Optical Fiber

INTRODUCTION TO OPTICAL FIBER:

Optical Fiber is a branch of physics which deals with the transmission & reception of light waves using optical fibers which acts as a guiding media. The transmission of light waves by fiber optics was first demonstrated by John Tyndall in 1870.

Optical Fiber:

Optical fiber is a thin & transparent guiding medium or material which guides the information carrying light waves. It is a cylindrical wave-guide system which propagates the data & speech signals in the optical frequency range.

A light beam acting as a carrier wave is capable of carrying more information than radio

waves & microwaves because of its high frequency as shown below.

Radio waves - 104 Hz , Micro waves - 1010 Hz, Light waves - 1015 Hz

Construction:

An optical fiber is a very thin, flexible transparent made with plastic or glass.

It has cylindrical shape consisting of three layers or sections.

(1) The Core

(2) The Cladding

(3) The Outer jacket or Buffer jacket

(1)The Core: It is the central layer surrounded by another layer called cladding. Light is  transmitted     within the core which has refractive index (n1). It is a denser medium. Core is made of silica (SiO2).

(2)The Cladding: It is the second layer, surrounded by a third layer called the outer jacket. It has   refractive index n2 which is less than the refractive index of core i.e (n1 >n2 ). It acts as a rarer   medium. It keeps the light within the core because n1>n2. To lower the refractive index of cladding   the silica is doped with phosphorous or bismuth material.

(3)The Outer or Buffer Jacket: It is the third layer it protects the fiber from moisture & abrasion. To      provide necessary toughness & tensile strength, a layer of strength member isarranged surrounding        buffer jacket. It is made of polyurethane material.

Working Principle of Optical Fiber:

Total Internal Reflection: The principle of optical fiber is total internal reflection.

Condition for Total Internal Reflection:

 (1) The light ray should move from denser to rarer medium.

 (2) The refractive index of core must be greater than cladding i.e. n1>n2

 (3) The angle of incidence (i) must be greater than the critical angle(θc) i.e. i>θc.

 (4) The critical angle𝜃𝑐 = 𝑠𝑖𝑛−1* 𝑛2/𝑛1.

TYPES OF OPTICAL FIBERS:

Based on Mode:

The rays travelling in the fiber by total internal reflection are called modes.

(1) Single mode fibers:

 If the thickness of the fiber is so small that it supports only one mode then the fiber is called single modefiberor mono mode fiber.

 The core diameter of this fiber is about 8 to 10μm and the outer diameter of cladding is 60 to 70μm.

(2)Multi mode fibers:

 If the thickness of the fiber is very large that it supports more than one mode then the fiber is called multi mode fiber.

 The core diameter of this fiber is about 50 to 200μm and the outer diameter of cladding is 100 to 250μm.

Based on refractive index profile:

(1) Step-Index Optical fiber:

 In a step-index optical fiber, the entire core has uniform refractive index n1 slightly greater than the refractive index of the cladding n2.

 Since the index profile is in the form of a step, these fibers are called step-index fibers.

 The transmission of information will be in the form of signals or pulses.

 These are extensively used because distortion and transmission losses are very less.

 Step-index optical fibers are of two types. They are

    (i) Single mode step-index fiber

    (ii) Multi-mode step-index fiber

(2) Graded index optical fiber:

 In this fiber, the refractive index of the core varies radially.

 It has maximum refractive index at its centre, which gradually falls with increase of radius and at the core-cladding interface matches with refractive index of cladding.

 Variation of refractive index of the core with radius is given by n(x) = 𝑛1[1 − 2Δ (x/a)p]2

   Where 𝑛1 -> refractive index at the centre of the core

    a-> radius of the core

    Δ-> Fractional index change,Δ = 𝑛1−𝑛2/𝑛1

    p-> grating profile index number

 This fiber divided into two types.

( i ) Single-mode graded index fiber

( ii ) Multi-mode graded index fiber

Based on types of materials:

(1) glass-glass optical fiber

(2) glass-plastic optical fiber

(3) plastic-plastic optical fiber

This information is for knowledge only, not to hurt anyone's feelings.