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Name: yje
Status: N/a
Age: N/A
Location: N/A
Country: N/A
Date: Around 1993


Question:
How is it that light from a laser can travel in a small glass fiber for many miles without regeneration of the original signal?



Replies:
Light may be conveyed down a length of a glass fiber via a process called total internal reflection. Suppose a beam of light is traveling in material A and strikes an interface between this material and another material B, where these two materials have different indices of refraction. Imagine a line at right angles to the interface passing through the point where the beam hits. The angle between this line and the beam is called the angle of incidence.If this angle is 0 degrees, all of the beam is transmitted & none is reflected. Now suppose the light source is moved so that the beam strikes more and more obliquely, then the amount of beam transmitted decreases, and more and more of it is reflected. If B's index of refraction is smaller than A's, we will eventually reach an angle of incidence called the critical angle at which all of the beam is reflected and none transmitted. This is total internal reflection. If the angle of incidence is further increased, the total reflection persists. This is what happens inside the glass fiber. The beam enters traveling approximately straight down the fiber. If there is a not-too-abrupt bend in the fiber, the beam will hit the fiber's inside wall (a glass-air interface) at a large angle of incidence and be totally reflected. This may occur thousands of times per meter of fiber. And this can be done with ordinary light as well as with laser light. Laser light works better because all parts of its beam start out traveling in essentially the same direction. Even if it is collimated, a "beam" of ordinary light will enter the fiber with some range of directions; some of the beam will not totally reflect, so some light is transmitted through the walls and is lost.

R.C. Winther



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