optical fiber ppt

OPTICAL FIBER

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Basic principle Total Internal Reflection in Fiber

An optical fiber (or fibre) is a glass or plastic fiber

that carries light along its length.

Light is kept in the "core" of the optical fiber by total

internal reflection.

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What Makes The Light Stay in Fiber

Refraction The light waves spread out along its beam. Speed of light depend on the material used called

refractive index. Speed of light in the material = speed of light in the free

space/refractive index Lower refractive index higher speed

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The Light is Refracted

This end travels further than the other hand

Lower Refractive index Region

Higher Refractive index Region

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Refraction When a light ray encounters a boundary separating two

different media, part of the ray is reflected back into the first medium and the remainder is bent (or refracted) as it enters the second material. (Light entering an optical fiber bends in towards the center of the fiber – refraction)

Refraction

LED or LASER Source

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Reflection

Light inside an optical fiber bounces off the cladding - reflection

Reflection

LED or LASER Source

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Critical Angle If light inside an optical fiber strikes the cladding too steeply,

the light refracts into the cladding - determined by the critical angle. (There will come a time when, eventually, the angle of refraction reaches 90o and the light is refracted along the boundary between the two materials. The angle of incidence which results in this effect is called the critical angle).

Critical Angle

n1Sin X=n2Sin90o

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Angle of Incidence

Also incident angle Measured from perpendicular Exercise: Mark two more incident angles

Incident Angles

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Angle of Reflection

Also reflection angle Measured from perpendicular Exercise: Mark the other reflection angle

Reflection Angle

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Reflection

Thus light is perfectly reflected at an interface between two materials of different refractive index if:

The light is incident on the interface from the side of higher refractive index.

The angle θ is greater than a specific value called the “critical angle”.

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Angle of Refraction

Also refraction angle Measured from perpendicular Exercise: Mark the other refraction angle

Refraction Angle

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Angle Summary

Refraction Angle

Three important angles The reflection angle always equals the incident

angle

Reflection Angle

Incident Angles

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Refractive Index

n = c/v c = velocity of light in a vacuum v = velocity of light in a specific medium

light bends as it passes from one medium to another with a different index of refraction air, n is about 1 glass, n is about 1.4

Light bends in towards normal - lower n to higher n

Light bends away from normal - higher n to lower n

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Snell’s Law

The amount light is bent by refraction is given by Snell’s Law: n1sin1 = n2sin2

Light is always refracted into a fiber (although there will be a certain amount of Fresnel reflection)

Light can either bounce off the cladding (TIR) or refract into the cladding

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Snell’s Law

Normal

Incidence Angle(1)

Refraction Angle(2)

Lower Refractive index(n2)

Higher Refractive index(n1)Ray of light

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Critical Angle Calculation

The angle of incidence that produces an angle of refraction of 90° is the critical angle n1sin(qc) = n2sin(90°)

n1sin(qc) = n2

qc = sin-1(n2 /n1)

Light at incident anglesgreater than the criticalangle will reflect backinto the core

Critical Angle, c

n1 = Refractive index of the coren2 = Refractive index of the cladding

OPTICAL FIBER CONSTRUCTION

Core – thin glass center of the fiber where light travels.Cladding – outer optical material surrounding the core

Buffer Coating – plastic coating that protect the fiber.

OPTICAL FIBER

The core, and the lower-refractive-index cladding, are

typically made of high-quality silica glass, though they

can both be made of plastic as well.

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NA & ACCEPTANCE ANGLE DERIVATION

In optics, the numerical aperture (NA) of an optical system is a dimensionless number that characterizes the range of angles over which the system can accept or emit light.”

optical fiber will only propagate light that enters the fiber within a certain cone, known as the acceptance cone of the fiber. The half-angle of this cone is called the acceptance angle, θmax.

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When a light ray is incident from a medium of refractive index n to the core of index n1, Snell's law at medium-core

interface gives

Substituting for sin θr in Snell's law we get:

By squaring both sides

Thus,

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from where the formula given above follows.

NUMERICAL APERATURE IS

ACCEPTANCE ANGLE

θmax =

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Definition:- Acceptance angle:- Acceptance angle is defined as the maximum angle of

incidence at the interface of air medium and core medium for which the light ray enters into the core and travels along the interface of core and cladding.

Acceptance Cone:- There is an imaginary cone of acceptance with an

angle .The light that enters the fiber at angles within the acceptance cone are guided down the fiber core

Numerical aperture:- Numerical aperture is defined as the light gathering capacity

of an optical fiber and it is directly proportional to the acceptance angle.

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Classification of Optical Fiber

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Three common type of fiber in terms of the material used:

• Glass core with glass cladding –all glass or silica fiber

• Glass core with plastic cladding –plastic cladded/coated silica (PCS)

• Plastic core with plastic cladding – all plastic or polymer fiber

Plastic and Silica Fibers

BASED ON MODE OF PROPAGATION

Two main categories of optical fiber used in fiber optic communications are

multi-mode optical fiber

single-mode optical fiber.

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Single-mode fiber Carries light pulses along single path

Multimode fiber Many pulses of light generated by LED travel at different

angles 29

Based on the index profile

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The boundary between the core and cladding may either be abrupt, in step-index fiber, or gradual, in graded-

index fiber

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Step Index Fibers

A step-index fiber has a central core with a uniform

refractive index. An outside cladding that also has a uniform

refractive index surrounds the core;

however, the refractive index of the cladding is less than

that of the central core.

