Optical Fiber Cable, Principle and Operation

FIBRE OPTICS

• Optical Fibre is new medium, in which information (voice, Data or Video) is transmitted through a glass or plastic fibre, in the form of light

• Transmission sequence:-• (1) Information is encoded into electrical signals. • (2) Electrical signals are converted into light signals. • (3) Light travels down the fibre. • (4) A detector changes the light signals into electrical

signals. • (5) Electrical signals are decoded into information

ADVANTAGES OF FIBRE OPTICS :

• (I) Optical Fibres are non conductive (Dielectrics)• - Grounding and surge suppression not required. • - 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)• - Loss is low and same at all operating speeds within the

fibre's specified bandwidth long, unrepeated links (>70km is operation).

• (v) 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 that can be received by a nearby antenna. • - 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 • - Serve all communication needs. • - Non-obsolescence.

APPLICATION OF FIBRE OPTICS IN COMMUNICATIONS

• 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.

Principle of Operation - Theory

Total Internal Reflection - The Reflection that Occurs when a Ligh Ray Travelling in One Material Hits a Different Material and Reflects Back into the Original Material without any Loss of Light.

THEORY AND PRINCIPLE OF FIBRE OPTICS

• Light travelling 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.

Total Internal Reflection

• As the angle of incidence increases, the angle of refraction approaches 90o to the normal. The angle of incidence that yields an angle of refraction of 90o is the critical angle. If the angle of incidence increases amore than the critical angle, the light is totally reflected 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.

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

90 o • Is c = arc sin (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 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 a core refractive index of 1.47 and a cladding index of 1.46.

• Fibre manufacturers control this difference to obtain desired optical fibre characteristics.

• Light striking the interface at less than 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 becomes attenuated 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.

• Meridional rays- those that pass through the fibre axis each time, they are reflected.

• Skew rays- travel down the fibre without passing through the axis.

• The path of a skew ray is typically helical wrapping around and around the central axis.

• Fortunately skew rays are ignored in most fibre optics analysis.

• The specific characteristics of light propagation through a fibre depends on many factors, including

• The size of the fibre. • The composition of the fibre. • The light injected into the fibre.

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

FIBRE GEOMETRY

Core (m) Cladding ( m)

8 125

50 125

62.5 125

100 140

Typical Core and Cladding Diameters

125 8 125 50 125 62.5 125 100

Core Cladding

FIBRE TYPES

• Step Index • Graded Index• The step index fibre has a core with uniform

index throughout. The profile shows a sharp step at the junction of the core and cladding. In contrast, the graded index has a non-uniform core. The Index is highest at the center and gradually decreases until it matches with that of the cladding. There is no sharp break in indices between the core and the cladding.

Classification There are Three Types of Fibres

• Multimode Step Index fibre (Step Index fibre)

• Multimode graded Index fibre (Graded Index fibre)

• Single- Mode Step Index fibre (Single Mode Fibre)

STEP INDEX MULTIMODE FIBRE

• This fibre is called "Step Index" because the refractive index changes abruptly from cladding to core.

• The cladding has a refractive index somewhat lower than the refractive index of the core glass. As a result, all rays within a certain angle will be totally reflected at the core-cladding boundary.

• Rays striking the boundary at angles grater than the critical angle will be partially reflected and partially transmitted out through the boundary.

• After many such bounces the energy in these rays will be lost from the fibre.

• 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.

• The maximum number of modes (N) depends on the core diameter (d), wavelength and numerical aperture (NA)

• N= 0.5 x ( x d x N A ) 2• ( )• This types of fibre results in considerable model

dispersion, which results the fibre's band width.

GRADED INDEX MULTI-MODE 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. 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.

Single Mode Step Index

n1n2

Dispersion

Multi mode Graded Index

n1

n2

High orderMode

Dispersion RefractiveIndex Profile

Low Order ModeMulti mode Step Index

InputPulse

OutputPulse

n1

n2

SINGLE MODE FIBRE.

• The single mode fibre has an exceedingly small core diameter of only 5 to 10 m.

• Standard cladding diameter is 125 m. Since this fibre carries only one mode, model dispersion does not exists. Single mode fibres easily have a potential bandwidth of 50to 100GHz-km.

• The core diameter is so small that the splicing technique and measuring technique are more difficult.

• High sources must have very narrow spectral width and they must be very small and bright in order to permit efficient coupling into the very small core dia of these fibres.

OPTICAL FIBRE PARAMETERS

• Wavelength.

• Frequency.

• Window.

• Attenuation.

• Dispersion. • 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.

WINDOW

Window Operational Wavelength

800nm - 900nm 850nm

1250nm - 1350nm 1300nm

1500nm - 1600nm 1550nm

ATTENUATION

• Attenuation is defined as the loss of optical power over a set distance, a fibre with lower attenuation will allow more power to reach a receiver than fibre with higher attenuation.

• Attenuation may be categorized as intrinsic or extrinsic

INTRINSIC ATTENUATION

• Absorption - Natural Impurities in the glass absorb light energy.

• 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

• 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

• 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• The spreading of light pulse as it travels down the

fibre. Because 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.

• System bandwidth is measured in MHz or GHz. In general, when we say that a system has bandwidth of 20 MHz, means that 20 million pulses of light per second will travel down the fibre and each will be distinguishable by the receiver.

NUMERICAL 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. We can also define as the angles at which rays will be propagated by the fibre. These angles form a cone called the acceptance cone, which gives the maximum angle of light acceptance. The acceptance cone is related to the NA

= arc sin (NA) or • NA = sin • where is the half angle of acceptance• The NA of a fibre is important because it gives

an indication of how the fibre accepts and propagates light. A fibre with a large NA accepts light well, a fibre with a low NA requires highly directional light.

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