Optical FibreOptical Fibre is an optical transmission device whichworks on the principle of total internal reflection. Principle of Optical FibreWhen a light signal is directed at one end of the fibre ata suitable angle, it undergoes repeated total internalreflection along the length of the fibre and finallycomes out at the other end.
Optical Fiber• Core – Glass or plastic with a higher index of refraction than the cladding – Carries the signal• Cladding – Glass or plastic with a lower index of refraction than the core• Buffer – Protects the fiber from damage and moisture• Jacket – Holds one or more fibers in a cable
Potential low cost: The glass which generally providesthe optical fiber transmission medium is made fromsand (SiO2).Enormous band width: The optical carrier frequency inthe range 1013 to 1016 Hz.In metallic cable systems band width is 500-700 MHzs.The amount of data that can be transmitted in a fixedamount of time.Signal security: Optical fibers provides the high degreeof signal security.Using an Optical fiber, 15000 independent speechescan be sent where as using a pair of copper wires,only 48 independent speeches can be sent
Types of Optical Fibre (Based on core size)Single Mode Step Multimode FibreIndex Fibre Multimode Multimode Step Index Fibres Graded Index Fibres
Single Mode Step Index Fiber• Single mode step index fiber has a core diameterof 8 to 9 microns, which only allows one light pathor mode• Single mode fibres have a lower signal loss and a higher information capacity or bandwidth than a multimode fibre. Index of refraction
Multimode Step-Index Fiber• Multimode fiber has a core diameter of 50 to 100 microns (sometimes even larger) – Allows several light paths (~100) or modes – This causes modal dispersion – some modes take longer to pass through the fiber than others because they travel a longer distance Index of refraction
Multimode Graded-Index Fiber• The index of refraction gradually changes across the core – Modes that travel further also move faster – This reduces modal dispersion so the bandwidth is greatly increased Index of refraction
Step-index and Graded-index• Step index multimode was developed first, but rare today because it has a low bandwidth (50 MHz-km)• It has been replaced by graded-index multimode with a bandwidth up to 2 GHz- km
Fiber Optic Specifications• Attenuation – Loss of signal, measured in dB• Dispersion – Blurring of a signal, affects bandwidth• Bandwidth – The number of bits per second that can be sent through a data link• Numerical Aperture – Measures the largest angle of light that can be accepted into the core
Acceptance AngleOptical fibre will only propagate light that enters the fibre withina certain cone, known as acceptance cone of the fibre. Thehalf-angle of this cone is called the acceptance angle (θi) Cladding ( µ 2 ) Air ( µo ) θ Core( µ1 ) φ i Axis From Snell’s law µo sin i = µ1 sin φ
φ = 90 − θ o µo sin i = µ1 sin(90o − θ ) µo sin i = µ1 cos θ µo sin i = µ1 1 − sin θ 2If i increases, Φ increases and θ decreases.There is a maximum value of i beyond which the light rayentering the fibre will fall on the core-cladding interface atan angle less than θc. Such a ray will be refracted into thecladding and will not propagate. This maximum value of i,say imax is called the acceptance angle of the fibre.
µo sin imax = µ1 1 − sin θ c 2When θ = θ c then according to Snell’s law µ1 sin 90o = µ 2 sin θ c µ2 sin θ c = µ1 µo sin imax = µ1 1 − ( µ 2 µ1 ) 2 µo sin imax = µ12 − µ 2 2
Taking µ o =1 for air, we get sin imax = µ − µ 2 1 2 2This equation gives a relation between the acceptanceangle and the refractive indices of the core and thecladding.The cone associated with the angle imax is called theacceptance cone.
Numerical ApertureIt is the measure of the light gathering capacity of the fibreand is defined as the product of sine of the acceptanceangle and the refractive index of the medium to which theend faces of the fibre are exposed. NA = µo sin imaxTaking µ o =1 for air, we get NA = sin imax = µ − µ 2 1 2 2 µ1 − µ 2Relative index ∆µ = µ1
ATTENUATION Attenuation or transmission loss is the loss of optical power as light travels down a fiber. The decrease in signal strength along a fiber optic waveguide caused by absorption and scattering is known as attenuation. Attenuation is usually expressed in dB/kmSince attenuation is the loss, therefore, it is always expressed as Pin αL = 10 log10 Pout where α is the attenuation coefficient of the fiber
DispersionDispersion, expressed in terms of the symbol ∆t, isdefined as pulse spreading in an optical fibre. As a pulseof light propagates through a fibre, elements such asnumerical aperture, core diameter, refractive indexprofile, wavelength, and laser line width cause the pulseto broaden. This poses a limitation on the overallbandwidth of the fibre.
V Number The number of modes of multimode fiber cable depends on the wavelength of light, core diameter and material composition. This can be determined by the Normalized frequency parameter (V). πd πd V= µ1 − µ 2 = 2 2 NA λ λ Where , d = fiber core diameter ; λ = wavelength of light NA=numerical apertureFor a single mode fiber, V ≤ 2.4 and for multimode fiber, V ≥2.4.Mathematically, the number of modes for a fiber is given by: For Step-index For Graded-index