www.nortonkit.com 18 अक्तूबर 2013
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Direct Links to Other Optics Pages:
Basic Concepts: [What Is Light?] [Light as a Wave] [Light as a Particle] [The Characteristics of a Photon] [The Photoelectric Effect] [The Transverse Electromagnetic Wave (TEM)]
Reflection and Refraction: [Introduction] [Reflection, Part 1] [Reflection, Part 2] [Refraction, Part 1] [Refraction, Part 2]
Lenses: [Introduction] [The Convex Lens]
Fiber Optics: [Introduction] [Fiber Optics, Part 2] [Fiber Optics, Part 3] [Fiber Optics, Part 4] [Fiber Optics, Part 5] [Fiber Optics, Part 6]
How Optical Fibers Work, Part 5

We said earlier that light is trapped inside the core of the optical fiber because the speed of light in the cladding is higher than the speed of light in the core. The obvious next questions are, "How much faster?" and, "How can we make it different for different parts of the glass fiber?"

The answer to both questions lies in the various additives that may be used in the glass-making process. While it is not our purpose here to discuss glass additives at any length, a few words are in order.

Glass is primarily made from molten silica (sand), although flint and quartz may also be used, and are used for specific purposes. However, if you just melt ordinary sand, the resulting glass is likely to be green or brown, due to the presence of iron in the mix. To eliminate that color, early glass workers learned to add manganese.

Most manufactured glass is actually soda-lime glass. This is commonly used for bottles, tableware, light bulbs, windows, and plate glass. Glass known as "crystal" is a potassium silicate mix that also includes lead oxide. Hence this is sometimes called "lead glass." Glass that must be resistant to chemicals and high temperatures has a fair amount of borax added. All additives will have an effect on the melting temperature of the glass, as well as on the density of (and therefore the speed of light inside) the glass.

When we speak of optical fibers, we don't normally discuss the actual speed of light in the fiber, but rather of the index of refraction of the glass itself.

Index of Refraction

The ratio of the speed of light in a vacuum to the speed of light in the specified medium. This is expressed as a positive real number greater than 1.

The speed of light in a vacuum such as open space is just slightly less than 300,000,000 meters per second (current measurements place it at very nearly 299,792,500 meters per second, give or take up to 200 m/s). For practical calculations, we'll just use the approximation.

The speed of light in water is 225,000,000 meters per second. Therefore, the index of refraction of water is:

300,000,000  = 1.33


In typical glass such as is used for optical fibers, the speed of light might well be about 200,000,000 m/s. The index of refraction for such glass would be:

300,000,000  = 1.50


In a real-world, practical fiber, this might describe the cladding, while the core would have a refractive index of perhaps 1.55 to 1.60. Such a combination is quite effective in keeping the bulk of the light energy inside the core, so that it can accomplish something when it reaches its destination.

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