OPTICAL FIBRE CABLEAn optical fiber cable is a cable containing one or more optical fibers.The optical fiber elements are typically individually coated with plasticlayers and contained in a protective tube suitable for the environmentwhere the cable will be deployed.DESIGN:Optical fiber consists of a core and a cladding layer, selected for totalinternal reflection due to the difference in the refractive index betweenthe two. In practical fibers, the cladding is usually coated with a layer ofacrylate polymer or polyimide. This coating protects the fiber fromdamage but does not contribute to its optical waveguide properties.A critical concern in outdoor cabling is to protect the fiber fromcontamination by water. This is accomplished by use of solid barrierssuch as copper tubes, and water-repellent jelly or water-absorbingpowder surrounding the fiber.Finally, the cable may be armored to protect it from environmentalhazards, such as construction work or gnawing animals. Undersea cablesare more heavily armored in their near-shore portions to protect themfrom boat anchors, fishing gear, and even sharks, which may be attractedto the electrical power that is carried to power amplifiers or repeaters inthe cable.CABLE TYPES: OFC: Optical fiber, conductive OFN: Optical fiber, nonconductive OFCG: Optical fiber, conductive, general use OFNG: Optical fiber, nonconductive, general use OFCP: Optical fiber, conductive, plenum
OFNP: Optical fiber, nonconductive, plenum OFCR: Optical fiber, conductive, riser OFNR: Optical fiber, nonconductive, riser OPGW: Optical fiber composite overhead ground wire ADSS: All-Dielectric Self-SupportingDATA TRANSMISSION:Data transmission fiber optics, simply put, is the sending and receivingof data from point-to-point via a network, thus the fundamental functionof all fiber systems from small to large. Data transmission requirementsrange from very simple cables connecting servers or storage arraysinside a network or telecommunications system, to large multi-fiberdistribution cables supporting intra-building connectivity and beyond.For smaller, localized data transmission applications, a multitude ofproducts are available to move data from place to place. Primarilymultimode, these applications use single fibers to move multiple signalsover distances, usually less than 300 meters. Depending on the particularapplication or system requirement, data transmission cabling can takemany forms from basic simplex (SX) or duplex (DX) cable assembliesto ribbon fiber distribution cables, and various combinations ofcustomized products.In larger data transmission applications, data transmission can bemultimode, single mode, or a combination of the two, depending onbandwidth and transmission distance requirements. These applicationsgenerally use a higher volume or longer lengths of cabling, or in somecase both, supporting data centers, building-to-building, campuses, andcarrier network communications.PRINCIPLE OF OPERATION:An optical fiber is a cylindrical dielectric waveguide (nonconductingwaveguide) that transmits light along its axis, by the process of totalinternal reflection. The fiber consists of a core surrounded by a claddinglayer, both of which are made of dielectric materials. To confine theoptical signal in the core, the refractive index of the core must be greaterthan that of the cladding.
Index of refractionThe index of refraction is a way of measuring the speed of light in amaterial. Light travels fastest in a vacuum, such as outer space. Thespeed of light in a vacuum is about 300,000 kilometers (186,000 miles)per second. Index of refraction is calculated by dividing the speed oflight in a vacuum by the speed of light in some other medium. The indexof refraction of a vacuum is therefore 1, by definition. The typical valuefor the cladding of an optical fiber is 1.52. The core value is typically1.62. The larger the index of refraction, the slower light travels in thatmedium.Total internal reflectionWhen light traveling in an optically dense medium hits a boundary at asteep angle (larger than the critical angle for the boundary), the light willbe completely reflected. This is called total internal reflection. Thiseffect is used in optical fibers to confine light in the core. Light travelsthrough the fiber core, bouncing back and forth off the boundarybetween the core and cladding. Because the light must strike theboundary with an angle greater than the critical angle, only light thatenters the fiber within a certain range of angles can travel down the fiberwithout leaking out. This range of angles is called the acceptance coneof the fiber. The size of this acceptance cone is a function of therefractive index difference between the fibers core and cladding.Multi-mode fiberFiber with large core diameter (greater than 10 micrometers) may beanalyzed by geometrical optics. Such fiber is called multi-mode fiber,from the electromagnetic analysis. In a step-index multi-mode fiber, raysof light are guided along the fiber core by total internal reflection. Raysthat meet the core-cladding boundary at a high angle greater than thecritical angle for this boundary, are completely reflected. The criticalangle (minimum angle for total internal reflection) is determined by thedifference in index of refraction between the core and claddingmaterials. Rays that meet the boundary at a low angle are refracted fromthe core into the cladding, and do not convey light and henceinformation along the fiber.
