometric optics. Instead, it must be analyzed as an electromagnetic structure, by solution of Maxwell's equations as reduced to the electromagnetic wave equation. The electromagnetic analysis may also be required to understand behaviors such as speckle that occur when coherent light propagates in multi-mode fiber. As an optical waveguide, the fiber supports one or more confined transverse modes by which light can propagate along the fiber. Fiber supporting only one mode is called single-mode or mono-mode fiber. The behavior of larger-core multi-mode fiber can also be modeled using the wave equation, which shows that such fiber supports more than one mode of propagation (hence the name). The results of such modeling of multi-mode fiber approximately agree with the predictions of geometric optics, if the fiber core is large enough to support more than a few modes.

The waveguide analysis shows that the light energy in the fiber is not completely

confined in the core. Instead, especially in single-mode fibers, a significant fraction of the energy in the bound mode travels in the cladding as an evanescent wave.

The most common type of single-mode fiber has a core diameter of 8–10 micrometers and is designed for use in the near infrared. The mode structure depends on the wavelength of the light used, so that this fiber actually supports a small number of additional modes at visible wavelengths. Multi-mode fiber, by comparison, is manufactured with core diameters as small as 50 micrometers and as large as hundreds of micrometers. The normalized frequencyV for this fiber should be less than the first zero of the Bessel functionJ0 (approximately 2.405). Special-purpose fiber

Some special-purpose optical fiber is constructed with a non-cylindrical core and/or cladding layer, usually with an elliptical or rectangular cross-section. These include polarization-maintaining fiber and fiberdesigned to suppress whispering gallery mode propagation.

Photonic-crystal fiber is made with a regular pattern of index variation (often in the form of cylindrical holes that run along the length of the fiber). Such fiber uses diffraction effects instead of or in addition to total internal reflection, to confine light to the fiber's core. The properties of the fiber can be tailored to a wide variety of applications.

Mechanisms of attenuation

Attenuation in fiber optics, also known as transmission loss, is the reduction in intensity of the light beam (or signal) with respect to distance traveled through a transmission medium. Attenuation coefficients in fiber optics usually use units of dB/km through the medium due to the relatively high quality of transparency of modern optical transmission media. The medium is usually a fiber of silica glass that confines the incident light beam to the inside. Attenuation is an important factor limiting the transmission of a digital signal across large distances. Thus, much research has gone into both limiting the attenuation and maximizing the amplification of the optical signal. Empirical research has shown that attenuation in optical fiber is caused primarily by both scattering and absorption. Light scattering

The propagation of light through the core of an optical fiber is based on total internal reflection of the lightwave. Rough and irregular surfaces, even at the molecular level, can cause light rays to be reflected in random directions. This is called diffuse reflection or scattering, and it is typically characterized by wide variety of reflection angles.Light scattering depends on the wavelength of the light being scattered. Thus, limits to spatial scales of visibility arise, depending on the frequency of the incident light-wave and the physical dimension (or spatial scale) of the scattering center, which is typically in the form of some specific micro-structural feature. Since visible light has a wavelength of the order of one micrometer (one millionth of a meter) scattering centers will have dimensions on a similar spatial scale. Thus, attenuation results from the incoherent scattering of light at internal surfaces

and interfaces. In (poly)crystalline materials such as metals and ceramics, in addition to pores, most of the internal surfaces or interfaces are in the form of grain boundaries that separate tiny regions of crystalline order. It has recently been shown that when the size of the scattering center (or grain boundary) is reduced below the size of the wavelength of the light being scattered, the scattering no longer occurs to any significant extent. This phenomenon has given rise to the production of transparent ceramic materials.Similarly, the scattering of light in optical quality glass fiber is caused by molecular level irregularities (compositional fluctuations) in the glass structure. Indeed, one emerging school of thought is that a glass is simply the limiting case of a polycrystalline solid. Within this framework, \degrees of short-range order become the building blocks of both metals and alloys, as well as glasses and ceramics. Distributed both between and within these domains are micro-structural defects that provide the most ideal locations for light scattering. This same phenomenon is seen as one of the limiting factors in the transparency of IR missile domes.[35]At high optical powers, scattering can also be caused by nonlinear optical processes in the fiber.[36][37] UV-Vis-IR absorption

In addition to light scattering, attenuation or signal loss can also occur due to selective absorption of specific wavelengths, in a manner similar to that responsible for the appearance of color. Primary material considerations include both electrons and molecules as follows:

1) At the electronic level, it depends on whether the electron orbitals are spaced (or \can absorb a quantum of light (or photon) of a specific wavelength or frequency in the ultraviolet (UV) or visible ranges. This is what gives rise to color.

