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Unit-2

Optical Fibers: Structures

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Basic Optical Laws and Definitions

Refractive index of a medium is defined as:

Typical values of n are 1.00 for air, 1.33 for water, 1.50 for glass, and 2.42 for

diamond.

The concepts of reflection and refraction can be interpreted most easily by

considering the behavior of light rays in a dielectric material.

When a light ray encounters a boundary separating two different media, part of 

the ray is reflected back into the first medium and remainder is bent or refractedas it enters the second material.

The bending or refraction of the light ray at the interface is a result of the

difference in speed of light in two materials that have different refractive indices.

mediuminwave)(EMlightovelocity

in vacuumwave)(EMlightovelocity!!

v

c

n

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Laws of Ref lection & Refr action

Snell¶s law of refraction:

2211 sinsin J J  nn !Optical Fiber communications, 3rd ed.,G.Keiser,McGrawHill, 2000

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Total internal ref lection, Critical angle

1

2sin

n

n

c!J 

1U

n2

n1> n

2

Incident

light

Tr ansmitted(refr acted) light

Ref lected

light

k t 

TIR 

k i

k r 

(a) (b) (c)

(a)Light wave tr avelling in a more dense medium strikes a less dense medium(n1>n2). Depending on

the incidence angle with res  pect to ,which is deter mined by the r atio of the refr active

indices, the wave may be tr ansmitted (refr acted) or ref lected. (a) (b) (c)and total internal ref lection (TIR).

2J 

1J c

J 

Q902 !J 

cJ J "1

cJ 

cJ J 1 cJ J !1

cJ J 1

Critical angle

1

2sin

n

n

c!J 

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Phase shift due to TIR 

The totally ref lected wave experiences a phase shift however 

which is given by:

Where ( p,N ) refer to the electric field com ponents par allel or 

nor mal to the plane of incidence res pectively.

2

1

1

1

22

1

1

22

sin

1cos

2tan;sin

1cos

2tan

n

n

n

nn

n

n  p N 

!

!

! U

UH 

U

UH 

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Optical Fiber s: odes and Configur ations

The propagation of light along a waveguide can be described in terms of a set

of guided electromagnetic waves called the modes of the waveguide.

Each guided mode is a pattern of electric and magnetic field distributions that

is repeated along the fiber at equal intervals.

The light or the optical signals are guided through the silica glass fibers by

total internal reflection.

A typical glass fiber consists of a central core glass (50 mm) surrounded by a

cladding made of a glass of slightly lower refractive index than the cores

refractive index.

Cladding is necessary to provide proper light guidance i.e. to retain the light

energy within the core as well as to provide high mechanical strength and

safety to the core from scratches.

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Optical Fiber s: Structure

1n 2n

21 nn "

Optical Fiber communications, 3rd ed.,G.Keiser,McGrawHill, 2000

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Step index fiber 

In the step index fiber, the refractive index of the core is uniform throughout

and undergoes an abrupt or step change at the core cladding boundary.

The light rays propagating through the fiber are in the form of meridional rays

which will cross the fiber axis during every reflection at the core claddingboundary and are propagating in a zig-zag manner as shown in figure below

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Graded index fiber 

In the graded index fiber, the refractive index of the core is made to vary in the

parabolic manner such that the maximum value of refractive index is at the

centre of the core.

The light rays propagating through it are in the form of skew rays or helicalrays which will not cross the fiber axis at any time and are propagating around

the fiber axis in a helical (or) spiral manner as shown in figure below

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Different Structures of Optical Fiber 

Optical Fiber communications, 3rd ed.,G.Keiser,McGrawHill, 2000

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Single mode fibers

In a single mode fiber, only one mode(Propagation path) can propagate

through the fiber . Normally the number of modes propagating through the

fiber is proportional to its V-number where

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Single mode fibers

In the case of a single mode fiber, V-number <=2.405.

The single mode fiber has a smaller core diameter (10 mm) and the

difference between the refractive indices of the core and the cladding is very

small.

Fabrication of single mode fibers is very difficult and so the fiber is expensive.

The launching of light into single mode fibers is also difficult.

Generally in the single mode fibers, the transmission loss and dispersion ordegradation of the signal are very small.

So the single mode fibers are very useful in long distance communication.

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Multi mode fibers

Multimode fibers allow a large number of modes (Propagation path) for the

light raystraveling through it.

Here the V-number is greater than 2.405.

Total number of modes N propagating through a given multimode step index

fiber is given by

where d is the diameter of the core of the fiber. For a multimode graded index  fiber having parabolic refractive index profile core,

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Multi mode fibers

The core diameter is generally larger than in the single mode fiber.

In the case of multimode graded index fiber, signal distortion is very low

because of self-focusing effects.

Here the light rays travel at different speeds in different paths of the fiber

because of the parabolic variation of refractive index of the core.

Launching of light into the fiber and fabrication of the fiber are easy.

These fibers are generally used in local area networks and applicationswhere high power must be transmitted.

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I ntermodal Dispersion

When an optical pulse is launched into a fiber, the optical power in the fiber 

is distributed over all the modes of the fiber.

Each of the mode that propagates in a multimode fiber travels at a slightly 

different velocity.

This means that the modes in a given optical pulse arrive at the fiber end 

at different times.

Thus causing the pulse to spreading out in time as it travels.This effect is

called as I ntermodal Dispersion.

The dispersion leads to the distortion (or) degradation of the signal quality

at the output end due to overlapping of the pulses.