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Optical fibre

2007 Schools Wikipedia Selection. Related subjects: General Physics

   An optical fibre is a glass or plastic fibre designed to guide light
   along its length by total internal reflection. Fibre optics is the
   branch of applied science and engineering concerned with such optical
   fibers. Optical fibers are widely used in fibre-optic communication,
   which permits digital data transmission over longer distances and at
   higher data rates than electronic communication. They are also used to
   form sensors, and in a variety of other applications.

   The operating principle of optical fibers applies to a number of
   variants including multi-mode optical fibers, single-mode optical
   fibers, graded-index optical fibers, and step-index optical fibers.
   Because of the physics of the optical fibre, special methods of
   splicing fibers and of connecting them to other equipment are needed. A
   variety of methods are used to manufacture optical fibers, and the
   fibers are also built into different kinds of cables depending on how
   they will be used.

   The light-guiding principle behind optical fibers was first
   demonstrated in Victorian times, but modern optical fibers were only
   developed beginning in the 1950's. Optical fibers became practical for
   use in communications in the late 1970's, and since then several
   technical advances have been made to extend the reach and speed
   capability of optical fibers, and lower the cost of fibre
   communications systems.

Applications

Optical fibre communication

   The optical fibre 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 transparent plastic, glass, or a
   combination of the two, 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 fibre 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.

Fibre optic sensors

   Optical fibers can be used as sensors to measure strain, temperature,
   pressure and other parameters. The small size and the fact that no
   electrical power is needed at the remote location gives the fibre optic
   sensor advantages to conventional electrical sensor in certain
   applications.

   Optical fibers are used as hydrophones for seismic or SONAR
   applications. Hydrophone systems with more than 100 sensors per fibre
   cable have been developed. Hydrophone sensor systems are used by the
   oil industry as well as a few countries' navies. Both bottom mounted
   hydrophone arrays and towed streamer systems are in use. The German
   company Sennheiser developed a microphone working with a laser and
   optical fibers.

   Optical fiber sensors for temperature and pressure have been developed
   for downhole measurement in oil wells. The fibre optic sensor is well
   suited for this environment as it is functioning at temperatures too
   high for semiconductor sensors ( Distributed Temperature Sensing).

   Another use of the optical fibre as a sensor is the optical gyroscope
   which is in use in the Boeing 767 and in some car models (for
   navigation purposes) and the use in Hydrogen microsensors.

Other uses of optical fibers

   A flying disc illuminated by fiber optics
   Enlarge
   A flying disc illuminated by fibre optics

   Fibers are widely used in illumination applications. They are 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.
   In some buildings, optical fibers are used to route sunlight from the
   roof to other parts of the building (see non-imaging optics). Optical
   fibre illumination is also used for decorative applications, including
   signs, art, and artificial Christmas trees. Swarovski boutiques use
   optical fibers to illuminate their crystal showcases from many
   different angles while only employing one light source. Optical fibre
   is an intrinsic part of the light-transmitting concrete building
   product, LiTraCon.

   Optical fibre is also used in imaging optics. A coherent bundle of
   fibers is used, sometimes along with lenses, for a long, thin imaging
   device called an endoscope, which is 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.

   An optical fibre doped with certain rare-earth elements such as erbium
   can be used as the gain medium of a laser or optical amplifier.
   Rare-earth doped optical fibers can be used to provide signal
   amplification by splicing a short section of doped fibre into a regular
   (undoped) optical fiber line. The doped fibre is optically pumped with
   a second laser wavelength that is coupled into the line in addition to
   the signal wave. Both wavelengths of light are transmitted through the
   doped fibre, which transfers energy from the second pump wavelength to
   the signal wave. The process that causes the amplification is
   stimulated emission.

   Optical fibers doped with a wavelength shifter are used to collect
   scintillation light in physics experiments.

   Optical fibre can be used to supply a low level of power (around one
   watt) to electronics situated in a difficult electrical environment.
   Examples of this are electronics in high-powered antenna elements and
   measurement devices used in high voltage transmission equipment.

Principle of operation

   An optical fibre is a cylindrical dielectric waveguide that transmits
   light along its axis, by the process of total internal reflection. The
   fibre consists of a core surrounded by a cladding layer. 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 fibre, or gradual, in
   graded-index fibre.

Multimode fibre

   The propagation of light through a multi-mode optical fiber.
   Enlarge
   The propagation of light through a multi-mode optical fibre.

