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【正文】 inrich 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 lowloss light transmission through a 75 cm long bundle which bined several thousand fibers. Their article titled A flexible fibrescope, using static scanning was published in the journal Nature in 1954.[7][8] The first fiber optic semiflexible 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 glassclad fibers。 previous optical fibers had relied on air or impractical oils and waxes as the lowindex cladding material. In 1880 Alexander Graham Bell and Sumner Tainter invented the 39。Photophone39。 at the Volta Laboratory in Washington, ., to transmit voice signals over an optical beam.[9] It was an advanced form of telemunications, but subject to atmospheric interferences and impractical until the secure transport of light that would be offered by fiberoptical systems. In the late 19th and early 20th centuries, light was guided through bent glass rods to illuminate body cavities.[10] Junichi Nishizawa, a Japanese scientist at Tohoku University, also proposed the use of optical fibers for munications 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 munications, such as the gradedindex optical fiber as a channel for transmitting light from semiconductor lasers.[12][13] The first working fiberoptical data transmission system was demonstrated by German physicist Manfred B246。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 pany 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 munication 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 lightloss 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 stepindex fiber, or gradual, in gradedindex 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 .[34] The core value is typically .[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 munication 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 munication 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 critical angle for the boundary), the light will be pletely 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 fiber39。s core and 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. Singlemode fiber has a small NA.Multimode fiberFiber with large core diameter (greater than 10 micrometers) may be analyzed by geometrical optics. Such fiber is called multimode fiber, from the electromagnetic analysis (see below). In