attenuation.pdf
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AttenuationSavera Tanwir
Fiber Optic Communication Systems 2
Topics
Attenuation Absorption
Intrinsic absorptionImpurities
Atomic Defects Scattering In homogeneities Geometric Effects Total Attenuation
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Fiber Optic Communication Systems 3
Attenuation
Attenuation is the reduction in amplitude andintensity of a signalThe attenuation of an optical fiber measures theamount of light lost between input and outputReceivers require some minimum power level,to accurately detect the transmitted signal
Fiber Optic Communication Systems 4
Attenuation
Losses occur at the channel coupler, splices,and within the fiber itself These transmission losses limit path lengthRequirements for fiber material
Low loss Ability to formed into hairlike long strands Capable of slight variations to achieve different RIs Continuous change in RI for Graded Index Fiber
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Fiber Optic Communication Systems 5
Glass - Attenuation
Glass fibers have low absorption than plasticfibersGlass fibers are preferred for long haulcommunicationsGlass in formed by fusing molecules of silica(silicon dioxide SiO 2) and other material(titanium, thallium, germanium, boron etc.)
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Glass - Attenuation
Objective: manufacture low-loss fiber Attenuation in Glass fiber Absorption Scattering Geometric Effects
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Absorption
Intrinsic Absorption: Glass absorbs heavily within specific wavelength
regions A natural property of glass It is very strong in the short-wavelength ultraviolet
portion Peak loss
Ultraviolet region but
unimportant
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Absorption
Intrinsic absorption peak also occur between 7 and 12m in the infrared
Infrared loss is associated with vibration of chemicalbonds
Intrinsic losses are mostly insignificant in wide regionwhere fiber systems can operate Intrinsic losses restrict the extension of fiber systems toward the
ultraviolet as well as toward longer wavelengths Fiber Optic Comm operates in a range of wavelengths from 0.5
to 1.6 m
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Impurities
Impurities are major source of loss inpractical fiber Types of impurities Transition-metal ions OH ions
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Rayleigh Scattering
Dominant loss in the low absorption window betweenultraviolet and infrared regions
Caused due to inhomogeneities of random nature onsmall scale compared to the wavelength
Inhomogeneities are fluctuations in refractive index dueto density and compositional variations in the glasslattice
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Rayleigh Scattering
Rayleigh scattering is proportional to -4
Loss is given by the following formula
L = 1.7 (0.85/ )4
The is in micrometers and L is in dB/km
Corresponding electric- field attenuation is =L/8.685 Where unit is km -1
Scattering restricts the use of fibers at short wavelengths. AsWavelength increases the scattering loss diminishes
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Inhomogeneities
Introduced un intentionally during manufacturingImperfect mixing and dissolution of chemicalsImperfect processing produces rough core-cladding interface
Scattering objects are larger than a wavelengthThese losses can be controlled by propermanufacturing techniques
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Fiber Optic Communication Systems 14
Geometric Effects
Two types of bends that cause attenuation Macroscopic Microscopic
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Macroscopic Bends
Refers to large-scale bending E.g occurs intentionally when wrapping the fiber on a spool or
pulling around a corner
125 m diameter fiber can be bent with radii of curvature 25 mm.
The fiber will not fracture unless the bend radius is less than 10 mm
The strength of fiber depends on the microscopic flaws, these flawsgrows over time
For commercial 125 m fiber, minimum bend radius is 25mm
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Geometric Effects
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Microscopic bends
Occurs when a fiber is sheathed within aprotective cable
The stresses set up during the cabling processcause small axial distortions ( microbends )
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Total Attenuation
Scattering and absorption combine to givetotal loss, or attenuation, which is theimportant number in communicationsystems.
Attenuation normally is measured indecibels per kilometer for communicationfibers.
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Total Attenuation
Minimum loss for silica fiber is 0.15dB at 1.5 m
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Total Attenuation
Spectral Attenuation for all-glass fibers
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Total AttenuationSpectral Attenuation for hard-clad silica fiber
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Total AttenuationSpectral Attenuation for an all-plastic fiber cable
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Measuring Losses
Optical Power Meter Measures power at the input and output Loss is ratio of two power measurements
divided by length of fiber Not practical for long-distance fibers
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Optical Time-Domain Reflectometer
Connected at one end onlyTransmits an optical pulse at one end andmeasures the backscattered optical power The backscattering occurs at the discontinuities
like splices, connectors, fiber breaksThe time delay of the reflection is measure oftheir location
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Sample Power Budget Calculation
6 dBAvailable fiber loss-26 dB26 dBTotal losses
4 dBPower margin6 dBConnector & Splices
16 dBCoupling loss32 dBLoss Budget-30 dBReceiver Sensitivity2 dBLED output power