   Fibre with large (greater than 10  μm) core diameter may be analyzed by
   geometric optics. Such fibre is called multimode fibre, from the
   electromagnetic analysis (see below). In a step-index multimode fibre,
   rays of light are guided along the fibre 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), greater than the
   critical angle for this boundary, are completely reflected. The
   critical angle (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, and do not convey light and
   hence information along the fibre. The critical angle determines the
   acceptance angle of the fibre, often reported as a numerical aperture.
   A high numerical aperture allows light to propagate down the fiber in
   rays both close to the axis and at various angles, allowing efficient
   coupling of light into the fibre. However, this high numerical aperture
   increases the amount of dispersion as rays at different angles have
   different path lengths and therefore take different times to traverse
   the fibre. A low numerical aperture may therefore be desirable.

   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 reflecting
   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 fibre. This ideal index
   profile is very close to a parabolic relationship between the index and
   the distance from the axis.

Singlemode fibre

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

   Fibre with a core diameter less than about ten times the wavelength of
   the propagating light cannot be modeled using geometric optics.
   Instead, it must be 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 fibre
   supports one or more confined transverse modes by which light can
   propagate along the fiber. Fibre supporting only one mode is called
   single-mode or mono-mode fiber. The behavior of larger-core multimode
   fiber can also be modeled using the wave equation, which shows that
   such fiber supports more than one mode of propagation (hence the name).
   The results of such modeling of multi-mode fiber approximately agree
   with the predictions of geometric optics, if the fibre core is large
   enough to support more than a few modes.

   The waveguide analysis shows that the light energy in the fibre is not
   completely confined in the core. Instead, especially in single-mode
   fibers, a significant fraction of the energy in the bound mode travels
   in the cladding as an evanescent wave.

   The most common type of single-mode fibre has a core diameter of 8 to
   10 μm and is designed for use in the near infrared. It is notable that
   the mode structure depends on the wavelength of the light used, so that
   this fiber actually supports a small number of additional modes at
   visible wavelengths. Multi-mode fibre, by comparison, is manufactured
   with core diameters as small as 50 microns and as large as hundreds of
   microns.

Special-purpose fibre

   Some special-purpose optical fibre is constructed with a
   non-cylindrical core and/or cladding layer, usually with an elliptical
   or rectangular cross-section. These include polarization-maintaining
   fibre and fibre designed to suppress whispering gallery mode
   propagation.

Materials

   Glass optical fibers are almost always made from silica, but some other
   materials, such as fluorozirconate, fluoroaluminate, and chalcogenide
   glasses are used for longer-wavelength infrared applications. Like
   other glasses, these glasses have a refractive index of about 1.5.
   Typically the difference between core and cladding is less than one
   percent.

   Plastic optical fibre (POF) is commonly step-index multimode fiber,
   with core diameter of 1 mm or larger. POF typically has much higher
   attenuation than glass fibre (that is, the amplitude of the signal in
   it decreases faster), 1 dB/m or higher, and this high attenuation
   limits the range of POF-based systems.

Fibre fuse

   At high optical intensities, above 2 megawatts per square centimetre,
   when a fibre is subjected to a shock or is otherwise suddenly damaged,
   a fibre fuse can occur. The reflection from the damage vaporizes the
   fibre immediately before the break, and this new defect remains
   reflective so that the damage propagates back toward the transmitter at
   1–3 meters per second ^,^,. The open fibre control system, which
   ensures laser eye safety in the event of a broken fiber, can also
   effectively halt propagation of the fibre fuse . In situations, such as
   undersea cables, where high power levels might be used without the need
   for open fiber control, a "fibre fuse" protection device at the
   transmitter can break the circuit to prevent any damage.

Manufacturing

   Optical fibre is made by first constructing a large-diameter preform,
   with a carefully controlled refractive index profile, and then pulling
   the preform to form the long, thin optical fibre. The preform is
   commonly made by three chemical vapor deposition methods: inside vapor
   deposition, outside vapor deposition, and vapor axial deposition.

   With inside vapor deposition, a hollow glass tube approximately 40 cm
   in length known as a "preform" is placed horizontally and rotated
   slowly on a lathe, and gases such as silicon tetrachloride (SiCl[4]) or
   germanium tetrachloride (GeCl[4]) are injected with oxygen in the end
   of the tube. The gases are then heated by means of an external hydrogen
   burner, bringing the temperature of the gas up to 1900 kelvins, where
   the tetrachlorides react with oxygen to produce silica or germania
   (germanium oxide) particles. When the reaction conditions are chosen to
   allow this reaction to occur in the gas phase throughout the tube
   volume, in contrast to earlier techniques where the reaction occurred
   only on the glass surface, this technique is called modified chemical
   vapor deposition.

