Fibers and Cables

Fiber optics is a technology in which signals are converted from electrical into optical signals, transmitted through a thin glass fiber, and re-converted into electrical signals. The basic optical fiber consists of two concentric layers differing in optical properties, and a protective outer coating.

Core: the inner light-carrying member.
Cladding: the middle layer, which serves to confine the light to the core.
Buffer: the outer layer which serves as a "shock absorber" to protect the core and cladding from damage.

The concentric layers of an optical fiber include the light-carrying core, the cladding and the protective buffer.

Total Internal Reflection
Light injected into the core and striking the core-to-cladding interface at an angle greater than the critical angle will be reflected back into the core. Since angles of incidence and reflection are equal, the light ray continues to zig-zag down the length of the fiber. The light is trapped within the core. Light striking the interface at less than the critical angle passes into the cladding and is lost.

Once light begins to reflect down a fiber it will continue to do so.

Rays of light do not travel randomly. They are channeled into modes, which are possible paths for a light ray traveling down the fiber. A fiber can support as few as one mode and as many as tens of thousands of modes. While we are normally not interested in modes per se, the number of modes in a fiber is significant because it helps determine the fiberOs bandwidth. More modes typically mean lower bandwidth. The reason is dispersion.

As a pulse of light travels through the fiber, it spreads out in time. While there are several reasons for such dispersion, two are of principal concern. The first is modal dispersion, which is caused by different path lengths followed by light rays as they bounce down the fiber. Some rays follow a more direct route than others. The second type of dispersion is material dispersion: different wavelengths of light travel at different speeds. By limiting the number of wavelengths of light, you limit the material dispersion.

Dispersion limits the bandwidth of the fiber. At high data rates, dispersion will allow pulses to overlap so that the receiver can no longer distinguish where one pulse begins and another ends.

 

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