The refractive index profile may be defined as

n(r) = n1 r < a (core) n2 r ≥ a (cladding)

GRADED-INDEX

In graded-index fiber, the index of refraction in the core decreases continuously between the axis and the cladding.

This causes light rays to bend smoothly as they approach the cladding, rather than reflecting abruptly from the core-cladding boundary.

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33Figure.2.6

(a)

(b)

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multimode step-index fiber the reflective walls of the fiber move the light pulses to

the receiver multimode graded-index fiber

acts to refract the light toward the center of the fiber by variations in the density

single mode fiber the light is guided down the center of an extremely

narrow core

Figure 2.10 Two types of fiber: (Top) step index fiber; (Bottom) Graded index fiber

Attenuation

Definition: a loss of signal strength in a lightwave, electrical or radio signal usually related to the distance the signal must travel.

Attenuation is caused by: Absorption Scattering Radiative loss

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Losses

Losses in optical fiber result from attenuation in the material itself and from scattering, which causes some light to strike the cladding at less than the critical angle

Bending the optical fiber too sharply can also cause losses by causing some of the light to meet the cladding at less than the critical angle

Losses vary greatly depending upon the type of fiber Plastic fiber may have losses of several hundred dB

per kilometer Graded-index multimode glass fiber has a loss of

about 2–4 dB per kilometer

Single-mode fiber has a loss of 0.4 dB/km or less37

Macrobending Loss: The curvature of the bend is much larger than fiber

diameter. Lightwave suffers sever loss due to radiation of the evanescent field in the cladding region. As the radius of the curvature decreases, the loss increases exponentially until it reaches at a certain critical radius. For any radius a bit smaller than this point, the losses suddenly becomes extremely large. Higher order modes radiate away faster than lower order modes.

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Microbending Loss

Microbending Loss: microscopic bends of the fiber axis that can arise when the fibers are incorporated into cables. The power is dissipated through the microbended fiber, because of the repetitive coupling of energy between guided modes & the leaky or radiation modes in the fiber.

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Dispersion

The phenomenon in an optical fibre whereby light photons arrive at a distant point in different phase than they entered the fibre.

Dispersion causes receive signal distortion that ultimately limits the bandwidth and usable length of the fiBer cable

The two main causes of dispersion are:

Material (Chromatic) dispersion

Waveguide dispersion

Intermodal delay (in multimode fibres)

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Dispersion in fiber optics results from the fact that in multimode propagation, the signal travels faster in some modes than it would in others

Single-mode fibers are relatively free from dispersion except for intramodal dispersion

Graded-index fibers reduce dispersion by taking advantage of higher-order modes

One form of intramodal dispersion is called material dispersion because it depends upon the material of the core

Another form of dispersion is called waveguide dispersion Dispersion increases with the bandwidth of the light source

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Advantages of Optical Fibre

Thinner Less Expensive Higher Carrying

Capacity Less Signal

Degradation& Digital Signals

Light Signals Non-Flammable Light Weight

Advantages of fiber optics

Much Higher Bandwidth (Gbps) - Thousands of channels can be multiplexed together over one strand of fiber

Immunity to Noise - Immune to electromagnetic interference (EMI).

Safety - Doesn’t transmit electrical signals, making it safe in environments like a gas pipeline.

High Security - Impossible to “tap into.”

Advantages of fiber optics

Less Loss - Repeaters can be spaced 75 miles apart (fibers can be made to have only 0.2 dB/km of attenuation)

Reliability - More resilient than copper in extreme environmental conditions.

Size - Lighter and more compact than copper. Flexibility - Unlike impure, brittle glass, fiber is

physically very flexible.

Fiber Optic Advantages greater capacity (bandwidth up

to 2 Gbps, or more)

smaller size and lighter weight

lower attenuation

immunity to environmental

interference

highly secure due to tap

difficulty and lack of signal

radiation

Disadvantages include the cost of interfacing equipment necessary to convert electrical signals to optical signals. (optical transmitters, receivers) Splicing fiber optic cable is also more difficult.

Disadvantages of fiber optics

Areas of Application

Telecommunications Local Area Networks Cable TV CCTV Optical Fiber Sensors

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Formula Summary

Index of Refraction

Snell’s Law

Critical Angle

Acceptance Angle

Numerical Aperture

v

cn

2211 sinsin nn

1

21sinn

nc

22

21

1sin nn

22

21sin nnNA

STUDENTS YOU CAN ALSO REFER IT……

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http://hank.uoregon.edu/experiments/Dispersion-in-Optical-Fiber/Unit_1.6%20(2).pdfhttp://www1.ceit.es/asignaturas/comuopticas/pdf/chapter4.pdf

http://course.ee.ust.hk/elec342/notes/Lecture%206_attenuation%20and%20dispersion.pdf

1 Engineering Physics by H Aruldhas, PHI India 2 Engineering Physics by B K Pandey , S. Chaturvedi, Cengage Learning 3Resnick, Halliday and Krane, Physics part I and II, 5th Edition John Wiely 4Engineering Physics by S.CHAND5Engineering Physics by G VIJIYAKUMARI