FIBER PROPERTIES

Transmission characteristics of a fiber depends on two important phenomena

Attenuation Dispersion

Attenuation or transmission lossMuch less than those of metallic conductorsExpressed in logarithmic unit of decibelCaused by absorption, scattering and bending losses

Attenuation - Loss of optical power as light travels along the fiberDefined as ratio of input (transmitted) optical power Pi into fiber to the output (received) power PoSignal attenuation = 10 log 10(Pi / Po) dB

Influenced by material composition, preparation and purification technique and the waveguide structure

Losses are categorized into

1. Material absorption lossesa. Intrinsicb. Extrinsic

2. Linear scattering losses3. Fiber bend loss

Material Absorption Losses

Related to material composition and fabrication process

Results in dissipation of transmitted power as heat

Absorption of light may be

Intrinsic – caused by interaction with one or more major components

of the glass

Extrinsic – caused by impurities within the glass

Intrinsic losses

Pure silicate glass has very little intrinsic absorption

In silica glass, the wavelength used is 700 nm to 1600 nm

This is between two intrinsic absorption regions

First in the UV region (< 400nm)

Second in the infrared region (>2000 nm)

Intrinsic absorption in the UV band

Due to electronic absorption (i.e.,) absorption occurs when a

light particle (photon) interacts with an electron and excites

it to a higher level.

Intrinsic absorption in infrared band

Absorption is caused by the vibration of Si-O bonds

Interaction between the vibration bond and the EM field of the optical

signal causes absorption.

Light energy transferred from the EM field to the bond

Extrinsic losses

Caused by impurities into the fiber material

Trace metal such as Iron, Nickel, Chromium

Electronic transition of these metal ions from one energy level to

another causes absorption

Also occurs where hydroxyl ions (OH-) are introduced into the fiber

and peak at 1383 nm, 1250 nm and 950nm

These absorptions define three windows of preferred operation, one

centered at 1300 nm, second at 850 nm and third around 1550 nm

Scattering Loss

Caused by the interaction of light with density fluctuations within a

fiber.

Density changes are produced when optical fibers are manufactured.

During manufacturing, regions of higher and lower molecular density

areas, relative to the average density of the fiber, are created.

Light traveling through the fiber interacts with the density areas

Light is then partially scattered in all directions.

LIGHT SCATTERING

RAYLEIGH SCATTERING

In commercial fibers operating between 700-nm and 1600-nm wavelength,

the main source of loss is called Rayleigh scattering.

Rayleigh scattering is the main loss mechanism between the ultraviolet and

infrared regions.

Rayleigh scattering occurs when the size of the density fluctuation (fiber defect)

is less than one-tenth of the operating wavelength of light.

As the wavelength increases, the loss caused by Rayleigh scattering decreases.

FIBER LOSSES

MIE SCATTERING

If the size of the defect is greater than one-tenth of the

wavelength of light - the scattering mechanism is called Mie

scattering.

Mie scattering, caused by these large defects in the fiber core,

scatters light out of the fiber core.

However, in commercial fibers, the effects of Mie scattering are

insignificant.

Optical fibers are manufactured with very few large defects.

BENDING LOSS

Bending the fiber also causes attenuation. Bending loss is classified according to the bend radius of curvature:

microbend loss macrobend loss

Macrobends are bends having a large radius of curvature relative to

the fiber diameterDuring installation, if fibers are bent too sharply, macrobend losses will

occur Microbend and macrobend losses are very important loss mechanisms.

MICROBEND LOSSESMicrobends are small microscopic bends of the fiber axis that

occur mainly when a fiber is cabledMicrobend losses are caused by small discontinuities or

imperfections in the fiber. Uneven coating applications and improper cabling procedures

increase microbend loss. External forces are also a source of microbends. An external force deforms the cabled jacket surrounding the

fiber but causes only a small bend in the fiber. Microbends change the path that propagating modes take up

MICROBEND LOSS

MACROBEND LOSSMacrobend losses are observed when a fiber

bend's radius of curvature is large compared to the

fiber diameter. Light propagating at the inner side of the bend

travels a shorter distance than that on the outer

side. To maintain the phase of the light wave, the mode

phase velocity must increase. When the fiber bend is less than some critical

radius, the mode phase velocity must increase to a

speed greater than the speed of light. However, it is impossible to exceed the speed of

light. This condition causes some of the light within

the fiber to be lost or radiated out of the fiber.

DISPERSION IN OPTICAL FIBERS

Two types of dispersion:

Intramodal dispersion

Intermodal dispersion

Dispersion leads to PULSE SPREADINGThe varying delay in arrival time between different components of a

signal "smears out" the signal in time.

This causes energy overlapping and limits information capacity

of the fiber

INTRAMODAL DISPESION

Pulse spreading that occurs within a single mode

Intramodal dispersion occurs because different colors of light travel through different materials and different waveguide structures at different speeds

Also called GROUP VELOCITY DISPERSION (GVD)

Occurs in all types of fibers

Two main causes : Material dispersion Waveguide dispersion

Material Dispersion

Arises from variations of the refractive index of the core material

as a function of wavelength

Spreading of a light pulse is dependent on the wavelengths

interaction with the refractive index of the fiber core

Different wavelengths travel at different speeds in the fiber

material and hence exit the fiber at different times

Material dispersion is a function of the source spectral width.

The spectral width specifies the range of wavelengths that can

propagate in the fiber.

Material dispersion is less at longer wavelengths.

Waveguide Dispersion

Arises because a Single Mode Fiber confines only 80% of the

optical power to the core

The other 20% tends to travel through the cladding and hence

travels faster

This results in spreading of the light pulses

The amount of dispersion depends on the fiber design and the

size of the fiber core relative to the wavelength of operation

In multimode fibers, waveguide dispersion and material

dispersion are basically separate properties.

Multimode waveguide dispersion is generally small compared

to material dispersion and is usually neglected.

INTERMODAL DISPERSION

Intermodal or modal dispersion causes the input light pulse

to spread.

The input light pulse is made up of a group of modes

(MULTIMODE)

As the modes propagate along the fiber, light energy

distributed among the modes is delayed by different amounts.

The pulse spreads because each mode propagates along

the fiber at different speeds.

Modes travel in different directions, some modes travel

longer distances.

Modal dispersion occurs because each mode travels a

different distance over the same time span

The modes of a light pulse that enter the fiber at one time

exit the fiber a different times.

These conditions causes the light pulse to spread.

As the length of the fiber increases, modal dispersion

increases

Optical Fiber Connection

Requires both jointing and termination of the transmission

medium

Number of connections or joints is dependent upon the link

length

Fiber to fiber connection with low loss and minimum distortion

is important

Two major categories of fiber joint currently in use:

1.Fiber splices

2. Fiber Connectors

Splices and Connectors – Ideally couple all light propagating in

one fiber into the adjoining fiber

Fiber Couplers

Branching devices that split all the light from the main fiber into

two or more fibers

Couple a proportion of the light propagating in the main fiber into

a branch fiber

Combine light from one or more branch fibers into a main fiber

Fiber alignment and Joint losses

Major consideration – Optical losses at the interface

Can be minimized if…..Jointed fiber ends are smoothPerpendicular to fiber axisTwo fiber axes are perfectly aligned

Still, a proportion of light – reflected back into the transmitting fiber

This phenomena is called FRESNEL REFLECTION

Magnitude of the partial reflection through the interface may beEstimated using the classical Fresnel formula

r = [(n1 – n) / (n1 +n)]2