Optic Fiber Attenuation

8/10/2019 Optic Fiber Attenuation

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optic fber attenuation

Loss o optical power o signal when it is transmitted through optic fber is called as

attenuation. Material dispersion, scattering, absorption, bending, splice loss,

waveguide dispersion, intermodal dispersion etc. are the main attenuation

mechanisms. In material dispersion, loss is due to the dispersion o white light. This

can be avoided by using monochromatic light. Scattering o light signal may causeattenuation. Here ayleigh scattering is signifcant. !ctual reason or scattering is

local variations in the reractive inde" in core part. !bsorption o optical signal by

the core material and impurities present in core material distorts the signal leading

to attenuation. #oth micro level and macro level bending o fber leads to fber loss

or attenuation. $onnectors, couplers and splicers in the fber optic communication

system diminish the signal and this loss is reerred as splice loss. %aveguide

dispersion which occurs in single mode fbers is the loss o signal due to reraction

towards cladding. Intermodal dispersion which occurs in multimode fbers is the

distortion o signal due to the di&erent paths o signal.

!bsorption o signal is a serious loss mechanism in an optical fber. !bsorption

occurs in optical fbers due to the presence o imperections in the atomic structure

o the fber material, due to some basic inherent intrinsic material properties and

due to some e"trinsic material properties. Imperections may appear in atomic

structure due to o"ygen defciencies and missing o certain molecules. 'i&usion o

hydrogen molecules may also induce absorption. #ut the contribution rom

imperections is relatively small in fber optic absorption losses. Inherent intrinsic

absorption is caused by basic fber material properties. I a material is ree rom

impurities and imperections, then entire absorption is due to intrinsic absorption.

Silica fbers possess very low intrinsic material absorption. Here absorption is

caused by the vibration o silicon(o"ygen bonds. The interaction between these

bonds and the electromagnetic feld o the optical signal is responsible or intrinsic

absorption. )resence o impurities in the fber material leads to e"trinsic absorption.

This is caused by the electronic transition o metal impurity ions rom one energy

level to another. !nother reason or e"trinsic absorption is the presence o hydro"yl

ions in the fber.

Scattering losses in optical fber

!ttenuation is the main loss mechanism in an optical fber. !bsorption and

scattering o signals results in attenuation. There are two types o scattering losses.

They are linear scattering and nonlinear scattering. In linear scattering, attenuation

occurs when optical power is transerred rom one mode to another *eepingre+uency unaltered. There are two categories in linear scattering. They are

ayleigh scattering and Mie scattering. ayleigh scattering is the main loss

mechanism in the visible range. It is proportional to the ourth power o the

operating wavelength. ayleigh scattering loss can be minimied by choosing

longest possible operating wavelength. I the sie o the deect is greater than one(

tenth o the wavelength o light, the scattering mechanism is called Mie scattering.

-on linear scattering occurs when re+uency is changed during optical power


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transer. The two types o nonlinear scattering are stimulated #rillouin scattering

and stimulate aman scattering. Stimulated #rillouin scattering is a cause o

concern in long distance systems, in wavelength division multiple"ing %'M/

systems and remote pumping o an erbium doped fber amplifer 0'1!/ through a

separate optical fber. Stimulated aman scattering creates problems in wavelength

division multiple"ing %'M/ systems.

2ptical fber splicing

Important methods employed or the splicing o optical fber cables are usion splice

method, v(groove butt splice method, elastic tube splice method and loose tube

splice method. In usion splice method, thermal bonding techni+ue is used. The ris*

o change in chemical composition does e"ist in this method. 3(groove splice

techni+ue uses a v(shaped groove which brings two fbres into mechanical

alignment. Then both the ends are epo"ied. 0lastic tube splice method is used or

the splicing o multimode fbres. Loose tube splice method is similar to v(groove

method. Here corner o a rectangular tube is used or the alignment.

Mode coupling in optic fbers

Mode is the path or light rays through an optical fber. I an optic fber supports only

one mode, it is called as single mode fber. Multimode fber supports more than one

mode. The electric feld distribution o various modes yields similar distributions o

light intensity within the fber core. These patterns are called mode patterns which

gives an idea o di&erent modes propagating in the fber. )ropagation

characteristics o a fber are very sensitive to deviations o the fber a"is rom

straightness, variations in the core diameter, irregularities in the core(cladding

interace and reractive inde" variations. Individual modes do not normally

propagate throughout the length o the fber. This result in a mode conversion which

is *nown as mode coupling. $oupled mode e+uations obtained rom Ma"well4s

e+uations can be used or the analysis o mode coupling. Mode coupling a&ects the

transmission properties o fber which is a serious cause or concern when used or

long distance communication. Mode coupling leads to intramodal dispersion li*e

material dispersion and waveguide dispersion and also intermodal dispersion.

2ptical detectors used in fber optic communication system

!valanche photodiode, )I- photodiode and pn 5unction photodiode are the main

optical detectors employed in fber optic communication. These devices converts

light signal to electrical waveorms. )n 5unction photodiode consists o a p(type

layer deposited on an n(type substrate. Minority carriers are responsible or

photocurrent. In the reverse biased mode, larger portion o the depletion layer

occupies the n(layer around the 5unction. The reason or this is attributed to smaller

impurity concentration in the n(layer. )ositive(Intrinsic(-egative )I-/ photodiode

consists o an intrinsic or lightly doped n(layer between two heavily doped n and p

layers. Here depletion region is wide and this enhances capture area o the photons.

!valanche photodiodes are variations o )I- photodiodes. More responsivity in

avalanche photodiodes is due to avalanche multiplication e&ect.

)osted by !M!$H!-'! 6!M!TH at 7899 !M


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each through avalanche photodiode !)'/

!valanche photodiodes are re+uired in fber optic communication at the receiving

end. )hotodiode detects the light signal and converts it into electrical orm. There

e"ist many orms o avalanche photodiodes. each through avalanche photodiode!)'/ is a promising candidate in this category. $onsider a positive(intrinsic(

negative semiconductor photodiode operating in reverse biased mode. %hen the

applied reverse voltage e"ceeds threshold value, photoelectrons generated as a

result o its e"posure to light, gets accelerated through the 5unction, collides with

other atoms to produce secondary electron(hole pairs. $arrier concentration

increases e"ponentially with the electric feld intensity. This phenomenon is *nown

as impact ioniation or avalanche e&ect. The reach through avalanche photodiode

consists o a high resistivity p(type material deposited on an epita"ial layer with an

e"tremely high order o impurity concentration. $ommonly used doping atoms to

achieve this are #oron and )hosphorous. 2peration o !)' is always in ully

depleted mode.

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Optic Fiber Attenuation