By

Ms. Monika SrivastavaDoctoral Scholar, AMR Group of Dr. Anurag Srivastava

ABV-IIITM, Gwalior

Unit 2

• Laser acronym

• Laser Vs ordinary light

• Characteristics of lasers

• Different processes involved in lasers

• Einstein’s coefficients

• Relation between Einstein’s coefficients

• Condition for lasing

• Basic working of laser

• Components of laser

• Types of laser

• Laser system

• Applications

The acronym LASER stands

Light Amplification by Stimulated Emission of

Radiation

Characteristics of LASERs

•Monochromatic

•Coherent

•Directional

•Highly intense beam

E1

E2

hu

(a) Absorption

hu

(b) Spontaneous emission

hu

(c) Stimulated emission

Inhu

Out

hu

E2 E2

E1 E1

Absorption, spontaneous (random photon) emission and stimulatedemission.

E2- Higher energy state

E1 - Lower energy state

Einstein’s coefficient

A21 = "coefficient of spontaneous emission"

B12 = "coefficient of absorption"

B21 = "coefficient of induced emissions"

Spontaneous emission = n2A21

Stimulated emission = n2B21 Jν0

Adsorption = n1B12 Jν0

B12

A21

B21

Relation between Einstein’s coefficient

𝐽𝑣0 =

2ħ𝑣03

𝑐3

𝑒(−ℎ𝑣0𝑘𝑇 ) − 1

By comparing equations

Population Inversion

When no. of atoms per unit volume in higher energy state becomes greaterthan that of no. of atoms per unit volume in lower energy state, this condition iscalled “Population inversion”. This sets stimulated emission and emits identicalphotons as of incident photon.

“Threshold Condition For Lasing”

Basic working of LASERs

Components of LASER

Types of LASER

• On the basis of medium there are different types of Lasers:

Solid Lasers

Liquid Lasers

Gases Lasers

Semiconductor Lasers.

Beam Generation Unit

Beam delivering unit

• Mirrors

• Beam splitters

Focusing lens

Fiber optic coupling

Applications:

Ruby Laser

Nd:YAG Laser

He-Ne Laser

Semiconductor laser-Construction

Optical Resonator:By cleaving two ends (110) optical resonator is obtained

Due to refractive indices difference between material and air ideally R=33%.

It is increased by coating both end with suitable materials such as Al2O3, TiO2 etc.

Active Medium: Heavily doped Semiconductor material such as GaAs, InP, etc.

Excitation Source : Current is pumping source.

Working

• When p-n junction is forward bias, depletion region is reduced and

hence causing diffusion of electrons and holes further and

recombination of these majority carrier takes place which leads to

emission of light.

p+

Eg

V

n+

EF C

eV

EF V

E cE c

Inversion region

Optical fiber

What are Fiber Optics?

• Optical fibers are long, thin strands of pure glass or plastic about thediameter of a human hair.

• They are designed to transmit light signals over long distances withless signal loss.

• Based on total internal reflection.

Structure of Optical fiber

• CoreQuartzRegion through which light passes.

• CladdingCoaxial middle region surrounding

the core.Provide confinement of light within

the core.ncladding < ncore

• JacketOuter most region made with

polymer.Protects core and cladding from

abrasions , contamination, moisture. Provide mechanical strength.

7.51

Types of Optical Fibre

Losses

• Losses due to material defects

•Surface scattering

•Bending loss

•Splice

Merits of Fiber Optics

Demerits/limitations of Fiber Optics

• Costly maintenance and installation.

• Splicing and connecting is more delicate:• Optical fiber can not be join together as easily as copper cable. It requires

training and expensive splicing and measurement equipment.

Fiber optic splicing and connectors

• A splice is a permanent connection

• A connector is removable

Optical Couplers and Switches

Power splitters - categorized :

As star couples with multiple inputs and outputs As tees, which have one input and two outputs

Optical fiber sensors

• A fiber optic sensor is a sensing device that uses fiber optictechnology for measuring physical quantities such astemperature, pressure, strain, voltages and acceleration.

• In particular, it uses an optical fiber as the sensing element,called an intrinsic sensor, or uses it to transport signals from theremote sensor to the signal processing module (extrinsicsensor).

• It can do sensing by measuring :• Wavelength shift

• Time delay

Applications of optical fibers

• Communication - Telephone transmission method uses fibre-optic cables.Optical fibres transmit energy in the form of light pulses.

• Medical uses - Optical fibres are well suited for medical use. They can bemade in extremely thin, flexible strands for insertion into the bloodvessels, lungs, and other hollow parts of the body. Optical fibres are usedin a number of instruments that enable doctors to view internal bodyparts without having to perform surgery.

• Simple uses - The simplest application of optical fibres is the transmissionof light to locations otherwise hard to reach.

• Military – Weight of aircraft are reduced.

In Industry

• Drilling Holes

• Welding

• Cutting

(i)

(ii) (iii)

In Every Day Life

• Bar Code Reader

• Hologram

In medicine

• Dental Treatment

• Treatment Of Kidney Stones

• Treatment Of Cancers etc