OPTICAL FIBRE CABLE

ADVANTAGES OF FIBRE OPTICS :

(I) Optical Fibres are non conductive

- Cables can be all dielectric.

(II) Electromagnetic Immunity :

- Immune to electromagnetic interference (EMI)

- No radiated energy.

- Unauthorised tapping difficult.

(III) Large Bandwidth (> 5.0 GHz for 1 km length)

- Future upgradability.

- Maximum utilization of cable right of way.

- One time cable installation costs.

(IV) Low Loss (5 dB/km to < 0.25 dB/km typical)

(vi) Small, Light weight cables. - Easy installation and Handling. - Efficient use of space.

(vi) Available in Long lengths (> 12 kms)- Less splice points.

(vii) Security -Extremely difficult to tap a fibre as it does not radiate energy

- Highly secure transmission medium. (viii) Security - Being a dielectric

- It cannot cause fire. - Does not carry electricity. - Can be run through hazardous areas.

(ix) Universal medium

Applications Common carrier nationwide networks. - Telephone Inter-office Trunk lines. - Customer premise communication

networks. - Undersea cables. - High EMI areas (Power lines, Rails, Roads). - Factory communication/ Automation. - Control systems. - Expensive environments. High lightening areas. - Military applications. - Classified (secure) communications.

  Transmission Sequence   

(1) Information is Encoded into Electrical Signals. (2) Electrical Signals are Coverted into light Signals. (3) Light Travels Down the Fiber. (4) A Detector Changes the Light Signals into Electrical Signals. (5) Electrical Signals are Decoded into Information.- Inexpensive light sources available. - Repeater spacing increases along with operating speeds because low loss fibres are used at high data rates

THEORY AND PRINCIPLE OF FIBRE OPTICS :

By Snell's law, n1 sin 1 = n2 sing 2 The critical angle of incidence c where 2

= 90 o Is c = arc sing (n2 / n1) At angle greater than c the light is

reflected, Because reflected light means that n1 and n2 are equal (since they are in the same material), 1 and 2 are also equal.

The angle of incidence and reflection are equal. These simple principles of refraction and reflection form the basis of light propagation through an optical fibre.

ø1

Angle of incidence

n1

n2

ø2

n1

n2

ø1

ø2

n1

n2

ø1 ø2

Angle ofreflection

Light is bent away from normal

Light does not enter second material

Propagation of light thro 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.

Jacket

CladdingCore

Cladding

Angle of reflection

Angle of incidence

Light at less thancritical angle isabsorbed in jacket

Jacket

Light is propagated by total internal reflection

Jacket

Cladding

Core

(n2)

(n2)

Fig. Total Internal Reflection in an optical Fibre

An Optical fibre consists of a core of optically transparent material usually silica or borosilicate glass surrounded by a cladding of the same material but a slightly lower refractive index.

Fibre themselves have exceedingly small diameters. Figure shows cross section of the core and cladding diameters of commonly used fibres. The diameters of the core and cladding are as follows

Core (m) Cladding ( m)

8 125

50 125

62.5 125

100 140

125 8 125 50 125 62.5 125 100

Core Cladding

Typical Core and Cladding Diameters

Fibre sizes are usually expressed by first giving the core size followed by the cladding size. Thus 50/125 means a core diameter of 50m and a cladding diameter of 125m.

Fibre types classification there are three types of fibres : (I) Multimode Step Index fibre

(Step Index fibre) (II) Multimode graded Index fibre

(Graded Index fibre) (III) Single- Mode Step Index fibre

(Single Mode Fibre)

Step index fibre This fibre is called "Step Index" because the refractive

index changes abruptly from cladding to core

The paths along which the rays (modes) of this step index fibre travel differ, depending on their angles relative to the axis.

As a result, the different modes in a pulse will arrive at the far end of the fibre at different times, resulting in pulse spreading which limits the bit-rate of a digital signal which can be transmitted.

This types of fibre results in considerable model dispersion, which results the fibre's band width.

GRADED INDEX FIBRE This fibre is called graded index because there are many changes in the refractive index with larger values towards the center. As light travels faster in a lower index of refraction.

So, the farther the light is from the center axis, the grater is its speed. Each layer of the core refracts the light. Instead of being sharply reflected as it is in a step index fibre, the light is now bent or continuously refracted in an almost sinusoidal pattern.

Those rays that follow the longest path by travelling near the outside of the core, have a faster average velocity. The light travelling near the center of the core, has the slowest average velocity.

