Fiber-optic Cable

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Fiber-optic Cable:- A fiber-optic cable is made of glass or plastic and transmits signals in the form of light:

Fiber-optic Cable :- A fiber -optic cable is made of glass or plastic and transmits signals in the form of light

Several aspects of the nature of light:

Several aspects of the nature of light Light travels in a straight line as long as it is moving through a single uniform substance. If a ray of light traveling through one substance suddenly enters another substance (of a different density), the ray changes direction.

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if the angle of incidence I (the arIgle the ray makes with the line perpendicular to the interface between the two substances) is less than the critical angle, the ray refracts and moves closer to the surface. If the angle of incidence is equal to the critical angle, the light bends along the interface. If the angle is greater than the critical angle, the ray reflects (makes a turn) and travels again in the denser substance. Note that the critical angle is a property of the substance, and its value differs from one substance to another.


Multimode Multimode is so named because multiple beams from a light source move through the core in different paths. How these beams move within the cable depends on the structure of the core. Many different rays will be bouncing around at different angles. Multi-mode fibers generally have a larger core diameter, and are used for short-distance communication links and for applications where high power must be transmitted.

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Fibers that carry more than one mode is called multimode.

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Light transmission in step index and graded index multimode fiber

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Multimode Step-Index Rays of light enter the fiber with different angles to the fiber axis, up to the fiber's acceptance angle (numerical aperture). Rays that enter with a shallower angle travel by a more direct path, and arrive sooner than those enter at steeper angles. The arrival of different modes of the light at different times is called Modal Dispersion . Modal Dispersion is also called modal distortion , multimode dispersion , intermodal distortion , intermodal dispersion , and intermodal delay distortion . Modal dispersion causes pulses to spread out as they travel along the fiber , the more modes the fiber transmits, the more pulses spread out. This significantly limits the bandwidth of step-index multimode fibers .

Multimode graded-index fibers:

Multimode graded-index fibers Different light modes in a graded-index multimode fiber still follow different lengths along the fiber , as in step-index multimode fiber .  However their speeds differ because the speed of guided light changes with fiber core’s refractive index. So the farther the light goes from the center of the fiber , the faster its speed. So the speed difference compensate for the longer paths followed by the light rays that go farthest from the center of the fiber . This equalizing of transit times of different modes greatly reduces modal dispersion. The bandwidth of a typical off-the-shelf graded-index multimode fiber with a 50 µm core may approach 1 GHz·km or more. Multimode graded-index fibers having bandwidths approaching 3 GHz·km have been produced.

Multimode step-index fiber:

Multimode step-index fiber In multimode step-index fiber , the density of the core remains constant from the center to the edges. A beam of light moves through this constant density in a straight line until it reaches the interface of the core and the cladding. At the interface, there is an abrupt change due to a lower density; this alters the angle of the beam's motion The term step index refers to the suddenness of this change, which contributes to the distortion of the signal as it passes through the fiber

Multimode graded-index fiber:

Multimode graded-index fiber A second type of fiber , called multimode graded-index fiber , decreases this distortion of the signal through the cable. The word index here refers to the index of refraction. the index of refraction is related to density. A graded-index fiber , therefore, is one with varying densities. Density is highest at the center of the core and decreases gradually to its lowest at the edge.


Single-mode Single-mode uses step-index fiber and a highly focused source of light that limits beams to a small range of angles, all close to the horizontal. The singlemode fiber itself is manufactured with a much smaller diameter than that of multimode fiber , and with substantiallY lower density (index of refraction). The decrease in density results in a critical angle that is close enough to 90° to make the propagation of beams almost horizontal.

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Single mode

Fiber-Optic Cable Connectors:

Fiber -Optic Cable Connectors There are three types of connectors for fiber -optic cables.

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The subscriber channel (SC) connector is used for cable TV. It uses a push/pull locking system. The straight-tip (ST) connector is used for connecting cable to networking devices. It uses a bayonet locking system and is more reliable than SC. MT-RJ is a connector that is the same size as RJ45.


Performance The performance is such that we need fewer (actually 10 times less) repeaters when we use fiber -optic cable.

Advantages of optical Fiber:

Advantages of optical Fiber Higher bandwidth Less signal attenuation :- A signal can run for 50 km without requiring regeneration. We need repeaters every 5 km for coaxial or twisted-pair cable. Immunity to electromagnetic interference Light weight.

Disadvantages of optical fiber:

Disadvantages of optical fiber Installation and maintenance Unidirectional light propagation. Cost.

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