Optical Communication 123

8/6/2019 Optical Communication 123

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OPTICAL COMMUNICATION

(READ THE FOLLOWING MATERIAL WITH REFERENCE TO NOTES DICTATED IN OFC REGULAR CLASS)

What is an Optical Fiber? Optical fibers are long strands of thin andflexible cylindrical transparent materials made of plastic or glass designed totransmit visible or infra red light across great distance.

Main Parts of an Optical Fiber Cable:

1. C ore: innermost part of the fiber, made of glass or plastic or glass2. C ladding: layer surrounding the core, made of glass or plastic,

refractive index is greater than that of core, constricts light within coreusing total internal reflection.

3. Jacket: outermost layer protecting the cladding from environmental

hazards, pollution and shocks, made of plastic or polymerD imension of fiber: C ore: 5 micrometer to 50 micrometerC ladding: about 125 micrometerJacket: about 250 micrometer


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Adv antages of fibers o v er Cables an d Wires:

y L ighter in weight and occupy less spacey M ore economical than cables and wires in the long runy N o physical connection is required from sender to receiver

y T

ransmission speed is very fast compared to wires and co axial cable.y T he electrical noise does not interfere with propagated light signals.T hus in optical fiber, cross talking can be avoided.

y T he attenuation in a fiber is markedly lowy T he bandwidth of the fiber is highery R aw materials required for production of optical fibers are available in

plentyy T he fibers last longer than copper wires as they are immune to

pollution and environmental hazardsy T he information carrying capacity of an optical fiber is several

(thousands) of times more than the copper cable or co-axial cablesused for transferring radio waves for telephones.

y

Working Principle: T he core is made of a material of refractive index n1while the cladding is made of a material of refractive index n2 (n1>n2).

When light enters one end of the fiber, it travels from a densermedium towards a rarer medium with a finite angle. T he light ray hits thecore-cladding interface with an angle greater than the critical angle for theinterface and thus gets reflected at the interface due to total internalreflection. It suffers multiple total internal reflections at the core-claddinginterface along the whole length of the fiber and finally emerges out of theother end of the fiber. T he transmission occurs without any loss of energy

even when the fiber is bent.

T he ability of light gathering of the fiber depends on:

1. core diameter2. numerical aperture


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Classification of optical Fiber:

1. S tep-in d ex optical Fiber: It has small core diameter (same order asthat of the wavelength of light) along with a core and cladding of uniform refractive index n1 and n2 respectively (n1>n2). It is called so

because a sudden change of refractive index occurs at the junction of the core-cladding interface in a single step to allow total internalreflection to take place.

Adv antage: Best for long distance communication as the pulse repetitioncan be high and maximum information can be send.

D isa dv antage: use of thin core creates mechanical difficulties inmanufacturing and handling of the fiber.

Utility: Used in under sea-level transmission of data where the expenditureis justified

2. G ra d e d -in d ex optical fiber: It is a multimode fiber with huge corediameter and with a core having non-uniform refractive index, therefractive index being a function of radial distance from the fiber axis.T he refractive index of the cladding is uniform. Since the refractiveindex towards the core-cladding interface is lower than that at thecenter, the light rays traveling along the edge travels faster. T hus, allrays arrive the end of the fiber at approximately the same time.


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Nu merical Apert u re an d Acceptance Angle

Acceptance Angle: T he maximum angle of incidence at the entranceaperture of the fiber for which the light ray undergo total internal reflectionat the core-cladding interface and propagates through the fiber is called the

acceptance angle.

D efine Nu merical apert u re.N umerical aperture ( N .A) of the fiber is the light collectingefficiency of the fiber and is the measure of the amount of lightrays that can be accepted by the fiber. It is equal to the sine of acceptance. Its a dimensionless quantity.

2

2

2

1sin nn NA

where n 1 and n 2 are the refractive indices of core andcladding respectively.

Nu merical apert u re d eri v ation u sing S nells Law

T he n u merical apert u re ( N A) of an optical system is a dimensionlessnumber that characterizes the range of angles over which the system canaccept or emit light .

N umerical aperture ( N .A) of the fiber is the light collecting efficiency of thefiber and is the measure of the amount of light rays that can be accepted bythe fiber.


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T he numerical aperture of an optical system such as an objective lens isdefined by

where n is the index of refraction of the medium and is the half-angle of

the maximum cone of light that can enter or exit the lens.

D eri v ation of Nu merical apert u re by u sing S nells law:

C onsider the optical fiber with refractive index of the fiber core is n 1 andrefractive index of the cladding is n 2. When a light ray is incident from a medium of refractive index n to the coreof index n 1 at the maximum acceptance angle, Snell's law at the medium

core interface gives

From the geometry of the above figure we have:

Where is the critical angle for total internal reflection.

