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| What do you understand by Attenuation? |
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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.
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| Why should we use Optical Fibres instead of our very own conventional Copper Cables? |
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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:
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- 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.
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| 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.
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What are the major factors that can dictate the choice of certain fibres for specific applications?
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- Low attenuation to maximize spacing of repeaters or amplifiers or to avoid them altogether.
- Maximization of transmission bandwidth or speed.
- Ease of collecting light from inexpensive large area emitters.
- Ease of making splices or attaching connectors in the field.
- Large tolerances to allow inexpensive connectors.
- Cost of Fibre
- Transmission wavelength
- Tolerance of high temperatures or other environmental conditions
- Strength and flexibility of the fibre.
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| What
is a mode? |
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A mode is a stable propagation state in an optical fibre. If light travels through an optical fibre along certain paths, the electromagnetic fields in the light waves reinforce each other to form a field distribution that is stable as it travels down the fibre. These stable operating points (standing waves) are modes. If the light tries to travel in other paths, a stable wave will not propagate down the fibre-thus no mode.
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| What are the characteristics of Step Index Fibres? |
- Step-Index multimode Fibres are simple types with core of 100-3000µm in diameter. They collect light efficiently from large area sources, but modal dispersion usually limits bandwidth to about 20Mhz-km
- The number of modes a step index fibre can transmit depends on its numerical aperture and core diameter as well as the wavelength. Single mode types have core diameters of about 8-10µm
- Step -Index multimode fibres come in all glass, plastic clad silica, and all plastic versions of various sizes.
- Single mode step index fibres, with core about 8-10mm across suffer no modal dispersion. The two components of their chromatic dispersion- material and waveguide dispersion-add to zero around 1300nm, giving them extremely high bandwidth with loss around 0.34-0.5 dB/km.
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| Give some reasons, that why cabling is needed ? |
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Cabling is the packaging of optical fibres for easier handling and protection. Uncabled fibres work fine in the laboratory and in certain applications such as sensors and the fibre optic systems for guiding missiles.However like wires, Fibres must be cabled for most communications uses.The major reasons for the same are:
- Ease of Handling
- Protection
- Crush Resistance
- Degradation
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| Can the environment for optical cable loosely classified ? |
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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.
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| 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.
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| 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.
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