Optical fiber

An optical fiber is a transparent thin fiber, usually made of glass or plastic, for transmitting light. Fiber optics is the branch of science and engineering concerned with such optical fibers.

Uses of optical fibers

The optical fiber can be used as a medium for telecommunication and networking because it is flexible and can be bundled as cables. Although fibers can be made out of either transparent plastic or glass, the fibers used in long-distance telecommunications applications are always glass, because of the lower optical absorption. The light transmitted through the fiber is confined due to total internal reflection within the material. This is an important property that eliminates signal crosstalk between fibers within the cable and allows the routing of the cable with twists and turns. In telecommunications applications, the light used is typically infrared light, at wavelengths near to the minimum absorption wavelength of the fiber in use.

Related Topics:
Telecommunication - Networking - Glass - Total internal reflection - Crosstalk - Infrared

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Fibers are generally used in pairs, with one fiber of the pair carrying a signal in each direction, however bidirectional communications is possible over one strand by using two different wavelengths (colors) and appropriate coupling/splitting devices.

Related Topics:
Bidirectional - Wavelength

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Fibers, like waveguides, can have various transmission modes. fibers used for long-distance communication are known as single mode fibers, as they have only one strong propagation mode. This results in superior performance compared to other, multi-mode fibers, where light transmitted in the different modes arrives at different times, resulting in dispersion of the transmitted signal. Typical single mode fibers can sustain transmission distances of 80 to 140 km (50 to 87 miles) between regenerations of the signal, whereas most multi-mode fibers have a maximum transmission distance of 300 to 500 metres. Note that single mode equipment is generally more expensive than multi-mode equipment. Fibers used in telecommunications typically have a diameter of 125 µm. The transmission core of single-mode fibers most commonly have a diameter of 9 µm, while multi-mode cores are available with 50 µm or 62.5 µm diameters. The refractive index,n, of the pure glass in the core is typically 1.5.

Related Topics:
Waveguide - Transmission modes - Single mode fiber - Multi-mode fiber - Dispersion

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Because of the remarkably low loss and excellent linearity and dispersion behavior of single-mode optical fiber, data rates of up to 40 gigabits per second are available in real-world use on a single wavelength. Wavelength division multiplexing can then be used to allow many wavelengths to be used at once on a single fiber, allowing a single fiber to bear an aggregate bandwidth measured in terabits per second.

Related Topics:
Linearity - Dispersion - Gigabit - Wavelength division multiplexing - Bandwidth - Terabit

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The fiber transmission loss is minimal for 1550 nm light and dispersion is minimal at 1310 nm making these the optimal wavelength regions for data transmission.

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Modern fiber cables can contain up to a thousand fibers in a single cable, so the performance of optical networks easily accommodate even today's demands for bandwidth on a point-to-point basis. However, unused point-to-point potential bandwidth does not translate to operating profits, and it is estimated that no more than 1% of the optical fiber buried in recent years is actually 'lit'.

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Modern cables come in a wide variety of sheathings and armor, designed for applications such as direct burial in trenches, installation in conduit, lashing to aerial telephone poles, submarine installation, or insertion in paved streets. In recent years the cost of small fiber-count pole mounted cables has greatly decreased due to the high Japanese and South Korean demand for Fiber to the Home (FTTH) installations.

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Recent advances in fiber technology have reduced losses so far that no amplification of the optical signal is needed over distances of hundreds of kilometers. This has greatly reduced the cost of optical networking, particularly over undersea spans where the cost reliability of amplifiers is one of the key factors determining the performance of the whole cable system. In the past few years several manufacturers of submarine cable line terminal equipment have introduced upgrades that promise to quadruple the capacity of older submarine systems installed in the early to mid 1990s.

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Longer-range systems still have to use optical amplifiers.

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