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Total Internal Reflection A ray of light incident upon the interface between two transparent optical materials of different indices of refraction will be totally internally reflected under certain conditions. When the ray is incident from the direction of the more dense material and the angle made by the ray with the normal to the interface is greater than the critical angle, the light will be reflected, not refracted. The critical angle is dependent only on the indices of refraction of the media.
An off-axis ray of light traversing a fiber 50mm in diameter may be reflected 3,000 times per foot of fiber length. This number increases in direct proportion to diameter decrease. In principal, total internal reflection between two transparent optical media results in a loss of zero loss per reflection; thus a useful quantity of illumination can be transported. This spectral reflectance differs significantly from that of metallic coatings shown graphically. An aluminum mirror cladding on a glass fiber core would sustain a loss of approximately 10 percent per reflection, a level that could not be tolerated in practical fiber optics. The reflection at the core-clad interface must be much higher in optical fiber to obtain useful light transmission over long distances.
Light is transmitted the length of a fiber at a constant angle with respect to the fiber axis. Scattering from the true geometric path can occur due to:
Light will be scattered in proportion to fiber length and depends on the angle of incidence. To be functional long fibers must have an optical quality superior to that of short fibers. Surface scattering occurs readily if optical polishing has not produced a surface that is perpendicular to the axis of the fiber; pits, scratches, and scuffs diffuse light very rapidly. Additional losses such as bending are covered in the Internal & External Losses section. The speed of light in matter is less than the speed of light in air. The change in velocity that occurs when light passes from one medium to another results in refraction. A portion of the light incident on a boundary surface from a higher index media to air is not transmitted but is instead reflected back into the air. The majority that is transmitted is totally reflected from the interface, assuming that the angle is less than the critical angle as in Figure 2-3. The relationship between the angle of incidence, I, and the angle of refraction, R, is expressed by Snell’s law as:
, where, ni is the index of refraction of first media (air in many cases) and nr the index of refraction in which the light continues to travel (in this case the core). When ni = 1(air) for all practical purposes, the refractive index of the core would be calculated from:
Figure 2-3 Refraction, Reflection and Numerical Aperture
The information given herein, including drawings, illustrations and schematics (that are intended for illustration purposes only), is believed to be reliable. However, Polymicro Technologies makes no warranties as to its accuracy or completeness and disclaims any liability in connection with its use. Polymicro Technologies only obligation shall be as set forth in Polymicro Technologies' standard terms and conditions of sale for this product and in no way will Polymicro Technologies be liable of any incidental, indirect or consequential damages arising out of the sale, resale, use or misuse of the product. Users of Polymicro Technologies' products should make their own evaluation to determine the suitability of each such product for the specific application.
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LEGAL NOTICES | PRIVACY POLICY |
Copyright © Polymicro Technologies, 1995 - 2008 All Rights Reserved
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Rays may be classified as meridional and skew.
Meridional rays are those that pass through the axis of a fiber while
being internally reflected. Skew rays are those that never intersect the
fiber axis although their behavior patterns resemble those of meridional
rays in all other respects. For convenience, we will deal only with the
geometric optics of meridional ray tracing. 
As
indicated in Figure 2-3, the angle of reflection is equal to the angle of
incidence. (By definition, the angle is that measured between the ray and
the normal to the interface at the point of reflection.)