2) At the atomic or molecular level, it depends on the frequencies of atomic or molecular vibrations or chemical bonds, how close-packed its atoms or molecules are, and whether or not the atoms or molecules exhibit long-range order. These factors will determine the capacity of the material transmitting longer wavelengths in the infrared (IR), far IR, radio and microwave ranges.

The design of any optically transparent device requires the selection of materials based upon knowledge of its properties and limitations. The Latticeabsorption characteristics observed at the lower frequency regions (mid IR to far-infrared wavelength range) define the long-wavelength transparency limit of the material. They are the result of the interactive coupling between the motions of thermally induced vibrations of the constituent atoms and molecules of the solid lattice and the incident light wave radiation. Hence, all materials are bounded by limiting regions of absorption caused by atomic and molecular vibrations (bond-stretching)in the far-infrared (>10 μm).

Thus, multi-phonon absorption occurs when two or more phonons simultaneously interact to produce electric dipole moments with which the incident radiation may couple. These dipoles can absorb energy from the incident radiation,

reaching a maximum coupling with the radiation when the frequency is equal to the fundamental vibrational mode of the molecular dipole (e.g. Si-O bond) in the far-infrared, or one of its harmonics.

The selective absorption of infrared (IR) light by a particular material occurs because the selected frequency of the light wave matches the frequency (or an integer multiple of the frequency) at which the particles of that material vibrate. Since different atoms and molecules have different natural frequencies of vibration, they will selectively absorb different frequencies (or portions of the spectrum) of infrared (IR) light.

Reflection and transmission of light waves occur because the frequencies of the light waves do not match the natural resonant frequencies of vibration of the objects. When IR light of these frequencies strikes an object, the energy is either reflected or transmitted. References

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Gambling, W. A., \Rise and Rise of Optical Fibers\IEEE Journal on Selected Topics in Quantum Electronics, Vol. 6, No. 6, pp. 1084–1093, Nov./Dec. 2000.

Hecht, Jeff, Understanding Fiber Optics, 4th ed., Prentice-Hall, Upper Saddle River, NJ, USA 2002 (ISBN 0-13-027828-9).

Mirabito, Michael M.A; and Morgenstern, Barbara L., The New Communications Technologies: Applications, Policy, and Impact, 5th. Edition. Focal Press, 2004. (ISBN 0-24-080586-0).

Nagel S. R., MacChesney J. B., Walker K. L., \Chemical Vapor Deposition (MCVD) Process and Performance\IEEE Journal of Quantum Electronics, Vol. QE-18, No. 4, p. 459, April 1982.

Ramaswami, R., Sivarajan, K. N., Optical Networks: A Practical Perspective, Morgan Kaufmann Publishers, San Francisco, 1998 (ISBN 1-55860-445-6). VDV Works LLC Lennie Lightwave's Guide To Fiber Optics, http://www.vdvworks.com/LennieLw/ ? 2002-6.

二、汉语译文

摘要：

光导纤维，简称光纤，是一种达致光在玻璃或塑料制成的纤维中的全反射原理传输的光传导工具。微细的光纤封装在塑料护套中，使得它能够弯曲而不至于断裂。通常光纤的一端的发射装置使用发光二极管或一束激光将光脉冲传送至光纤，光纤的另一端的接收装置使用光敏元件检测脉冲。包含光纤的线缆称为光缆。由于光在光导纤维的传输损失比电在电线传导的损耗低得多，更因为主要生产原料是硅，蕴藏量极大，较易开采，所以价格便宜，促使光纤被用作长距离的信息传递工具。随着光纤的价格进一步降低，光纤也被用于医疗和娱乐的用途。