High orderMode

Dispersion RefractiveIndex Profile

Low Order ModeMulti mode Step Index

InputPulse

OutputPulse

n1

n2

Single Mode Step Index

n1n2

Dispersion

Multi mode Graded Index

n1

n2

Optical fibre systems have the following parameters.

(I) Wavelength. (II) Frequency. (III) Window. (IV) Attenuation. (V) Dispersion. (VI) Bandwidth.

WAVELENGTH It is a characterstic of light that is emitted from the

light source and is measures in nanometers (nm). In the visible spectrum, wavelength can be described as the colour of the light.

For example, Red Light has longer wavelength than Blue Light, Typical wavelength for fibre use are 850nm, 1300nm and 1550nm all of which are invisible.

FREQUENCY It is number of pulse per second emitted from a

light source. Frequency is measured in units of hertz (Hz). In terms of optical pulse 1Hz = 1 pulse/ sec.

A narrow window is defined as the range of wavelengths at which a fibre best operates. Typical windows are given below :

Window Operational Wavelength

800nm - 900nm 850nm 1250nm - 1350nm 1300nm 1500nm - 1600nm 1550nm

Gamma rays

Rontgen rays

U.V. rays

Visible Light

Infra Red

Thermal Rays

U.H.F.

M.F.

L.F.

Ra

dio

Fre

qu

en

cie

s

10

- 12

10

-810

- 610

- 410

010

410

610

210

- 210

- 10

1M

m1K

m1m

1m

m1μ

m1n

m1p

m

WA

VE

LE

NG

TH

IN N

M

ATTENUATION INTRINSIC ATTENUATION It is loss due to inherent or within the fibre.

Intrinsic attenuation may occur as (I) Absorption - Natural Impurities in the glass

absorb light energy. (II) Scattering - Light rays travelling in the core

reflect from small imperfections into a new pathway that may be lost through the cladding.

Absorption - Natural Impurities in the Glass Absorb Light Energy.

LightRay

(2) Scattering - Light Rays Travelling in the Core Reflect from small Imperfections into a New Pathway that may be Lost through the cladding.

LightRay

Light is lost

EXTRINSIC ATTENUATION

It is loss due to external sources. Extrinsic attenuation may occur as –

Macrobending - The fibre is sharply bent so that the light travelling down the fibre cannot make the turn & is lost in the cladding.

Microbending - Microbending or small bends in the fibre caused by crushing contraction etc. These bends may not be visible with the naked eye.

Micro bend

Micro bend

Fig. Loss and Bends

Micro bend

DISPERSION It is defined as the spreading of light

pulse as it travels down the fibre. ecause of the spreading effect, pulses tend to overlap, making them unreadable by the receiver.

BANDWIDTH It is defined as the amount of

information that a system can carry such that each pulse of light is distinguishable by the receiver.

NUMBERICAL APERTURE

Numerical aperture (NA) is the "light - gathering ability" of a fibre. Light injected into the fibre at angles greater than the critical angle will be propagated. The material NA relates to the refractive indices of the core and cladding.

NA = n12 - n22

where n1 and n2 are refractive indices of core and cladding respectively.

NA is unitless dimension.

Dispersion Dispersion is the spreading of light pulse as its travels down the length of an optical fibre. Dispersion limits the bandwidth or information carrying capacity of a fibre.

There are three main types of dispersion in a fibre -

(I) Modal Dispersion (II) Material dispersion (III) Waveguide dispersion

BANDWIDTH AND DISPERSION A bandwidth of 400 MHz -km means that a 400 MHz-signal

can be transmitted for 1 km. It means that the product of frequency and the length must be 400 or less. We can send a lower frequency for a longer distance, i.e. 200 MHz for 2 km or 100 MHz for 4 km.

Multimode fibres are specified by the bandwidth-length product or simply bandwidth.

Single mode fibres on the other hand are specified by dispersion, expressed in ps/km/nm.

BANDWIDTH AND DISPERSION A bandwidth of 400 MHz -km means that a 400 MHz-signal

can be transmitted for 1 km. It means that the product of frequency and the length must be 400 or less. We can send a lower frequency for a longer distance, i.e. 200 MHz for 2 km or 100 MHz for 4 km.

Multimode fibres are specified by the bandwidth-length product or simply bandwidth.

Single mode fibres on the other hand are specified by dispersion, expressed in ps/km/nm.