Substituting cos c for sin r in Snell's law we get:

By squaring both sides


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Solving, we find the formula stated above:

T his has the same form as the numerical aperture in other optical systems

Write the expression for the refracti v e in d ex in gra d e d in d ex fibers.

n(r)= n1 [1 -2 (r/a) ] 1 /2 ; for O


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d) T o protect the core from scratches and other mechanicaldamages

What are the u ses of optical fibers?a) T o transmit the information which are in the form of codedsignals of the telephone communication, computer data, etc.b) T o transmit the optical images (Example : Endoscopy)c) T o act as a light source at the inaccessible places.d) T o act as sensors to do mechanical, electrical and magneticmeasurements.

What is the principle u se d in the working of fibers as light g u id es?T he phenomenon of total internal reflection is used to guide

the light in the optical fiber. T o get total internal reflection, the rayshould travel from denser to rarer i.e. from core to clad region of the fiber and the angle of incidence in the denser medium should begreater than the critical angle of that medium.

What are step in d ex an d gra d e d in d ex fibers?In the case of step index fiber, the refractive index of a core

is a constant and is larger than the refractive index of the cladding.T he light propagation is mainly by meridional rays. In the case of graded index fiber (G R IN fiber) the refractive index of the corevaries parabolically from the centre of the core having maximumrefractive index to the core-cladding interface having constantminimum refractive index. Here the light propagation is by skew

rays.

D efine acceptance angle.T he maximum angle max with which a ray of light can

enter through the entrance end of the fiber and still be totallyinternally reflected is called acceptance angle of the fiber.

Why d o we prefer step in d ex single mo d e fiber for long d istancecomm u nication?Step index single mode fiber has

a)low attenuation due to smaller core diameterb) higher bandwidth andc) very low dispersion.

What are meri d ional rays?M eridional rays are the rays following Zig Zag path when


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they travel through fiber and for every reflection it will cross thefiber axis.

What are skew rays?Skew rays are the rays following the helical path around the

fiber axis when they travel through the fiber and they would notcross the fiber axis at any time.

What is V n u mber of fiber or normalize d freq u ency of fiber?V number of fiber or normalized frequency of fiber is used to

find the number of propagating modes through the fiber.

V= 2 a ( N .A)

In step index fiber number of modes propagating through the fiber=V 2 /2In graded index fiber number of modes propagating through the fiber=V 2 /4.

What are the con d itions for total internal reflection?a) Light should travel from denser medium to rarer medium.b) T he angle of incidence should be greater than the critical angle of the denser medium.

G iv e the relation between n u merical apert u re of skew rays an d meri d ional rays.

](NA).COS =[(NA) Mer

d

o

alSkew K , when the fiber is placed in air.Here, K is the half of the angular change in every reflection.

When d o yo u ha v e phase shift d u ring total internal reflection of light.

When the light ray travels from denser medium to rarer medium, if theangle of incidence is greater than the critical angle of core medium, thereis a phase shift for both T E and TM waves.

D efine c u toff wa v elength of the fiber .T he cutoff wavelength is defined as the maximum value of wavelength

that can be transmitted through the fiber. T he wavelengths greater than thecutoff wavelength can not be transmitted through the optical fiber.

)(cutoff NAV aT

P


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Mention the r u le d isting u ishing mo d e an d or d er.T he rule states that the smaller the modes propagating angle, the

lower the order of the mode. T hus the mode traveling precisely along thefibers central axis is zero mode.

G iv e the expression for n u merical apert u re in gra d e d in d ex fibers.

N .A(r)= N .A.(0) (1 -(r/a) ) 1 /2 for r


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Comparison between S ingle Mo d e Fiber ( S MF ) an d Mu lti Mo d e Fibers(MMF ) :

T he following are the difference between Single M ode and M ultimode fiberoptics:

1 ) Single M ode carries only a single ray of light whereas multiple rays of light can travel through M ultimode fiber optics.

2 ) Single mode fibers do not exhibit any dispersion unlike M ultimode fibers.

3 ) M ultimode fibers have multiple spatial modes unlike single mode fibers.

4 ) Single mode fibers are better at retaining the fidelity of light pulse overlong distances than multimode fibers.

5 ) Single mode fibers have higher bandwidth than multimode.

6 ) Single mode fiber equipment is more expensive than the equipment formultimode.

7 ) Single mode fiber is cheaper than multimode fiber.

8 ) M ultimode fibers have higher capacity and reliability over short distancesthan single mode.