光纤主要分为两类，渐变光纤与突变光纤。前者的折射率是渐变的，而后者的折射率是突变的。另外还分为单模光纤及多模光纤。近年来，又有新的光子晶体光纤问世。

光导纤维是双重构造，核心部分是高折射率玻璃，表层部分是低折射率的玻璃或塑料，光在核心部分传输，并在表层交界处不断进行全反射，沿“之”字形向前传输。这种纤维比头发丝还细，这样细的纤维要有折射率截然不同的双重结构分布，是一个非常惊人的技术。各国科学家经过多年努力，创造了内附着法、MCVD法、VAD法等等，制成了超高纯石英玻璃，特制成的光导纤维传输光的效率有了非常明显的提高。现在较好的光导纤维，其光传输损失每公里只有零点二分贝；也就是说传播一公里后只损失4.5％。 【关键词】光纤多模折射全反射散射

光纤虽然在现代世界中广泛使用，但仍是一个相当简单陈旧的技术。最早在19世纪40年代初由丹尼尔·Colladon和巴比涅雅克首次在巴黎展示了通过折射导光的光纤的导光原理。12年后约翰·廷德尔在他的伦敦的公共讲座中作了示范。在1870年左右廷德尔还写了关于自然光在内部全反射的理论的入门书：“当光线从空气中传递入水，折射光正朝着垂直弯曲，当光线从水到空气垂直弯曲通过......如果在水中的光与表面垂直包围的角度大于48度，射线一点也不会穿出水面，将被水面完全反射.这标志着限制全反射开始的角度被称为介质的限制角。水，

这个角度是48°27'，火石玻璃，它是38°41'，而钻石，它是23°42'”。未染色的人的头发也被作为光纤。

实际应用出现在20世纪初的牙科密切内部照明，在20世纪20年代显像管传输图像通过被无线电实验者克拉伦斯·汉塞尔和电视的先驱约翰·洛吉贝尔德独立证实。该原则最早是由海因里希·拉姆在其后的十年内体检。

10年之后出现现代光学纤维是以玻璃纤维涂用透明覆面，以提供更适合的折射率。这之后的发展则侧重于图像传输的纤维束。哈罗德·霍普金斯大学和伦敦帝国学院的纳林德·辛格Kapany通过一个75厘米长束结合几千纤维，实现低损耗光传输。他们于1954年在“自然”杂志发表了题为“一个灵活采用静态扫描的纤维内窥镜”的文章。在1956年第一个光纤半灵活的胃镜由罗勒Hirschowitz，C.威尔伯·彼得斯，和Lawrence E.柯蒂斯在密歇根大学的研究人员获得专利。在胃镜发展的过程中，柯蒂斯生产出第一包层玻璃纤维;而以前的光纤依赖于空气或不切实际的油脂和蜡为低折射率的包层材料。

1880年亚历山大·格雷厄姆·贝尔和萨姆纳弧形在华盛顿特区沃尔特实验室Photophone发明了通过光束传输语音信号。这是电子通信的一种先进形式，但是要受大气干扰，是不切实际的，直到光纤系统提供的可靠的光的传输。在19世纪末和20世纪初，光被用来透过弯曲的玻璃棒引导灯老照亮体腔。西泽俊一，一个日本东北大学的科学家，还提出在1963年使用的光纤通信，正如他于2004年在印度出版的书上所说。西泽发明的其他技术，如半导体激光器的光传输通道的梯度折射率光纤，对光纤通信的发展作出了贡献。第一台运作的光纤数据传输系统被证实是德国物理学家曼弗雷德B?rner于1965年在乌尔姆的德律风根实验室研制成功，这项技术于1966年获得了第一项专利。高锟和英国公司的标准电话和电缆（STC）的乔治·A·Hockham率先推广光纤的衰减可以减少每公里低于20分贝（分贝/公里）的想法，使光纤成为实用的通讯媒介。他们提出可用纤维的衰减是由应该被去除的杂质而不是由基本物理效应如散射所致。他们正确和系统的分析了光纤的光损耗特性，并指出了生产这种纤维所使用的正确材料 - 高纯度石英玻璃。这一发现使得高锟在2009年获得诺贝尔物理学奖。 运作原理：

