Fibre optics

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Slide 1: 

Fiber Optics

Introduction : 

Introduction Communications systems that carry information through a guided fiber cable are called fiber optic systems. Use of optical fibers to replace conventional transmission lines and microwave wave-guide in telecommunication systems. Light is effectively the same as RF radiation but at a much higher frequency, theoretically the information-carrying capacity of a fiber is much greater than that of microwave radio systems.

Introduction - (continued) : 

Introduction - (continued) As they are not electrically conductive, hence very suitable for use in areas where electrical isolation and interference are severe problems. An attenuation of less than 2dB/Km.

Advantages of Optical Communications : 

Advantages of Optical Communications 1) Extremely wide system bandwidth - up to 10 GHz is possible 2) Immunity to EMI - such as lightning, electric motors, fluorescent lights 3) Virtual elimination of cross-talk 4) Lower signal attenuation than other propagation systems

Advantages of Optical Communications : 

Advantages of Optical Communications 5) Lighter weight and smaller size 6) Lower cost 7) Conversion of the earth’s resources - principal ingredient in glass is sand which is cheap and virtually unlimited supply 8) Safety - no hazard of short circuits nor spark will exist in optical fiber

Fundamentals of Fiber Optic Systems : 

Fundamentals of Fiber Optic Systems Optical fibers guide light waves within the fiber material because light rays bend or change direction when they pass from one medium to another. This phenomenon is called refraction. Optical fiber is a thin, transparent strand of material, usually glass or plastic or a combination of the two, that is used to carry light beams.

Reflection in Optical Fiber : 

Reflection in Optical Fiber Fig 1: Fiber for light beam propagation

Reflection in Optical Fiber - (continued) : 

Reflection in Optical Fiber - (continued) From fig. 1, the light rays are reflected from the inner walls as they propagate lengthwise along the fiber. A single light beam can be modulated simultaneously by hundreds, or even thousands, of independent signals.

Refraction of Light : 

Refraction of Light Light travels at approximately 3x108 m/s in free space and slower in a material denser than free space. This reduction in speed as it passes from free space into a denser material results in refraction of the light. Fig. 2 shows the light is bent at the interface. The degree to which the ray is bent depends on the index of refraction n of the denser material. n is defined as the ratio between speed of light in free space and speed of light in given material.

Refraction of Light - (continued) : 

Refraction of Light - (continued) Fig. 2: Light refraction at an interface

Physics of Light : 

Physics of Light The normal is an imaginary line perpendicular to the interface of the 2 materials. The angle of incidence is the angle of incident ray to the normal. The angle of refraction is the angle of refracted ray to the normal. The critical angle is the angle of incidence that will produce a 900 angle of refraction.

Physics of Light - (continued) : 

Physics of Light - (continued) 3 specific conditions are shown in Fig. 3. The angle of incidence, A1 and the angle of refraction, A2. Material 1 is more dense than material 2, so n1 is greater than n2.

Physics of Light - (continued) : 

Physics of Light - (continued) Fig.3: Index of refraction

Physics of Light - (continued) : 

Physics of Light - (continued) Fig. 3A shows how a light ray passing from material 1 to material 2 is refracted in material 2 when A1 is less than the critical angle. Fig. 3B shows the condition that exists when A1 is at the critical angle and angle A2 is at 900. The light is directed along the boundary between the 2 materials. Fig. 3C shows that any light ray incident at an angle greater than A1 of Fig. 3B will be reflected back into material 1 with A2 equal to A1.

Total Internal Reflection of Light : 

Total Internal Reflection of Light Fig. 4: Total internal reflection in optical fibers Total internal reflection forms the basis for light propagation in optical fibers.

Fiber Composition : 

Fiber Composition Fig. 5: Optical fiber construction

Fiber Composition - (continued) : 

Fiber Composition - (continued) An optical fiber consists of 3 distinct parts: 1) the core 2) the cladding 3) the sheath (jacket or coating). The core and cladding act as an optical wave-guide. Core - it is a transmission area of fiber. - typical core diameters range from 50 to 500 m Cladding - it surrounds the core and has a different index of refraction than the core. - it defines the optical boundary of the core and makes sure that total internal reflection occurs at the core outer skin.

Fiber Composition - (continued) : 

Fiber Composition - (continued) Fibers are specified by the outer diameters of the core and cladding. A 50/150 fiber means that the core is 50 m while the outside dimension of both the core plus cladding together is 150 m. The core and cladding are surrounded by the sheath.

Fiber Composition - (continued) : 

Fiber Composition - (continued) Sheath - it has a specially plastic coating that provides shock and abrasive resistance for the fiber. - its thickness ranges from 250 to 1000 m. - it is not involved in the actual transmission of light. The index of refraction of the assembly varies across the radius of the cable, with the core being more dense and having a constant or smoothly varying index of refraction, designated , and the cladding region being less dense and having another constant index of refraction, designated

Mode of Propagation : 

Mode of Propagation Mode simply means path from which light is propagated. If there is only one path for light to take down the cable, it is called single mode. If there is more than one path, it is called multi-mode.

Index Profile : 

Index Profile It is a graphical representation of the value of the refractive index across the fiber. The refractive index is plotted on the horizontal axis and the radial distance from the core axis is plotted on the vertical axis. There are 2 basic types of index profiles: step and graded

Index Profile - (continued) : 

Index Profile - (continued) A step-index fiber has a central core with a uniform refractive index. The core is surrounded by an outside cladding with a uniform refractive index less than that of the central core. In a graded-index fiber there is no cladding and the refractive index of the core is non-uniform; it is highest at the center and decreases gradually with distance toward the outer edge.

Single-Mode Step-Index Fiber : 

Single-Mode Step-Index Fiber It has a central core that is sufficiently small so that there is essentially only one path that light may take as it propagates down the cable. The refractive index of the cladding is slightly less than that of the central core and is uniform throughout the cladding. Consequently, all light rays follow approximately the same path down the cable and take approximately the same amount of time to travel the length of the cable.

Multi-mode Step-Index Fiber : 

Multi-mode Step-Index Fiber The light rays that strike the core/cladding interface at an angle greater than the critical angle are propagated down the core in a zigzag fashion, continuously reflecting off the interface boundary. There are many paths that a light ray may follow as it propagates down the fiber. As a result, all light rays do not follow the same path and hence do not take the same amount of time to travel the length of the fiber.

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