Feb 22, 2009

OPTICAL FIBER

OPTICAL FIBER

A bundle of optical fibers. Theoretically, using advanced techniques such as DWDM, the modest number of fibers seen here could have sufficient bandwidth to easily carry the sum of all types of current data transmission needs for the entire planet. (~100 terabits per second per fiber )

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

Optical fibers are commonly used in telecommunication systems, as well as in illumination, sensors, and imaging optics.

Principle of operation

An optical fiber (American spelling) or fibre (British spelling) is a cylindrical dielectric waveguide that transmits light along its axis, by the process of total internal reflection. The fiber consists of a denser core surrounded by a cladding layer. For total internal reflection to confine the optical signal in the core, the refractive index of the core must be greater than that of the cladding. The boundary between the core and cladding may either be abrupt, in step-index fiber, or gradual, in graded-index fiber.



A diagram which illustrates the propagation of light through a multi-mode optical fiber.

Fiber with large (greater than 10 μm) core diameter may be analyzed by geometric optics. Such fiber is called multi-mode fiber, from the electromagnetic analysis (see below). In a step-index fiber, rays of light are guided along the fiber core by total internal reflection. Rays that meet the core-cladding boundary at a high angle (measured relative to a line normal to the boundary) are completely reflected. The minimum angle for total internal reflection is determined by the difference in index of refraction between the core and cladding materials. Rays that meet the boundary at a low angle are refracted from the core into the cladding, where they are not useful for conveying light along the fiber. In this way, the minimum angle for total internal reflection determines the acceptance angle of the fiber, often reported as a numerical aperture. A high numerical aperture makes it easier to efficiently couple a transmitter or receiver to the fiber. However, by allowing light to propagate down the fiber in rays both close to the axis and at various angles, a high numerical aperture also increases the amount of multi-path spreading, or dispersion, that affects light pulses in the fiber.

In graded-index fiber, the index of refraction in the core decreases continuously between the axis and the cladding. This causes light rays to bend smoothly as they approach the cladding, rather than reflect abruptly from the core-cladding boundary. The resulting curved paths reduce multi-path dispersion because high angle rays pass more through the lower-index periphery of the core, rather than the high-index center. The index profile is chosen to minimize the difference in axial propagation speeds of the various rays in the fiber. This ideal index profile is very close to a parabolic relationship between the index and the distance from the axis.

Fiber with a core diameter narrower than a few wavelengths of the light carried, is analyzed as an electromagnetic structure, by solution of Maxwell's equations, as reduced to the electromagnetic wave equation. The electromagnetic analysis may also be required to understand behaviors such as speckle that occur when coherent light propagates in multi-mode fiber. As an optical waveguide, the fiber supports one or more confined transverse modes by which light can propagate along its axis. Fiber supporting only one mode is called single-mode or mono-mode fiber, while fiber that supports more than one mode is called multi-mode fiber. By the waveguide analysis, it is seen that the light energy in the fiber is not completely confined in the core, but, especially in single-mode fibers, a significant fraction of the energy in the bound mode travels in the cladding as an evanescent wave.



A typical single-mode optical fiber, showing diameters of the component layers.



Optical fiber communication

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 attenuation. Both multi-mode and single-mode fibers are used in communications, with multi-mode fiber used mostly for short distances (up to 500 m), and single-mode fiber used for longer distance links. Because of the tighter tolerances required to couple light into and between single-mode fibers, single-mode transmitters, receivers, amplifiers and other components are generally more expensive than multi-mode components.

The light used is typically infrared light, at wavelengths near to the minimum absorption wavelength of the fiber in use. The fiber absorption is minimal for 1550 nm light and dispersion is minimal at 1310 nm making these the optimal wavelength regions for data transmission. A local minimum of absorption is found near 850 nm, a wavelength for which low cost transmitters and receivers can be designed, and this wavelength is often used for short distance applications. Fibers are generally used in pairs, with one fiber of the pair carrying a signal in each direction.


SYNCHRONOUS OPTICAL NETWORKING

The synchronous optical network, commonly known as SONET, is a standard for communicating digital information using lasers or light emitting diodes (LEDs) over optical fiber as defined by GR-253-CORE from Telcordia. It was developed to replace the Plesiochronous Digital Hierarchy (PDH) system for transporting large amounts of telephone and data traffic and to allow for interoperability between equipment from different vendors. The more recent synchronous digital hierarchy (SDH) standard developed by the International Telecommunication Union (ITU) is built on experience in the development of SONET. It is documented in standard G.707 and its extension G.708. Both SDH and SONET are widely used today; SONET in the U.S. and Canada, SDH in the rest of the world. SDH is growing in popularity and is currently the main concern with SONET now being considered as the variation.

SONET differs from PDH in that the exact rates that are used to transport the data are tightly synchronized across the entire network, made possible by atomic clocks. This synchronization system allows entire inter-country networks to operate synchronously, greatly reducing the amount of buffering required between each element in the network.

Both SONET and SDH can be used to encapsulate earlier digital transmission standards, such as the PDH standard, or used directly to support either ATM or so-called Packet over SONET networking. As such, it is inaccurate to think of SONET as a communications protocol in and of itself, but rather as a generic and all-purpose transport container for moving both voice and data.


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