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Fiber Optics

Mr. Krishna Muthyam

Asst.prof.of Physics

Griet

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�Fiber Optics

Topics :

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  • Introduction
  • Principle and Construction of an optical fiber
  • Acceptance angle and Numerical aperture
  • Types of Fibers
  • Losses associated with optical fibers
  • Basic components in optical fiber communication system
  • Application of optical fibers.

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������������������������������������� ������������Introduction��

  • Fibre optics is a branch of physics which deals with the transmission and reception of light waves using optical fibers, which acts as a guiding media.

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  • Due to atmospheric conditions like rain, fog etc. the efficient communication is disturbed. Hence to have an efficient communication system the light which carries information requires a guiding media known as optical fibre.

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  • The optical fibre is a flexible transparent fiber which works on the principle of total internal reflection(TIR) by which light signals can be transmitted from one place to another with a negligible loss of energy.

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Features of Optical Fibers :

  • It is light in weight

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  • It is smaller in size and is flexible, so that it can bend to any position

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  • It is non-conductive , non-radiative and non-inductive

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  • It has high bandwidth and low loss.

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  • There is no short circuiting as in metal wires.

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  • There is no internal noise / cross talks.

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  • It can withstand to any range of temperatures and moisture condition.

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  • There is no need to ground and hence no voltage problem occurs.

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Structure of Optical Fiber

  • It is made up of transparent dielectrics

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  • It has three coaxial regions i.e. the core, cladding and outer jacket.
  • Core: It is the innermost cylindrical region. It is also the light guiding region.
  • Cladding: The core is surrounded by the cladding.
  • Outer jacket: It is a plastic coating to protect the cladding.

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NOTE: Glass or Plastic is used as Dielectric material.

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PRINCIPLE AND PROPAGATION IN OPTICAL FIBERS :�

  • For optical fibers, the process of propagation of light (optical signal) is simple.
  • because once the light enters the fiber, the rays do not encounter any new surfaces, but repeatedly they hit the same surface.
  • The reason of confining the light beam inside the fibers is the total internal reflection (TIR).
  • Even for a bent fiber, the light guidance takes place by multiple internal reflection all over the length of the fiber as shown in fig.

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Total Internal Reflection

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Denser Medium

Rarer Medium

 

 

 

r = 90°

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Numerical Aperture: �

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Acceptance angle and Numerical Aperture

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Acceptance Angle: �

  • All right rays falling on optical fiber are not transmitted through the fiber. Only those light rays making θi>θc at the core-cladding interface are transmitted through the fiber by undergoing TIR. For which angle of incidence, the refraction angle is greater than 900 will be propagated through TIR.

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  • There by Acceptance Angle is defined as: The maximum angle of incidence to the axis of optical fiber at which the light ray may enter the fiber so that it can be propagated through TIR.

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Fiber axis

Core n1

Cladding n2

A

B

θr

θi

θr

θ

Consider the optical fiber with core refractive index n1 and cladding refractive index n2 and refractive index of air is n0. Light is incident at the air-core interface at an angle θi.

Cladding n2

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Applying Snell’ s law for Air-Core media

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This maximum angle is called the acceptance angle

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Numerical Aperture

  • Light collecting capacity of the fiber is expressed in terms of acceptance angle using the terminology ‘Numerical Aperture’

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  • Sine of the maximum acceptance angle is called is called the Numerical Aperture of the fiber.

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Types of Optical Fibers

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Optical fibers are classified into 3 major categories based on�1)Material�2)No.of modes and�3)Refractive Index Profile�

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Glass Fibers and Plastic Fibers

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Based on the materials in which the fibers arc made it is classified into two types as follows

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Glass fibers : If the fibers are made up of mixture of metal oxides and silica glasses.

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Plastic fibers : If the fibers are made up of plastics and they are called plastic optical fibers.

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On the basis of RI & MODE

Based on variation in the core refractive index (n1), optical fibers are divided in to two types

1. Step index fiber

2. Graded index fiber

Based on mode of propagation, fibers are further classified in to

1. Single mode Fiber

2. Multi mode Fiber

MODE : Mode is the one which describes the nature of propagation of electromagnetic waves in a wave guide.

All together in total three (3) types of fibers

1. Single mode step index fiber

2. Multi mode step index fiber

3. Multi mode graded index fiber

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1. Single mode step index fiber � 2. Multi mode step index fiber� 3. Multi mode graded index fiber

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Step Index Fiber : The refractive index of core material is uniform throughout and undergoes a sudden change in the form of step at the core-clad interface.

