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FAQ

What do you understand by Attenuation ?

Transmission of light by optical fibres is not 100% efficient. Some light is lost, causing attenuation of the signal. Several mechanisms are involved, including absorption by materials within the fibre, scattering of light out of the fibre core and leakage of light out of the core caused by environmental factors. The degree of attenuation depends on the wavelength of light transmitted. Attenuation measures the reduction in signal strength by comparing output power with the input power. Measurements are made in decibels (dB), a very useful unit. A decibel is a logarithmic unit measuring the ratio of o/p power to I/p power. Loss in decibels is defined as dBloss = -10 X log10 (POWER OUT /POWER IN), Thus, if O/P power is 0.001 of I/P power, the signal has experienced a 30db loss. The minus sign is added to avoid negative numbers in attenuation measurements. It is not used in systems where the sign of the logarithm indicates if the signal has decreased (minus) or increased (plus). Each optical fibre has a characteristic attenuation that is measured in decibels per unit length, normally decibels per kilometer. The total attenuation (in decibels) in the fibre equals the characteristic attenuation times the length. The basic attenuation mechanisms in a fibre are absorption, scattering and radiative losses of the optical energy. Absorption is related to the fibre material, whereas scattering is associated both with the fibre material and with structural imperfections in the optical waveguide. Attenuation owing to radiative effects originates from perturbations (both microscopic and macroscopic) of the fibre geometry.

Why should we use Optical Fibres instead of our very own conventional Copper Cables ?

There are various advantages, which make Optical Fibres better than Copper systems. The crucial operating difference between a fibre optic communication system and other types is that signals are transmitted as light. Conventional electronic communication relies on electrons passing through wires. Radio frequency and microwave communications rely on radio waves and microwaves travelling through open space. The major points which distinguish it on positive side are:

Low transmission loss and wide Bandwidth: Optical Fibres have lower transmission losses and wider bandwidth than copper wires. This means that with optical fibre cable systems more data can be sent over longer distances, thereby decreasing the number of wires and reducing the number of repeaters needed for these spans. This reduction in equipment and components decreases the system cost and complexity.
Small size and Weight: The low weight and the small (hair sized) dimensions of fibres offer a distinct advantage over heavy, bulky wire cables in crowded underground city ducts or in ceiling-mounted cable trays. This is also of importance in aircraft, satellites, and ships, where small, lightweight cables are advantageous, and in tactical military applications, where large amounts of cable must be unreeled and retrieved rapidly.
Immunity to interference: An especially important feature of optical fibres relates to their dielectric nature. This provides optical waveguides with immunity to electromagnetic interference (EMI), such as inductive pickup from signal carrying wires and lightning. It also ensures freedom from electromagnetic pulse (EMP) effects, which is of particular interest in military applications.
Electrical Isolation: Since optical fibres are constructed of glass, which is an electrical insulator, there is no need to worry about ground loops, Fibre-to-Fibre crosstalks is very low and equipment interface problems are simplified. This also makes the use of fibres attractive in electrically hazardous environment, since the fibre creates no arcing or sparking.
Signal Security: By using an optical fibre a high degree of data security is afforded, since the optical signal is well confined within the waveguide (with any emanations being absorbed by an opaque jacketing around the fibre). This makes fibre attractive in applications where information security is important, such as banking, computer networks and military systems.
Abundant Raw materials: Of additional importance is the fact that silica is the principal material of which optical fibres are made. This raw material is abundant and inexpensive, since it is found in ordinary sand. The expense in making the actual fibre arises in the process required to make ultrapure glass from this raw material.
Fibres are not prone to Thefts: Last but not the least, that Optical Fibres cables has a distinctive advantage of not being prone to Thefts, as it is the most common practice in conventional copper cables.

What do you understand by Refractive Index (R.I) ?

The most important optical measurement for any transparent material is its refractive index (n). The refractive index is the ratio of the speed of light in a vacuum to the speed of light in the medium (n= Cvac / Cmat). The Light always travels more slowly in a material than in vaccum, so the refractive index is always greater than 1.0 in the optical part of the spectrum. In practice R.I is measured by comparing the speed of light in the material to that in the air rather than in a vaccum.

Light is bent as it is passes through a surface where the refractive index changes- for example, as it passes from air to glass. The amount of bending depends on the refractive indices of the two media and the angle at which the light strikes the surface between them. The angles of incidence and refraction are measured not from the plane of the surface but from a line normal (perpendicular) to the surface. The relationship is known as Snell's Law, which is written ni sin I = nr sin R where ni , nr are the refractive Indexes of the initial medium and the medium into which the light is refracted, and the medium into which the light is refracted, and I and R are the angles of incidence and refraction respectively.