光纤是圆柱形的介质波导，应用全反射原理来传导光线。光纤的结构大致分为里

面的核心部分与外面的包覆部分。为了要局限光信号于核心，包覆的折射率必须小于核心的折射率。渐变光纤的折射率是缓慢改变的，从轴心到包覆，逐渐地减小；而突变光纤在核心-包覆边界区域的折射率是急剧改变的。 折射率

折射率可以用来计算在物质里的光线速度。在真空里，及外太空，光线的传播速度最快，大约为 3 亿米／秒。一种物质的折射率是真空光速除以光线在这物质里传播的速度。所以，根据定义，真空折射率是 1 。折射率越大，光线传播的速度越慢。通常光纤的核心的折射率是 1.48 ，包覆的折射率是 1.46 。所以，光纤传导信号的速度粗算大约为 2 亿米／秒。电话信号，经过光纤传导，从纽约到悉尼，大约 12000 公里距离，会有最低 0.06 秒时间的延迟。 全反射

当移动于密度较高的介质的光线，以大角度入射于核心-包覆边界时，假若这入射角（光线与边界面的法线之间的夹角）的角度大于临界角的角度，则这光线会被完全地反射回去。光纤就是应用这种效应来局限传导光线于核心。在光纤内部传播的光线会被边界反射过来，反射过去。由于光线入射于边界的角度必须大于临界角的角度，只有在某一角度范围内射入光纤的光线，才能够通过整个光纤，不会泄漏损失。这角度范围称为光纤的受光锥角，是光纤的核心折射率与包覆折射率的差值的函数。更简单地说，光线射入光纤的角度必须小于受光角的角度，才能够传导于光纤核心。受光角的正弦是光纤的数值孔径。数值孔径越大的光纤，越不需要精密的熔接和操作技术。单模光纤的数值孔径比较小，需要比较精密的熔接和操作技术。

多模光纤

核心直径较大的光纤（大于 10 微米）的物理性质，可以用几何光学的理论来分析，这种光纤称为多模光纤，用于通信用途时，线材会以橘色外皮做为辨识。 在一个多模突变光纤内，光线靠着全反射传导于核心。当光线遇到核心-包覆边界时，假若入射角大于临界角，则光线会被完全反射。临界角的角度是由核心折

射率与包覆折射率共同决定。假若入射角小于临界角，则光线会折射入包覆，无法继续传导于核心。临界角又决定了光纤的受光角，通常以数值孔径来表示其大小。较高的数值孔径会允许光线，以较近轴心和较宽松的角度，传导于核心，造成光线和光纤更有效率的耦合。但是，由于不同角度的光线会有不同的光程，通过光纤所需的时间也会不同，所以，较高的数值孔径也会增加色散。有些时候，较低的数值孔径会是更适当的选择。

渐变光纤的核心的折射率，从轴心到包覆，逐渐地减低。这会使朝着包覆传导的光线，平滑缓慢地改变方向，而不是急剧地从核心-包覆边界反射过去。这样，大角度光线会花更多的时间，传导于低折射率区域，而不是高折射率区域。因此，所形成的曲线路径，会减低多重路径色散。工程师可以精心设计渐变光纤的折射率分布，使得各种光线在光纤内的轴传导速度差值，能够极小化。这理想折射率分布应该会非常接近于抛物线分布。

单模光纤

核心直径小于传播光波波长约十倍的光纤，不能用几何光学理论来分析其物理性质。替而代之，必须改用麦克斯韦方程组来分析，导出相关的电磁波方程。视为光学波导，光纤可以传播多于一个横模的光波。只允许一种横模传导的光纤称为单模光纤。用于通信用途时，线材会以黄色外皮做为辨识[来源请求]。大直径核心、多横模的光纤的物理性质，也可以用电磁波波动方程分析。结果会显示出，这种光纤允许多于一个横模的光波。这样的解析多模光纤，所得到的结果，与几何光学的解析结果大致相同。