Step Index fibers are available in both Single mode and Multi modes form.

a) Single Mode Step Index Fibers:

The variation of the refractive index of a step index fiber as a function of distance can be mathematically represented as longitudinal cross section and the diameter of the core is smaller so that it can allow one mode of propagation.

Structure:

i) Core Diameter: 8 to 12 μm, usually 8.5μm

ii) Cladding Diameter: Around 125μm

iii) Sheath Diameter: 250 to 1000 μm

iv) NA: 0.08 to 0.15 usually 0.10

Performance Characteristics:

i) Band Width: Greater than 500 MHZ Km.

ii) Attenuation: 2 to 5 dB / Km.

iii) Applications: These fibers are ideally suited for high band width applications using single mode injection coherent (LASER) sources.

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b) Multi Mode Step Index Fibers:

These fibers have reasonably large core diameters and large NA to facilitate efficient transmission to incoherent or coherent light sources.

These fibers allow finite number of modes.

Structure:

i) Core Diameter: 50 to 200 μm

ii) Cladding Diameter: 125 to 400 μm

iii) Sheath Diameter: 250 to 1000 μm

iv) NA: 0.16 to 0.5

Performance Characteristics:

i) Band Width: 6 to 50 MHZ Km.

ii) Attenuation: 2.6 to 50 db/km.

iii) Applications: These fibers are ideally suited for limited band width and relatively low cost applications.

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Graded index fiber�

In graded index multimode fiber the refractive index of the core varies radially.

Refractive index increases from one end of core diameter to center and attains maximum value at the center. Again refractive index decreases as moving away from center to towards the other end of the core diameter.

Structure:

i) Core Diameter: 30 to 100 μm

ii) Cladding Diameter: 105 to 150 μm

iii) Sheath Diameter: 250 to 1000 μm

iv) NA: 0.2 to 0.3

Performance Characteristics:

i) Band Width: 300 MHZ Km to 3 GHZ Km.

ii) Attenuation: 2 to 10 dB/km.

iii) Applications: These are ideally suited for medium to high band width applications using incoherent and coherent multimode sources.

Pulse broadening is overcome in graded index fiber.

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Attenuation in Optical Fibre

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Attenuation in optical Fibre.

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Absorption

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Absorption

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Scattering

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Scattering

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Dispersion

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Bending Losses

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Optical fiber in Communication System

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  • An efficient optical fiber communication system requires high information carrying capacity such as voice signals, video signals over long distances with a minimum number of repeaters. It essentially consists of following parts.

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  • 1.Encoder 2. Transmitter 3.Wave guide

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  • 4.Receiver 5.Decoder

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  • 1.Encoder: It converts electric signal corresponding to analog information such as voice, figures, objects etc into a binary data. This binary data comes out in the form of stream of electrical pulses.

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  • 2.TRANSMITTER: It mainly consists of drive circuit and a light source. Drive circuit supplies the electric pulses to the light source from the encoder.

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  • 3. Receiver: Receiver consists of a detector followed by amplifier. This combination converts light pulses in to electrical pulses.
  • �4. Decoder: Electrical pulses containing information are fed to the electronic circuit called decoder.�Decoder converts binary data of electrical pulses in to analog information signals. �

  • NOTE: LED or diode laser is used as light source and it converts electrical signals into optical signals. These optical signals are injected into wave guide.

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Electrical

signal

ENCODER

DRIVE SOURCE

LIGHT SOURCE

TRANSMITTER

OPTICAL SIGNAL

AMPLIFIER

PHOTO DETECTOR

SIGNAL

RESTORER

DECODER

Wave guide

receiver

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Transmission of signal in fibers

  • In MMSTF ray paths are different so that they reach at different times.

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  • So that pulsed signal received at other end is broadened is called intermodal dispersion. This causes reducing the transmission rate & capacity.

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  • In MMGRF, though the rays travel longer distances they possesses relatively lesser RI & hence both the pulses reach the other end simultaneously.

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  • Thus the intermodal dispersion can be overcome by graded index fiber.

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Types of rays

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Types of rays

  • Meridional ray: The ray propagates through the core of the fiber undergoing total internal reflection.
  • Skew rays: They describe angular helical path as they propagate through the fiber.