9 ) M ultimode fibers support more than one propagation mode unlike singlefiber.

10 ) M ultimode fibers are limited by modal dispersion whereas single modeis not.


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E LE CTRO M AGN E TIC S P E CTRUM

Fiber MaterialsReq u irements for optical fiber material

y It must be possible to make long thin, flexible fibers from the materialy M aterial must be transparent at a particular optical wave length in

order for fiber to guide light efficientlyy P hysically compatible materials that have slightly different refractive

indices for core and cladding must be available

M aterials that satisfy these requirements are glasses and plastic.M ajority of fibers are made of glass consisting of either silica orsilicate. P lastic fibers are less widely used because of their higherattenuation. P lastic fibers are used for short distance applications(several hundred meters) and abusive environments.


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G lass Fiber:

Glass is made by fusing mixture of metal oxides, sulfides, or selenides.T he resulting material is a randomly connected molecular network rathera well defined structure as found in crystalline materials.

A consequence of this random order is glass does not have a well definedmelting point. When glass is heated, it gradually begins to soften until itbecomes a viscous liquid. Optical fibers are made from oxide glasses andmost popular is silica (SiO2) which has refractive index of 1.458 at 850nm.

T o produce two similar materials with slightly different refraction indicesfor core and cladding, either fluorine or other oxides (dopants) are addedto silica. Sand is the principle raw material for silica. G lass composed of

pure silica is referred to as either silica glass, fused glass, or vitreoussilica.


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D esire d properties of glass fibers are,

y R esistance to deformation at temperatures as high as 1000 C y High resistance to breakage from thermal shocky Good chemical durability

y

High transparency in both visible and infrared regions of interest.

Plastic Optical Fibers

Growing demand for delivering high-speed services to workstationsHave greater optical signal attenuations than glass fiberT hey tough and durableC ore diameter is 10-20 times larger.

Fiber FabricationT wo basic techniques

Vapor-phase oxi d ation processOutside vapor phase oxidationVapor phase axial depositionM odified chemical vapor depositionP lasma activated chemical vapor deposition

D irect-melt metho d s.

D irect melt metho d

Follows traditional glass making procedures Optical fiber are made directly from molten state of purified

components of silicate glass

Vapor phase oxi d ation

Highly pure vapors of metal galides (Si C l4) react with oxygen toform white powder of SiO2 particles

P articles are collected on surface of bulk glass by above methodsand are transformed to a homogenous glass by heating withoutmelting to form a clear glass rod or tube. T his rod is called

perform P reform is 10-25 mm in diameter and 60-120 cm long.


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Prefrom is fed into circular heater called drawing furnace.

P reform end is softened to the point where it can be drawn into a

very thin filament which becomes optical fiber

T he speed of the drum at the bottom of draw tower determines

how fast and in turn how thick the fiber is

An elastic coating is applied to protect the fiber.


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O u tsi d e Vapor Phase Oxi d ation

C ore layer is deposited on a rotating ceramic rod

C ladding is deposited on top of core layer

C eramic rod is slipped out (different thermal expansion coefficient)

T he tube is heated and mounted in a fiber drawing tower and made

into a fiber

T he central hole collapses during this drawing process.


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Vapor Phase Axial D eposition

Similar to outside vapor deposition

Starts with a seed which is a pure silica rod

T he preform is grown in the axial direction by moving rod upward

R od is also rotated to maintain cylindrical symmetery

As preform moves upward it is transformed into a solid transparent

rod preform by zone melting (heating in a narrow localized zone)

Adv antages

N o central hole is formed.


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Mo d ifie d Chemical Vapor D eposition

P ioneered at Bell Labs, and adopted to produce low loss graded index

fiber

Glass vapor particles, arising from reaction of constituent metal halide

gasses and oxygen flow through inside of revolving silica tube

As SiO2 particles are deposited, they are sintered to a clear glass layer

by an oxyhydrogen torch which travels back and forth

When desired thickness of glass have been deposited, vapor flow is

shut off

T ube is heated strongly to cause it to collapse into a solid rod prefrom

Fiber drawn from this prefrom rod will have a core that consists of

vapor deposited material and a cladding that consists of original silica

tube.


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D o u ble Cr u cible Metho d ( D irect melt metho d)

Silica and halide glass fiber can all be made using a direct-melt double

crucible technique

Glass rods for the core and cladding materials are first made

separately by melting mixtures of purified powders

T hese rods are then used as feedstock for each of two concentric

crucibles

Advantage of this method is being a continuous processC areful attention must be paid to avoid contaminants during melting.

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Optical Communication 123

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