FIBER OPTICS

Light propagation through thin glass fibers

FIBER OPTICS

Advantages

Low Attenuation (less signal degradation)

Very High Bandwidth over long distances

Small Size and Low Weight

No Electromagnetic Interference

Non-Conductive Cables

Easy installation

Eliminates Spark Hazards (non-flammable)

Data Security

PRINCIPLE OF FIBER OPTICS

Optical fibers are used as dielectric waveguides for guiding the electromagnetic waves at optical frequencies

Light is guided through transparent glass fibers by total internal reflection

TOTAL INTERNAL REFLECTION

Snell’s law

When the transmitted (refraction angle) reaches 90 , the incidence angle is ⁰

called as the critical angle

1

2

t

i

n

n

sin

sin

TOTAL INTERNAL REFLECTION

Angle of incidence less than the critical angle

Angle of incidence equal to the critical angle

Angle of incidence greater than the critical angle

Total internal refection takes place only when the light

travels from denser to rarer medium and angle of

incidence should be greater than the critical angle of

the denser medium

TOTAL INTERNAL REFLECTION

Structure Fiber Optic Cable

Glass fiber consists of a central core

glass (≈ 50 μm) surrounded by a cladding

Refractive index of cladding is slightly

lower than the refractive index of core

Core + Cladding ≈ 125 – 200 μm

Fiber is made of silica (SiO2) glass

Cladding is also made of silica but the

outer layer is added with small amount of

B, Ge or P to decrease the refractive index

Structure Fiber Optic Cable

Cladding provides

Proper light guidance (to retain the light

wave within the core)

High mechanical strength

High safety to the core from the scratch

Buffer Jacket

Made of plastic and protects the fiber

from moisture and abrasion

Structure 0f Fiber Optic Cable

Strength Members

Provides necessary toughness and

tensile strength

Fiber optic cable withstands during hard

pulling, bending, stretching or rolling

without fracture even though the fiber is

made from brittle glass

Black Polyurethane Outer Jacket

Avoids mechanical crushing

Consider the light propagation through a step index fiber

Fiber is placed in a medium of refractive index “no”

Propagation of Light through Optical Fibers

Let ɸmax is the incident angle at the entrance end of the fiber with respect to the axis of the fiber

Let θr be the corresponding angle of refraction

By Snell’s law

Propagation of Light through Optical Fibers

r0

1max

0

1

r

max

sin n

nsin

n

n

sin

sin

)190sin (since n

nsin

n

n

90sin

sin

1

2c

1

2c

In the limit of total internal reflection

Since

Propagation of Light through Optical Fibers

1

2r

rc

)(90sin

90

n

n

(or)

1

2r cosn

n rc 90

Angle of incidence at the core-cladding interface

Propagation of Light through Optical Fibers

1/22

1

2r 1sin

n

n

1cossin 22

1

2r cosn

n

r0

1max sin sin

n

n

2/1

20

22

21

1/22

1

2

0

1max 1sin

n

nn

n

n

n

n

Propagation of Light through Optical Fibers

2/1

20

22

21

maxsin

n

nnIf the fiber is placed in air (n0 = 1)

2/122

21maxsin nn

Acceptance Angle

Maximum value of the angle of

incidence at the entrance end

of the fiber, at which the angle

of incidence at the core-

cladding surface is equal to

the critical angle of the core

medium

2

221

1-max

2/122

21

1-max

2/122

21max

sin

sin

sin

nn

nn

nn

Acceptance Angle

Acceptance angle of the fiber is the maximum angle of incidence

up to which a light ray can enter into the fiber and still be totally

internally reflected

2

221

1-max

2/122

21

1-max

2/122

21max

sin

sin

sin

nn

nn

nn

Numerical Aperture

Numerical Aperture (N.A.) is the light collecting efficiency of the

fiber and is the measure of amount of light rays that can be

accepted by the fiber

2/122

21max

2/122

21 sin N.A nnnn

2/12121

2/122

21max sinN.A. nnnnnn

If n1 ≈ n2,

then (n1+n2) = 2n1

2/11

2/1211 2 2N.A. nnnn

Relative refractive index

difference of the fiber (Δ) 1

2121

22

21

2 n

nn

n

nn

Step Index Fiber

The refractive index of the core of the step index fiber is constant through out the core

Graded Index fiber

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