PRESENTATIONPRESENTATION

Transmission Characteristics Of Transmission Characteristics Of

Optical FibersOptical Fibers

PREPARED BYPREPARED BY::

AIBAD AHMEDAIBAD AHMED

TRANSMISSION CHARECTERISTICSTRANSMISSION CHARECTERISTICS

Transmission characteristics of fiber material Transmission characteristics of fiber material interact with the optical signal.interact with the optical signal.

Optical signals while transmitting through fiber Optical signals while transmitting through fiber suffers power losses (Attenuation).suffers power losses (Attenuation).

Signal attenuation is defined as the ratio of optical input Signal attenuation is defined as the ratio of optical input

power (Ppower (Pii) to the optical output power (P) to the optical output power (Poo).).

TRANSMISSION CHARECTERISTICSTRANSMISSION CHARECTERISTICS

Attenuation may occur due to following factors:Attenuation may occur due to following factors:

1.1. AbsorptionAbsorption2.2. ScatteringScattering3.3. Macro bendingMacro bending4.4. Micro bendingMicro bending Some other factors are:Some other factors are: Connectors ,splicing ,imperfect light Connectors ,splicing ,imperfect light

coupling, radiation and leaky modes.coupling, radiation and leaky modes.

ABSORPTIONABSORPTION

Some energy of the optical signal is Some energy of the optical signal is absorbed by the fiber material and absorbed by the fiber material and converted to heat.converted to heat.

It is a It is a cumulative cumulative (depends on length) (depends on length) effect.effect.

Absorption in fiber material are of two Absorption in fiber material are of two types.types.

1.1. Intrinsic (pure)Intrinsic (pure)

2.2. Extrinsic (impure)Extrinsic (impure)

ABSORPTIONABSORPTION Intrinsic:Intrinsic:

This loss is due to interaction of optical signal with This loss is due to interaction of optical signal with electrons & atoms of fiber in it’s pure state.electrons & atoms of fiber in it’s pure state.

Electrons and atoms of fiber material & optical signal has Electrons and atoms of fiber material & optical signal has frequency and wavelength as well.frequency and wavelength as well.

f = frequency of optical signalf = frequency of optical signal

f1= frequency of electronsf1= frequency of electrons f2= frequency of atomsf2= frequency of atoms

Then process of matching frequencies occurThen process of matching frequencies occur known known as “RESONANCE”.as “RESONANCE”.

CONTINUEDCONTINUED

If If ff~f1 ~f1 then electrons of fiber absorbs optical then electrons of fiber absorbs optical signal energy called signal energy called “Electronic Resonance“Electronic Resonance”.”.

If If ff~f2 ~f2 then atoms of fiber absorbs optical signal then atoms of fiber absorbs optical signal energy called energy called “Atomic Resonance“Atomic Resonance”.”.

Extrinsic:Extrinsic:Unintentionally injected impurities in fiber Unintentionally injected impurities in fiber material during fabrication causes absorption.material during fabrication causes absorption.

e.g.e.g. Metal ions like Fe, Cr, Ni absorbs light as well Metal ions like Fe, Cr, Ni absorbs light as well as OH atoms bonded in fiber absorbs light. as OH atoms bonded in fiber absorbs light.

PROBLEMPROBLEM

Q) When the injected optical power into Q) When the injected optical power into an 8 km length of fiber is 120 an 8 km length of fiber is 120 µW, the µW, the optical power at the fiber output is 3 optical power at the fiber output is 3 µW. Determine the signal attenuation µW. Determine the signal attenuation due to absorption in fiber per km.due to absorption in fiber per km.

CONTINUEDCONTINUEDData:Data: L= 8 kmL= 8 kmPi= 120 µWPi= 120 µWPo=3 µWPo=3 µWFormula:Formula:

Solution:Solution:

αα = 10 log10 (pi / po) = 10 log10 (120 *10^-6/3*10^-6) = 10 log10 (pi / po) = 10 log10 (120 *10^-6/3*10^-6) αα = 16 db = 16 dbnow ,now ,αα = (16db)/L = (16db)/Lαα = 16/8 db/km = 16/8 db/kmαα = 2 db/km = 2 db/km

CONTINUEDCONTINUED

now ,now ,

αα = (16db)/L = (16db)/L

αα = 16/8 db/km = 16/8 db/km

αα = 2 db/km = 2 db/km

SCATTERINGSCATTERING

It is the change in the direction of optical signal in the It is the change in the direction of optical signal in the core of optical fiber.core of optical fiber.