   The oxide particles then agglomerate to form large particle chains,
   which subsequently deposit on the walls of the tube as soot. The
   deposition is due to the large difference in temperature between the
   gas core and the wall causing the gas to push the particles outwards
   (this is known as thermophoresis). The torch is then traversed up and
   down the length of the tube to deposit the material evenly. After the
   torch has reached the end of the tube, it is then brought back to the
   beginning of the tube and the deposited particles are then melted to
   form a solid layer. This process is repeated until a sufficient amount
   of material has been deposited. For each layer the composition can be
   varied by varying the gas composition, resulting in precise control of
   the finished fibre's optical properties.

   In outside vapor deposition or vapor axial deposition, the glass is
   formed by flame hydrolysis, a reaction in which silicon tetrachloride
   and germanium tetrachloride are oxidized by reaction with water (H[2]O)
   in an oxyhydrogen flame. In outside vapor deposition the glass is
   deposited onto a solid rod, which is removed before further processing.
   In vapor axial deposition, a short seed rod is used, and a porous
   preform, whose length is not limited by the size of the source rod, is
   built up on its end. The porous preform is consolidated into a
   transparent, solid perform by heating to about 1800 kelvins.

   The preform, however constructed, is then placed in a device known as a
   drawing tower, where the preform tip is heated and the optic fiber is
   pulled out as a string. By measuring the resultant fiber width, the
   tension on the fiber can be controlled to maintain the fibre thickness.

   This manufacturing process is accomplished by several fibre optic
   companies, including 3M, Corning Inc., and Molex. In addition, various
   fiber optic component manufacturers, assembly houses, and custom fibre
   optic providers exist.

Optical fibre cables

   In practical fibers, the cladding is usually coated with a tough resin
   buffer layer, which may be further surrounded by a jacket layer,
   usually plastic. These layers add strength to the fibre but do not
   contribute to its optical wave guide properties.

   For indoor applications, the jacketed fibre is generally enclosed, with
   a bundle of flexible fibrous polymer (e.g. Kevlar) strength members, in
   a lightweight plastic cover to form a simple cable. Each end of the
   cable may be terminated with a specialized optical fibre connector to
   allow it to be easily connected and disconnected from transmitting and
   receiving equipment.

   For use in more strenuous environments, a much more robust cable
   construction is required. In loose-tube construction the fiber is laid
   helically into semi-rigid tubes, allowing the cable to stretch without
   stretching the fiber itself. This protects the fiber from tension
   during laying and due to temperature changes. Alternatively the fibre
   may be embedded in a heavy polymer jacket, commonly called "tight
   buffer" construction. These fibre units are commonly bundled with
   additional steel strength members, again with a helical twist to allow
   for stretching.

   Another critical concern in cabling is to protect the fibre from
   contamination by water, because its component hydrogen ( hydronium) and
   hydroxyl ions can diffuse into the fiber, reducing the fiber's strength
   and increasing the optical attenuation. Water is kept out of the cable
   by use of solid barriers such as copper tubes, water-repellant jelly,
   or more recently water absorbing powder, surrounding the fibre.

   Finally, the cable may be armored to protect it from environmental
   hazards, such as construction work or gnawing animals. Undersea cables
   are more heavily armored in their near-shore portions to protect them
   from boat anchors, fishing gear, and even sharks, which may be
   attracted to the electrical power signals that are carried to power
   amplifiers or repeaters in the cable.

   Modern fiber cables can contain up to a thousand fibers in a single
   cable, so the performance of optical networks easily accommodates 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 fibre 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, dual use as power
   lines , installation in conduit, lashing to aerial telephone poles,
   submarine installation, or insertion in paved streets. In recent years
   the cost of small fibre-count pole mounted cables has greatly decreased
   due to the high Japanese and South Korean demand for Fibre to the Home
   (FTTH) installations.

Termination and splicing

   ST Fiber connector
   Enlarge
   ST Fibre connector

   Optical fibers are connected to terminal equipment by optical fibre
   connectors. These connectors are usually of a standard type such as FC,
   SC, ST, or LC.

   Optical fibers may be connected to each other by connectors or by
   splicing, that is, joining two fibers together to form a continuous
   optical waveguide. The generally accepted splicing method is arc fusion
   splicing, which melts the fibre ends together with an electric arc. For
   quicker fastening jobs, a "mechanical splice" is used.

   Fusion splicing is done with a specialized instrument that typically
   operates as follows: The two cable ends are fastened inside a splice
   enclosure that will protect the splices, and the fibre ends are
   stripped of their protective polymer coating (as well as the more
   sturdy outer jacket, if present). The ends are cleaved (cut) with a
   precision cleaver to make them perpendicular, and are placed into
   special holders in the splicer. The splice is usually inspected via a
   magnified viewing screen to check the cleaves before and after the
   splice. The splicer uses small motors to align the end faces together,
   and emits a small spark between electrodes at the gap to burn off dust
   and moisture. Then the splicer generates a larger spark that raises the
   temperature above the melting point of the glass, fusing the ends
   together permanently. The location and energy of the spark is carefully
   controlled so that the molten core and cladding don't mix, and this
   minimizes optical loss. A splice loss estimate is measured by the
   splicer, by directing light through the cladding on one side and
   measuring the light leaking from the cladding on the other side. A
   splice loss under 0.1 dB is typical. The complexity of this process is
   the major thing that makes fibre splicing more difficult than splicing
   copper wire.

