step index and graded index fiber

8/13/2019 Step Index and Graded Index Fiber

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STEP INDEX AND GRADED

INDEX FIBER

Made by-

Aparna Garg

Ashish Arya

Abhinav Agrawal

Aftab Ali Rai


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Single mode and Multimode Fiber

Core has uniform refractive index

Refractive index of cladding is also uniform butsmaller than Core

R.I.core>R.I.claddingStep change in Refractive Index at core cladding

interface

STEP INDEX FIBER


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STEP INDEX FIBER

Refractive Index variation in Step Index Fiber


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Fabricated from doped silica

Normalized Frequency (V)


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2 Types:

Matched Cladding

Depressed Cladding

Matched Cladding:

Core: Constant refractive index

Cladding: Single homogeneous layer of dielectric

SINGLE MODE STEP INDEX FIBER


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Depressed Cladding:

Core: Constant Refractive Index

Cladding: Made of two regions

SINGLE MODE STEP INDEX FIBER


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Fabricated from multicomponent glass or doped Silica

Large core diameter and numerical aperture

MULTIMODE STEP INDEX FIBER


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Normalized Frequency (V)>2.405

Attainable bandwidth: 300MHz- 3GHz km

High attenuation

Numerical aperture,

MULTIMODE STEP INDEX FIBER


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GRADED INDEX MULTIMODE FIBER

Graded index fiber is an optical fiber whose core has arefractive index that decreases with increasing radialdistance from the fiber axis.


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Higher Order ModesLarger propagation length

Travel farther into the claddingSpeed increases with distance away from the core(decreasing index of refraction)

Relative difference in propagation speed is less


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Refractive index profile

where, = Relative refractive index difference

arnn

ara

r

n

rn

21

2

1

21

21


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If , = Step index profile

= 2 Parabolic profile= 1 Triangular profile

Variations in R.I. with


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Light rays follow Sinusoidal paths down the fiber.

The intermodal dispersion is smaller than for step index multimodefiber

Shorter wavelengths of rays travelling near fiber axis

4

2

1

inter

c

nD

g


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Propagation of Ray

Propagation through Reflection and Total Internal

Reflection Due to gradual decrease in cores R.I. from the center of

the fiber ray reflects many times


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The ray that travels directly along the axis reaches the end ofthe fiber at the same time as the higher order ray .

Modal dispersion

speed of guided light changes with fiber cores refractive index.

Farther the light goes from the centre of the fiber, the faster it is.

Speed difference compensate for the longer paths followed bythe light rays that go farthest from the centre of the fiber. This

equalizing of transit times of different modes greatly reducesmodal dispersion.


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Step model of graded index fiber

Model shows the refraction at different layers of core

Ray refracts at higher angle at every layer and total internal

reflection occurs before it reaches at boundary of core and

cladding


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NA(Numerical aperture) of gradedindex fiber

NA is at its maximum value at fiber axis

Axial Numerical Aperture [NA(0)]= n1(2 )1/2

NA decreases with increase in r

Becomes zero for r>a


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No. of modes in graded index fiber

Depends upon NA, Relative R.I. difference, Profileparameter(alpha) and normalized frequency (V).

Have typically over one hundred modes

Accept less light than multimode single index fiber for samecore and delta


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Basic design parameter

Standard multimode graded index fiber core and cladding

sizes are 50/125 m, 62.5/125 m, 85/125 m and 100/400 m.

Typical values of delta are around 0.01 to 0.02

62.5/125 m design with 0.02 delta offers the best overallperformance


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Difference between Step index and Graded index

1. Step index fiber is of two types viz; mono mode fiber and multi mode fiber.Graded index fiber is of only of one type that is multi mode fiber.

2. The refractive index of the core of the step index fiber is constant throughout the core.

The refractive index of the core of the graded index fiber is maximum at thecentre of the core and then it decreases towards core-cladding interface.


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3. Number of modes for step index fiber N = V2/2, where V is cut offfrequency or normalized frequency or V- numberNumber of modes for graded index fiber is N = V2/ 4.

4. V number can be less that 2.405 or more that 2.405 for step indexfiberV number is always more than 2.405 for graded index fiber.


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5. Step-index multimode fibers are mostly used for imaging andillumination.

Graded-index multimode fibers are used for data communications andnetworks carrying signals moderate distances - typically no more

than a couple of kilometers.


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step index and graded index fiber

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