Attenuation.pdf

8/10/2019 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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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


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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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.)


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


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


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


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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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


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 )


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


Total Attenuation

Spectral Attenuation for all-glass fibers


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Total AttenuationSpectral Attenuation for hard-clad silica fiber


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


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

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