Single-mode fiberFiber with a core diameter less than about ten times the wavelength ofthe propagating light cannot be modeled using geometric optics.The electromagnetic analysis may also be required to understandbehaviors such as speckle that occur when coherent light propagates inmulti-mode fiber. As an optical waveguide, the fiber supports one ormore confined transverse modes by which light can propagate along thefiber. Fiber supporting only one mode is called single-mode or mono-mode fiber. The most common type of single-mode fiber has a corediameter of 8–10 micrometers and is designed for use in the nearinfrared. The mode structure depends on the wavelength of the lightused, so that this fiber actually supports a small number of additionalmodes at visible wavelengths. Multi-mode fiber, by comparison, ismanufactured with core diameters as small as 50 micrometers and aslarge as hundreds of micrometers.Special-purpose fiberSome special-purpose optical fiber is constructed with a non-cylindricalcore and/or cladding layer, usually with an elliptical or rectangularcross-section. These include polarization-maintaining fiber and fiberdesigned to suppress whispering gallery mode propagation.Disadvantages of Optical fibres:Price - Even though the raw material for making optical fibres, sand, isabundant and cheap, optical fibres are still more expensive per metrethan copper. Although, one fibre can carry many more signals than asingle copper cable and the large transmission distances mean that fewerexpensive repeaters are required.Fragility - Optical fibres are more fragile than electrical wires.Affected by chemicals - The glass can be affected by various chemicalsincluding hydrogen gas (a problem in underwater cables.)Opaqueness - Despite extensive military use it is known that most fibresbecome opaque when exposed to radiation.Requires special skills - Optical fibres cannot be joined together as aeasily as copper cable and requires additional training of personnel andexpensive precision splicing and measurement equipment.
Advantages Of Fiber Optics Immunity to Electromagnetic Interference Data Security Non Conductive Cables Eliminating Spark Hazards Ease Of Installation High Bandwidth Over Long DistancesUses of Optical FibresUntil the optical fibre network was developed, telephone calls weremainly sent as electrical signals along copper wire cables. As demandfor the systems to carry more telephone calls increased, simple copperwires did not have the capacity, known as bandwidth, to carry theamount of information required.Systems using coaxial cables like TV aerial leads were used but as theneed for more bandwidth grew, these systems became more and moreexpensive especially over long distances when more signal regeneratorswere needed. As demand increases and higher frequency signals arecarried, eventually the electronic circuits in the regenerators just cannotcope.Optical fibres offer huge communication capacity. A single fibre cancarry the conversations of every man, woman and child on the face ofthis planet, at the same time, twice over. The latest generations of opticaltransmission systems are beginning to exploit a significant part of thishuge capacity, to satisfy the rapidly growing demand for datacommunications and the Internet. The main advantages of using optical fibres in the communicationsindustry are: - A much greater amount of information can be carried on an opticalfibre compared to a copper cable. - In all cables some of the energy is lost as the signal goes along thecable. The signal then needs to be boosted using regenerators. Forcopper cable systems these are required every 2 to 3km but with opticalfibre systems they are only needed every 50km.
- Unlike copper cables, optical fibres do not experience any electricalinterference. Neither will they cause sparks so they can be used inexplosive environments such as oil refineries or gas pumping stations.- For equal capacity, optical fibres are cheaper and thinner than coppercables which makes them easier to install and maintain.