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OPTICAL FIBRE : Optical Fibre is new medium, in which information (voice, Data or Video) is transmitted through a glass or plastic fibre, in the form of light. An optical fiber is a flexible , transparent fiber made of glass or plastic slightly thicker than human hair. Fiber are used instead of metal wires because signals travel along them with less loss. Optical fiber include a transparent core surrounded by a transparent cladding material with a lower indx of refraction.lifht is kept in the core by the total internal reflection.this cause the fiber to act as a waveguide. Based on number of propagation we have two types of fibers: (1) Multimode fiber (2) Singlemode fiber (1) MULTIMODE FIBER: FIBER THAT SUPPORT MANY PROPAGATION PATHS CALLED MULTIMODE FIBER (MMF), Multimode fibers are generally have a wider core diameter,and are used for short distance communication links and for applications where high power must be transmitted.These fibers are generally used in local are network. MMF allow a large number of modes for the light rays traveling through it. (2) SINGLEMODE FIBER: FIBER THAT ONLY SUPPORTS A SINGLE MODE ARE KNOWN AS SINGLE MODE FIBER (SMF). Singlemode fibers are used for most communication links longer than 1,050 metres. The singlemode fiber has smaller core diameter (10mm) and the difference between core and cladding is very small. Fabrication of SMF is very difficult so the fibers are expensive, futher the launching of light into the SMF is also difficult. So the SMF are very useful in long distance communication.

The transmission sequence of optical fiber is given below: (1) Information is encoded into electrical signals. (2) Electrical signals are converted into light signals. (3) Light travels down the fibre. (4) A detector changes the light signals into electrical signals. (5) Electrical signals are decoded into information.

TYPES OF OPTICAL FIBER : There are different types of optical fiber depending upon the different factors : (1) Refractive index (2) Number of modes Based on the refractive index we have two types of fibers : (1) Step Index (2) Graded Index

(1) STEP-INDEX MULTIMODE FIBER : has a large core, up to 100 microns in diameter. As a result, some of the light rays that make up the digital pulse may travel a direct route, whereas others zigzag as they bounce off the cladding. These alternative pathways cause the different groupings of light rays, referred to as modes, to arrive separately at a receiving point. This type of fiber is best suited for transmission over short distances, in an endoscope, for instance. (2) GRADED-INDEX MULTIMODE FIBER : contains a core in which the refractive index diminishes gradually from the center axis out toward the cladding. The higher refractive index at the center makes the light rays moving down the axis advance more slowly than those near the cladding.

ADVANTAGES OF OPTICAL FIBRE : Optical Fibre has the following advantages: (1) Optical Fibres are non conductive (Dielectrics). (2) Electromagnetic Immunity. (3) Large Bandwidth. (4) Low Loss. (5) Small, Lightweight cables. (6) Available in Long lengths (> 12 kms). (7) Security. (8) Large speed. (9) Easy to maintain.

FIBRE OPTIC SYSTEM: Optical Fibre is new medium, in which information (voice, Data or Video) is transmitted through a glass or plastic fibre, in the form of light.

Geometry of Fibre: A hair-thin fiber consist of two concentric layers of high-purity silica glass the core and the cladding, which are enclosed by a protective sheath. Light rays modulated into digital pulses with a laser or a light-emitting diode move along the core without penetrating the cladding.

Principle of Operation: TOTAL INTERNAL REFLECTION: The reflection that occurs when a light ray travelling in one material hits a different material and reflects back into the original material without any loss.

CRITICAL ANGLE: The angle of incidence that yields an angle of refraction of 90o is the critical angle. If the angle of incidence increases more than the critical angle, the light is totally reflected back into the first material so that it

does not enter the second material. The angle of incidence and reflection are equal and it is called Total Internal Reflection.

The inner core is the light carrying part. The surrounding cladding provides the difference refractive index that allows total internal reflection of light through the core. The index of the cladding is less than 1%, lower than that of the core. Light injected into the fibre and striking core to cladding interface at greater than the critical angle, reflects back into core, since the angle of incidence and reflection are equal, the reflected light will again be reflected. The light will continue zigzagging down the length of the fibre. Jacket

Jacket C ladding C ore

C ladding(n 2) C ore (n 2)

C ladding Jacket Light at lessthan A ngleof A ngleof critical angleis incidence reflection absorbedinjacket Light ispropagatedby total internal reflection Fig. Total Internal R eflectioninanoptical Fibre

Testing of Optical Fibers using OTDR: An Optical Time-Domain Reflectometer is an optoelectronic instrument used to characterize an optical fibre. It injects a series of optical pulses into the fibre under test and extracts, from the same end of the fibre, light that is scattered or reflected back from points along the fibre.

The scattered or reflected light that is gathered back is used to characterize the optical fibre. This is equivalent to the way that an electronic time-domain meter measures reflections caused by changes in the impedance of the cable under test.

The strength of the return pulses is measured and integrated as a function of time, and plotted as a function of fibre length.

APPLICATION OF OPTICAL FIBER IN COMMUNICATION: (1) Common carrier nationwide networks. (2) Telephone Inter-office Trunk lines. (3) Customer premise communication networks. (4) Undersea cables. (5) High EMI areas (Power lines, Rails, Roads). (6) Factory communication/ Automation. (7) Control systems. (8) Expensive environments. (9) High lightening areas.