What are the major factors that can dictate the choice of certain fibre for specific applications ?

What is a mode ?

What are the characteristics of Step Index Fibres ?

Give some reasons, that why cabling is needed ?

Can the environment for optical cable loosely classified ?


The major types of environment for optical cable can be loosely classified as follows: Inside devices (e.g., inside a telephone switching system or computer). Intraoffice or horizontal (e.g., across a room or under a raised floor in a computer room; often to individual terminals or work groups.) Intrabuilding (e.g., between walls or above suspended ceilings between offices in a structure; typically between distribution nodes in a building.) Plennum installations (i.e, through air spaces in a building; must meet building codes). Interbuilding or campus links (short exterior connections;link distribution nodes in separate building). Tolerance of high temperatures or other environmental conditions Temporary light duties cables(e.g. remote news gathering. Temporary heavy duty cables(e.g. military battlefield communication) Aerial cables (e.g. strung from utility poles outdoors). May be supported by lashing to support wires or other cables. All di-electric self-supporting cables. Cable installed in Plastic Ducts buried underground. Direct burial cables(i.e., laid directly in a trench or plowed into the ground). Submarine cables(i.e., submerged in ocean water or sometimes fresh water) Instrumentation Cables, which may have to meet special requirements (e.g., withstand high temperatures, corrosive vapors, or nuclear radiation.) Composite cables, which include fibres and copper wires that carry signals (used in buildings). Hybrid power-fibre cables, which carry electric power (or serve as the ground wire for an electric power system) as well as optical signals.

Define Loose tube, Tight buffered, Ribbon Cables.


Loose tube Cable: In the simplest loose tube design, a coated fibre is contained in along tube, with inner diameter much larger than the fibre diameter. The fibre is installed in a loose helix inside the tube, so it can move freely with respect to the tube walls. This design protects the fibre from the stresses applied to the cable in installation or service, including effects of changing temperature. Such Stresses can cause bending losses as well as damage the fibre. Loose tubes can be used without any filling. However if they are to be used outdoors, they are normally filled with a jelly like material. The gel acts as a buffer, keeping out moisture and letting the fibres move in the tube. A single tube contains upto a dozen fibres, making it possible to achieve high fibre densities in a compact cable. A tightly buffered fibre: It is encased (after coating) in a plastic layer. The coating is a soft plastic that allows deformation and reduces forces applied to the fibre. The surrounding buffer is a harder plastic, to provide physical protection. Tightly buffered fibres may be stranded in conventional cables. Tight buffering tolerances assure that the fibres are in precisely predictable positions, making it easier to install connectors. A major advantage of tight buffered cable for indoor use is its compatibility with fire and electrical codes. Although losses are somewhat higher than in loose tube cables, indoor transmission distances are short enough that it's not a problem. Ribbon cables: It is in some ways a variation on the tight-buffered cable. A group of coated fibres is arranged in parallel, then coated with plastic to form a multifibre ribbon. This differs from the Tight buffered cables in that one plastic layer encases many parallel fibres. The flat ribbon looks something like flat 4-wire cables used for household telephones. Typical ribbons contain 5 to 12 fibres. Upto 12 ribbons can be stacked together to form the core of a cable. The simple structure makes a ribbon cable easy to splice in the field; a single splice can connect multiple fibres. Multifibers connectors can also be installed readily.

What are some Fibre Optic Applications ?


The wide variety of fibre optic systems that have come into use because of the advantages of optical fibres are discussed as under : Long Haul telecommunication systems on land and at sea to carry many simultaneous telephone calls(or other signals) over long distances.These include ocean spanning submarine cables and national backbone networks for telephone and computer data transmission . Interoffice trunks that carry many telephone conversations simultaneously between local and regional switching facilities . Connections between the telephone N/W and antennas for mobile telephone service. Links among computers and high resolution video-terminals used for such purposes as computer aided design. Transmission of signals within ships and aircraft . Local area Networks operating at high speeds or over large areas, and backbone systems connecting slower local area Networks. High speed interconnections between computer and peripherals devices, or between computers, or even within segments of single large computers. Moderate speed transmission of computers data in places where fibres is most economical to install. Fibre-optics bundles for illumination and imaging, endoscopes to view inside the body and treat diseases with light and without surgery and optical sensors to measure rotation, pressure, sound waves, magnetic.