Optical fibre communication basics

OPTICAL FIBER COMMUNICATION

Semiconductor Physics

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Processes involved

Generation Modulation Transmission Amplification Reception by photo detectors


General communication System

Information Source

Transmitter

(Modulated)

Transmission

medium

Receiver(Demodulato

r)

Destination


Types of optical fibers

Step-index single mode fiber Step-index multi mode fiber Graded index multi mode fiber


Absorption & Emission of radiation Absorption Spontaneous emission Stimulated emission


Diagrammatic representation

hv= E2-E1

Absorption

E2

E1

hv= E2-E1

Spontaneous Emission

E2

E1

hv= E2-E1

Stimulated Emission

E2

E1


Population Inversion

Necessary condition for stimulated emission

Process of accumulation of e- at E2 level and vacuum states at E1 level

Required to: Enhance radioactive recombination To move fermi level into CB & VB. To cause accumulation of e- in CB & VB Responsible for laser action


Layers surrounding Fiber cables Primary coating

Gives additional mechanical strength Protects fiber from stress Protection against chemical attacks

Buffer jacket Avoids the formation of microbends Bending stress is relieved Fiber losses are reduced


Layers surrounding Fiber cables… Outer Jacket

To reduce abrasion and to provide extra protection against external mechanical strength

Filler materials To avoid moisture absorption by fibers Prevents corrosion


Layers of Fiber cables

Core 8µm

Cladding125µm

Buffer250 µm

Jacket400 µm


1. Single Fiber cable

Fiber is made from brittle glass Buffer jackets made of plastic Silicon coating between buffer jacket

and optical fiber Final cover by black polyurethane


2. Multi fiber Cable

Used in telecommunication Kevlar is used to provide tensile

strength Outer sheath for protection against

impact forces (forces generated due to brittleness)


Advantages of optical fiber as waveguides Wider band-width Low transmission loss Signal security Small size & weight Flexibility Easy maintenance Low cost


Materials used to manufacture optical fibers Glass fibers Crystalline and fluoride fibers Plastic clad glass fibers Plastic fibers


Application of optical fibers Fiber optic delay lines

By using low loss optical fibers Fiber optic sensors

By utilizing external influences, like phase sensitivity, micro bending losses and modal noise

Fiber endoscopes Using large diameter and short length

silica fibers


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Optical fibre communication basics