波导分析显示，在光纤内的光波的能量，并不是全部局限于核心里。令人惊讶地，特别是在单模光纤里，有很大一部分的能量是以衰减波的形式传导于包覆。 最常见的一种单模光纤，核心直径大约为 7.5–9.5 微米，专门用于传导近红外线。多模光纤的核心直径可以小至 50 微米，或者大至几百微米。

特用光纤

有些特用光纤的核心或包覆会特别地制作成非圆柱形，通常像椭圆形或长方形。这包括维护偏极化光纤。

光子晶体光纤是一种新型的光纤，其折射率以规律性的模式变化（通常沿着光纤的轴向会有圆柱空洞）。光子晶体光纤应用衍射效应（单独的或加上全反射效应）来局限光波于光纤核心。

衰减机制

在介质内，光纤的衰减，又称为传输损失，指的是随着传输距离的增加，光束（或信号）强度会减低。由于现代光传输介质的高质量透明度，光纤的衰减系数的单位通常是 dB/km （每公里长度介质的分贝）。因为硅石玻璃纤维能够满足严格的规定，局限光束于内部，传输介质材料大多是由硅石玻璃纤维制成的。 阻碍数字信号远距离传输的一个重要因素就是衰减。因此，减少衰减是光纤光学研究的必然目标。经过多次实验得到的结果，显示出光散射和吸收是造成光纤衰减的主要原因之一。 光散射

因为光线的全反射，光线可以传输于光纤核心。粗糙、不规则的表面，甚至在分子层次，也会使光线往随机方向反射，称这现象为漫反射或光散射，其特征通常是多种不同的反射角。大多数物体因为表面的光散射，可以被人类视觉侦测到。光散射跟入射光波的波长有关。可见光的波长大约是 1 微米。人类视觉无法侦测到超小于这尺寸的物体。所以，位于可见物体表面的散射中心也有类似的空间尺寸。光波入射于内部的边界面时，会因为不同调散射而造成衰减。对于结晶材料或多晶材料，像金属或陶瓷，除了细孔以外，大部分内部接口的形式乃晶界，分隔了晶粒尺寸的微小区域。材料学专家发现，假若能将散射中心（或晶界）的尺寸减小到低于入射光波的波长，则光散射的影响会减小很多，可以被忽略。这发现引起更多有关透明陶瓷材料的研究。类似地，在光学光纤内，光散射是由分子层次的不规则玻璃结构所造成的。很多材料学专家认为玻璃无疑是多晶材料的极限案例。而其展现出短距离现像的畴域，则是金属、合金、玻璃、陶瓷等等的基础建筑材料。散布在这些畴域之间，有很多微结构缺陷，是造成光散射的最理想地点。当光学倍率变高时，光纤的非线性光学行为也可能会造成光散射。

紫外线和红外线吸收

除了光散射以外，光纤材料会选择性地吸收某些特定波长的光波，这也会造成衰减或信号损失。吸收光波的机制类似颜色显现的机制。

1.在电子层次，光纤材料的每种组成原子，其不同的电子轨域的能级差值，决定了光纤材料能否吸收某特定频率或频率带的光子。这些特定频率或频率带的光子，大多属于紫外线或可见光的频区。这就是很多可见物质显示出颜色的机制。 2.在原子或分子层次，振动频率、堆积结构、化学键强度等等，这些重要因素共同决定了材料传输红外线，远红外线，无线电波，微波等等长波的能力。 在设计任何透明光学元件前，必须先知道材料的性质和限制，然后才能选择适当的材料。任何材料在低频率区域的晶格吸收特性，也赋予了这材料对于这低频率光波的透明限制。这是组成的原子或分子的热感应振动，和入射光波之间，相互耦合的结果，在。因此，在红外线频区（＞ 1 微米），每一种材料都要避开这些由于原子或分子振动机制而产生的吸收区域。因为某特定频率的红外线光波，恰恰好匹配了，某种材料的原子或分子的自然振动频率，这种材料会选择性地吸收这特定频率的光波。由于不同的原子或分子有不同的自然振动频率，它们会选择性地吸收不同频率（或不同频率带）的红外线光波。由于光波频率不匹配光纤材料的自然振动频率，会造成光波的反射或透射。当红外线光波入射于这不匹配的光纤材料，一部分能量会被反射，另一部分能量会被透射。

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