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Wednesday, December 3, 2008

Fiber Optic Cable

By John Kapoor

Think of discrete packets of light propagating through the waveguide. Most of the light rays making up these packets will be guided through the waveguide within the limits of its physical boundaries. The path of light will follow in a straight line, bouncing off the side of the waveguide as a result of reflection. Some of the rays will get loss in the waveguide, but most will continue to reflect.

Optical Fiber is a very thin and flexible medium having a cylindrical shape. It is the light equivalent of microwave wave-guide. The fiber waveguide is constructed of two layers of glass or plastic. It consists of following three sections:

  1. The inner most layer or the core, is where the light travels. It is made of glass or plastic and has refractive index.
  2. The fiber core is surrounded by a transparent sheath called cladding. It is a glass or plastic coating having optical properties much different from those of the core. The refractive index of cladding is lower than fiber core and denoted by µc. This layer is kept thick enough about one-tenth of the cross-sectional area in order to provide isolation.
  3. These layers are protected by an outer section called jacket or coating. It is made of polymer or any other material that provides protection against moisture, crushing and other environmental dangers.

The core acts like a continuous layer of two parallel mirrors. It has higher refractive index then cladding ie. µf > µc which results in total internal reflection. A signal is first encoded into light beam which is then passed in between the boundaries and propagated as a result of multiple internal reflection.

If the light wave enters at one end of a fiber at an angle less than acceptance angle then it strikes the fiber cladding medium at an angle more than critical angle and is totally reflected the totally internal reflected light stroke the opposite wall at an angle more than critical angle and is again totally reflected this process continues through out the length of the fiber and emerges from the other end without appreciable loss of energy

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