An Introduction

The first step on the road to laser was the publication of paper by Albert Einstein in 1916 –describing how atoms could interact with light. -

1. Monochromaticity 2. Directionality/ Collimation 3. Coherence

* Monochromaticity

* Directionality Radiation comes out of the laser in a certain direction, and spreads at a defined divergence angle (  ) This angular spreading of a laser beam is very small compared to other sources of electromagnetic radiation, and described by a small divergence angle (of the order of milli-radians)

* Coherence Common light source  Light beams of random phase varying with time and position Laser source  Light of in step waves of identical frequency, phase and polarization

COMPONENTS OF LASER 1. Gain Medium (Something to amplify the light) 2. Pump source ( something to create population inversion ) 3.Resonator( something to provide suitable optical feedback)

* The gain medium uses the pump energy to amplify or increase gain of laser output. * The gain media should have the ability to absorb the pump energy and store it in the form of excited electrons and have at least one energy transition capable of producing the laser emission at the desired frequency- Major determining factor of wavelength of the operation of laser. * The gain medium can be solid, liquid, gas, semiconductor, free electrons, plasma etc. the gain medium is material with properties that allow it to amplify the light by stimulated emission.

* Pump Source  Provides energy to the laser system – a source of energy to excite electrons in the gain medium to high energy states. Examples :electrical discharges,flash lamps, arc lamps and chemical reactions. The type of pump source used depends on the gain medium. A helium –neon (He –Ne) laser uses an electrical discharge in the helium –neon gas mixture.

* If the gain of the medium is capable of storing energy in the form of excited electrons long time before decaying back to the ground state, and increase in proportion of electrons will assume a high energy state as the medium is pumped, until a population inversion occurs. means more atoms in the upper state than the ground state Stimulated emission is the essence of Laser action

A population inversion must be sustained for the laser action to occur

* The simplest is the case of a two level system. * The two level systems are impractical because they strongly absorb the emitted photons and hence it is very difficult to maintain population inversion to operate in a continuous fashion. * Most of the laser operate as three level or four level systems

* Initially excited to a short-lived high- energy state. * Then quickly decay to the intermediate metastable level. * Population inversion is created between lower ground state and a higher-energy metastable state.

 Laser transition takes place between the third and second excited states.  Rapid depopulation of the lower laser level.

* In terms of numbers, the diode laser is the most common laser today. The 2 common families of diode lasers are composed of: * Ga AIAs (Gallium/Aluminum/Arsenide) with a wavelength output in the 750 to 950 nanometers (used in CD &CD/ROM players), * InGaAsP (Indium/Gallium/Arsenide/Phosphide) with a wavelength output in the 1100 – 1650 nm range (used in optical telecommunications). * Another family of diode lasers like AlGaInP (Aluminum/Gallium/Arsenide/Phosphide) operates in the visible part of the spectrum, primarily red. Semiconductor lasers are actually solid-state lasers, too, but because semiconductor lasers have a different mode of laser operation, they have a different name.

Recording 1 Reconstruction 2 HOLOGRAPHY Two step process

Recording Process A laser beam with sufficient coherence length is split into two parts by a beam splitter. One part of the wave illuminates the object, scattered and reflected to the recording medium. The other one acting as the reference wave illuminates the light sensitive medium directly. Both waves interfere. The resulting interference pattern is recorded and chemically developed.

Reconstruction Process The developed photographic plate is illuminated by the reference wave Two images are seen,one is due to reflected light and other is due to transmitted light Image formed due to transmitted light is real and some what distorted and due to reflected light is virtual

Transmitted beam Reference beam Virtual object Real object Photographic plate

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