   Mechanical fiber splices are designed to be quicker and easier to
   install, but there is still the need for stripping, careful cleaning
   and precision cleaving. The fibre ends are aligned and held together by
   a precision-made sleeve, often using a clear gel ( index matching gel)
   that enhances the transmission of light across the joint. Such joints
   typically have higher optical loss, and are less robust than fusion
   splices, especially if the gel is used. All splicing techniques involve
   the use of an enclosure into which the splice is placed for protection
   afterward.

   Fibers are terminated in connectors so that the fiber end is held at
   the end face precisely and securely. A fiber optic connector is
   basically a rigid cylindrical barrel surrounded by a sleeve that holds
   the barrel in its mating socket. It can be push and click, turn and
   latch, or threaded. A typical connector is installed by preparing the
   fiber end and inserting it into the rear of the connector body. Quick
   set glue is usually used so the fiber is held securely, and a strain
   relief is secured to the rear. Once the glue has set, the end is
   polished to a mirror finish. Various types of polish profile are used,
   depending on the type of fiber and the application. For singlemode
   fiber, the fiber ends are typically polished with a slight curvature,
   such that when the connectors are mated the fibers touch only at their
   cores. This is known as a "physical contact" (PC) polish. The curved
   surface may be polished at an angle, to make an angled physical contact
   (APC) connection. Such connections have higher loss than PC
   connections, but greatly reduced backreflection, because light that
   reflects from the angled surface leaks out of the fibre core.

   Various methods to align two fiber ends to each other or one fibre to
   an optical device ( VCSEL, LED, waveguide etc.) have been reported.
   They all follow either an active fiber alignment approach or a passive
   fibre alignment approach.

History

   The history of dielectric optical lightguides goes back to Victorian
   times, when the total internal reflection principle was used to
   illuminate streams of water in elaborate public fountains. Later
   development, in the early-to-mid twentieth century, focused on the
   development of fibre bundles for image transmission, with the primary
   application being the medical gastroscope. The first fibre optic
   semi-flexible gastroscope was patented by Basil Hirschowitz, C. Wilbur
   Peters, and Lawrence E. Curtiss, researchers at the University of
   Michigan, in 1956. In the process of developing the gastroscope,
   Curtiss produced the first glass-clad fibers; previous optical fibers
   had relied on air or impractical oils and waxes as the low-index
   cladding material. A variety of other image transmission applications
   soon followed.

   In 1965, Charles K. Kao and George A. Hockham of the British company
   Standard Telephones and Cables were the first to recognize that
   attenuation of contemporary fibers was caused by impurities, which
   could be removed, rather than fundamental physical effects such as
   scattering. They demonstrated that optical fibre could be a practical
   medium for communication, if the attenuation could be reduced below 20
   dB per kilometer (Hecht, 1999, p. 114). By this measure, the first
   practical optical fibre for communications was invented in 1970 by
   researchers Robert D. Maurer, Donald Keck, Peter Schultz, and Frank
   Zimar working for American glass maker Corning Glass Works. They
   manufactured a fibre with 17 dB optic attenuation per kilometer by
   doping silica glass with titanium.

   On 22 April, 1977, General Telephone and Electronics sent the first
   live telephone traffic through fibre optics, at 6 Mbit/s, in Long
   Beach, California.

   The erbium-doped fibre amplifier, which reduced the cost of
   long-distance fibre systems by eliminating the need for
   optical-electrical-optical repeaters, was invented by David Payne of
   the University of Southampton, and Emmanuel Desurvire at Bell
   Laboratories in 1986. The two pioneers were awarded the Benjamin
   Franklin Medal in Engineering in 1998.

   The first transatlantic telephone cable to use optical fibre was TAT-8,
   based on Desurvire optimized laser amplification technology. It went
   into operation in 1988.

   In 1991, the emerging field of photonic crystals led to the development
   of photonic crystal fibre (Science (2003), vol 299, page 358), which
   guides light by means of diffraction from a periodic structure, rather
   than total internal reflection. The first photonic crystal fibers
   became commercially available in 1996 . Photonic crystal fibers can be
   designed to carry higher power than conventional fibre, and their
   wavelength dependent properties can be manipulated to improve their
   performance in certain applications.

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