Optic Fibre

8/8/2019 Optic Fibre

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Course Material Prepared By:

Transmission Faculty, NSCBTTC, Kalyani

OPTICAL FIBER CABLE, CHARACTERISTICS,

CONSTRUCTION AND SPLICING

1.0 A Brief History of Fiber-Optic Communications

Optical communication systems date back to the 1790s, to the optical semaphoretelegraph invented by French inventor Claude Chappe. In 1880, Alexander Graham Bell

patented an optical telephone system, which he called the Photophone. However, hisearlier invention, the telephone, was more practical and took tangible shape.

By 1964, a critical and theoretical specification was identified by Dr. Charles K.Kao for long-range communication devices, the 10 or 20 dB of light loss per kilometer

standard. Dr. Kao also illustrated the need for a purer form of glass to help reduce lightloss. By 1970 Corning Glass invented fiber-optic wire or "optical waveguide fibers"

which was capable of carrying 65,000 times more information than copper wire, throughwhich information carried by a pattern of light waves could be decoded at a destination

even a thousand miles away. Corning Glass developed an SMF with loss of 17 dB/km at

633 nm by doping titanium into the fiber core. By June of 1972, multimode germanium-doped fiber had developed with a loss of 4 dB per kilometer and much greater strengththan titanium-doped fiber. Prof. Kao was awarded half of the 2009 Nobel Prize in

Physics for " groundbreaking achievements concerning the transmission of light in fibers for optical communication". In April 1977, General Telephone and Electronics tested and

deployed the world's first live telephone traffic through a fiber-optic system running at 6Mbps, in Long Beach, California. They were soon followed by Bell in May 1977, with an

optical telephone communication system installed in the downtown Chicago area,covering a distance of 1.5 miles (2.4 kilometers). Each optical-fiber pair carried the

equivalent of 672 voice channels and was equivalent to a DS3 circuit. Today more than80 percent of the world's long-distance voice and data traffic is carried over optical-fiber

cables.

2.0 Fiber-Optic Applications

FIBRE OPTICS: The use and demand for optical fiber has grown tremendously

and optical-fiber applications are numerous. Telecommunication applications arewidespread, ranging from global networks to desktop computers. These involve the

transmission of voice, data, or video over distances of less than a meter to hundreds of kilometers, using one of a few standard fiber designs in one of several cable designs.



Fig. 1

5.0 Principle of Operation - Theory

·

y Total Internal Reflection - The Reflection that Occurs when a Ligh RayTravelling in One Material Hits a Different Material and Reflects Back into

the Original Material without any Loss of Light.

Fig. 2

Speed of light is actually the velocity of electromagnetic energy in vacuum such

as space. Light travels at slower velocities in other materials such as glass. Lighttravelling from one material to another changes speed, which results in light changing its

direction of travel. This deflection of light is called Refraction.



The amount that a ray of light passing from a lower refractive index to a higher one is bent towards the normal. But light going from a higher index to a lower one

refracting away from the normal, as shown in the figures.

ø1

Angle of incidence

n1

n2

ø2

n1

n2

ø1

ø2

n1

n2

ø1 ø2

Angle of reflection

Light is bent away

from normal

Light does not enter

second material

Fig. 3

As the angle of incidence increases, the angle of refraction approaches 90o

to thenormal. The angle of incidence that yields an angle of refraction of 90

ois the critical

angle. If the angle of incidence increases amore than the critical angle, the light is totallyreflected back into the first material so that it does not enter the second material. The

angle of incidence and reflection are equal and it is called Total Internal Reflection.

6.0 PROPAGATION OF LIGHT THROUGH FIBRE

The optical fibre has two concentric layers called the core and the cladding. The

inner core is the light carrying part. The surrounding cladding provides the difference

refractive index that allows total internal reflection of light through the core. The index of the cladding is less than 1%, lower than that of the core. Typical values for example are acore refractive index of 1.47 and a cladding index of 1.46. Fibre manufacturers control

this difference to obtain desired optical fibre characteristics. Most fibres have anadditional coating around the cladding. This buffer coating is a shock absorber and has no

optical properties affecting the propagation of light within the fibre. Figure shows theidea of light travelling through a fibre. Light injected into the fibre and striking core to

cladding interface at grater than the critical angle, reflects back into core, since the angleof incidence and reflection are equal, the reflected light will again be reflected. The light

will continue zigzagging down the length of the fibre. Light striking the interface at lessthan the critical angle passes into the cladding, where it is lost over distance. The

cladding is usually inefficient as a light carrier, and light in the cladding becomesattenuated fairly. Propagation of light through fibre is governed by the indices of the core

and cladding by Snell's law.

Such total internal reflection forms the basis of light propagation through a optical

fibre. This analysis consider only meridional rays- those that pass through the fibre axiseach time, they are reflected. Other rays called Skew rays travel down the fibre without

passing through the axis. The path of a skew ray is typically helical wrapping around andaround the central axis. Fortunately skew rays are ignored in most fibre optics analysis.



The specific characteristics of light propagation through a fibre depends on manyfactors, including

- The size of the fibre.

- The composition of the fibre.

y The light injected into the fibre.

Jacket

Claddi

C r e

Claddi

le f r eflecti

le f i cidence

Li t at less t an

critical angle is

absor bed in jacket

Jacket

Light is pr opagated by

total inter nal r eflection

Jacket

Cladding

Cor e

(n2)

(n2)

Fig. Total Inter nal Reflection in an optical Fibr e

Fig. 4 Propagation of light through fiber

7.0 Geometry of Fiber

A hair-thin fiber consist of two concentric layers of high-purity silica glass thecore and the cladding, which are enclosed by a protective sheath as shown in Fig. 5. Light

rays modulated into digital pulses with a laser or a light-emitting diode moves along thecore without penetrating the cladding.

Fig. 5 Geometry of fiber

The light stays confined to the core because the cladding has a lower refractive

index²a measure of its ability to bend light. Refinements in optical fibers, along with thedevelopment of new lasers and diodes, may one day allow commercial fiber-optic

networks to carry trillions of bits of data per second.

The diameters of the core and cladding are as follows.

Optic Fibre

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