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Optical fiber

Optical fibers
Optical fibers

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 (POF = plastic optical fibers) 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.

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.

Fibers, like waveguides, can have various transmission modes. The 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 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.

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.

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.

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'.

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.

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.

Longer-range systems still have to use optical amplifiers.

Refracted rays

A diagram which illustrates the propogation of light through an optical fiber.
A diagram which illustrates the propogation of light through an optical fiber.

In an optical fiber, a refracted ray is one that is refracted from the core into the cladding. Specifically a ray having direction such that where r is the radial distance from the fiber axis, φ(r ) is the azimuthal angle of projection of the ray at r on the transverse plane, θ(r ) is the angle the ray makes with the fiber axis, n (r ) is the refractive index at r, n (a ) is the refractive index at the core radius, a . Refracted rays correspond to radiation modes in the terminology of mode descriptors.

For the fiber to guide the optical signal, the refractive index of the core must be slightly higher than that of the cladding. In different types of fibers, the core and core-cladding boundary function slightly differently in guiding the signal. Especially in single-mode fibers, a significant fraction of the energy in the bound mode travels in the cladding.

Source: from Federal Standard 1037C

Following are the two major types of fiber-optic cable:

  • Single-mode: uses a specific light wavelength. The cable's core diameter is 8 to 10 micrometres. Single-mode fiber is often used for intercity telephone trunks and video applications.
  • Multi-mode: uses a large number of frequencies (or modes). The cable's core is larger than that of single-mode fiber. Multimode fiber is the type usually specified for LANs and WANs.

Advantages of optical fibers over wires

  • Lower cost in the long run
  • Low loss of signal (typically less than 0.3 dB/km), so repeater-less transmission over long distances is possible
  • Large data-carrying capacity (thousands of times greater, reaching speeds of up to 1.6 Tb/s in field deployed systems and up to 10 Tb/s in lab systems)
  • Immunity to electromagnetic interference, including nuclear electromagnetic pulses (but can be damaged by alpha and beta radiation)
  • No electromagnetic radiation; difficult to eavesdrop
  • High electrical resistance, so safe to use near high-voltage equipment or between areas with different earth potentials
  • Low weight
  • Signals contain very little power
  • No crosstalk between cables
  • No sparks (e.g. in automobile applications)
  • Difficult to place a tap or listening device on the line, providing better phyisical network security

Disadvantages of optical fibers compared to wires

  • High investment cost
  • Need for more expensive optical transmitters and receivers
  • More difficult and expensive to splice than wires
  • At higher optical powers, is susceptible to "fiber fuse" wherein a bit too much light meeting with an imperfection can destroy as much as 1.5 kilometers of wire at several metres per second [1][2][3][4] . A "Fiber fuse" protection device at the transmitter can break the circuit to prevent damage, if the extreme conditions for this are deemed possible.
  • Cannot carry electrical power to operate terminal devices. However, current telecommunication trends greatly reduce this concern: availability of cell phones and wireless PDAs; the routine inclusion of back-up batteries in communication devices; lack of real interest in hybrid metal-fiber cables; and increased use of fiber-based intermediate systems.

Almost all these disadvantages have been surmounted or bypassed in contemporary telecommunications usage, and communication systems are now unthinkable without fiber optics. Their cost is much more economic than old coaxial cables because the transmitters and receivers (laser and photodiodes) turn out cheaper than electric circuitry as their capacity is much superior. The cost of regeneration in electrical long distance transmission systems is completely impractical for modern communications.

History

Charles Kao's 1966 PhD thesis estimated that glass fibers need to have an optic signal attenuation of less than 20 dB per kilometer to be useful for long distance communication. The first useful optical fiber was invented in 1970 by researchers Maurer, Keck, Schultz, and Zimar working for American glass maker Corning Glass Works. They manufactured a fiber with 17 dB optic attenuation per kilometer by doping silica glass with titanium.

The first transatlantic telephone cable to use optical fiber was TAT-8, which went into operation in 1988.

Other uses of optical fibres

  • Fibers can be used as light guides in medical and other applications where bright light needs to be brought to bear on a target without a clear line-of-sight path.
  • Optical fibers can be used as sensors to measure strain, temperature, pressure and other parameters.
  • Bundles of fibers are used along with lenses for long, thin imaging devices called endoscopes, which are used to view objects through a small hole. Medical endoscopes are used for minimally invasive exploratory or surgical procedures (endoscopy). Industrial endoscopes (see fiberscope or borescope) are used for inspecting anything hard to reach, such as jet engine interiors.
  • In some high-tech buildings, optical fibers are used to route sunlight from the roof to other parts of the building.
  • Optical fibers have many decorative applications, including signs and art, artificial Christmas trees, and lighting.
  • A few communities have Fiber to the Home technology which provides subscribers with Ultra High Speed Internet, Telephone, and Television services.
  • The German company Sennheiser developed a microphone working with a Laser and optical fibres. German article about this microphone

Optical fiber in waveguides

Waveguides are silicon chips with extremely thin and extremely flexed optical fibers on them. Companies like JDS Uniphase manufacture these waveguides for use in computers and in splitting boxes. A waveguide separates the different colors of light, and allows it to have the same signal sent in many directions. (see waveguides)

See also

External links


 

 

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