Scattering occurs when an optical signal hits a local RI Scattering occurs when an optical signal hits a local RI variation in the core.variation in the core.

Such local RI variations act like small objects in the core Such local RI variations act like small objects in the core to scatter light.to scatter light.

Scattering may change a guided mode into a radiation or Scattering may change a guided mode into a radiation or leaky mode.leaky mode.

TYPES OF SCATTERINGTYPES OF SCATTERING

RAYLEIGH SCATTERINGRAYLEIGH SCATTERING

MIE SCATTERINGMIE SCATTERING

RAYLEIGH SCATTERING RAYLEIGH SCATTERING It occurs when size of local RI variations is less than It occurs when size of local RI variations is less than

the wavelength of optical signal (about 1/10 of the wavelength of optical signal (about 1/10 of λλ ) .) .

CONTINUEDCONTINUED

Attenuation due to Rayleigh scattering:Attenuation due to Rayleigh scattering:

Ars = e^(-Ars = e^(-γγLL ) )

Where “Where “γγ” is Rayleigh Scattering ” is Rayleigh Scattering coefficient coefficient

αα 1/ 1/λλ^4.^4.

and “L” is the length of fiber. and “L” is the length of fiber.

PROBLEMPROBLEM

Q) Determine the attenuation in Q) Determine the attenuation in db/km due to rayliegh scattering in db/km due to rayliegh scattering in silica fiber at optical wavelength of silica fiber at optical wavelength of 630 nm. Assume the value of 630 nm. Assume the value of constant is 1.895*10^-28.constant is 1.895*10^-28.

CONTINUEDCONTINUED

Solution:Solution: γγ=constant (1/ =constant (1/ λλ^4)^4) γγ= 1.895*10^-28/(630*10^-9)= 1.895*10^-28/(630*10^-9) γγ= 1.99*10^-3/m= 1.99*10^-3/m

For 1 kmFor 1 km;;Ars= Ars= e^(-e^(-γγLL )=e^[-1.99*10^-3*10^3] )=e^[-1.99*10^-3*10^3]Ars= 0.301/kmArs= 0.301/km

In db attenuation =10 log10 (1/ In db attenuation =10 log10 (1/ Ars)Ars) =10 log 10 (1/0.301)=10 log 10 (1/0.301) = 5.12 db/km= 5.12 db/km

CONTINUEDCONTINUED

In db attenuation =10 log10 (1/ In db attenuation =10 log10 (1/ Ars)Ars)

=10 log 10 (1/0.301)=10 log 10 (1/0.301)

= 5.12 db/km= 5.12 db/km

MIE SCATTERINGMIE SCATTERING

When diameter of the local variations in RI When diameter of the local variations in RI > > wavelengths of guided modeswavelengths of guided modes

Imperfections in core-cladding interface causes Imperfections in core-cladding interface causes MIE SCATTERING .MIE SCATTERING .

Now a days optical fibers highly purified so MIE Now a days optical fibers highly purified so MIE SCATTERING is impossibleSCATTERING is impossible

MACRO BENDINGMACRO BENDING

It is a large scale bending which occurs It is a large scale bending which occurs intentionally by:intentionally by:

Wrapping the fiber on spool or pulling around Wrapping the fiber on spool or pulling around a corner.a corner.

EFFECTS:EFFECTS:

1.1. Under limited conditions, loss of powerUnder limited conditions, loss of power

2.2. Under extreme conditionsUnder extreme conditions , , fracture of fiber.fracture of fiber.

MICROBENDINGMICROBENDING

It s a microscopic bending and it is unintentional.It s a microscopic bending and it is unintentional.

It may occur during manufacturing, cable installation and at It may occur during manufacturing, cable installation and at service.service.

Factors:Factors:

During sheathed of fiber in cable, due to stress micro bends During sheathed of fiber in cable, due to stress micro bends occurs.occurs.

Due to frequent change of temperature.Due to frequent change of temperature.

Irregular external pressure cause micro bends e.g. Heavy Irregular external pressure cause micro bends e.g. Heavy vehicle run over it.vehicle run over it.

.. MICROBENDINGMICROBENDING

Irregular external pressure cause micro bends Irregular external pressure cause micro bends e.g. Heavy vehicle run over it.e.g. Heavy vehicle run over it.