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An Introduction LASERS AND HOLOGRAPHY

LASER the light fantastic 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. -

What makes Laser light different

MonochromaticityDirectionality/ CollimationCoherence

The unique properties of Laser lightMonochromaticity

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)

CoherenceCommon light sourceLight beams of random phase varying with time and positionLaser sourceLight of in step waves of identical frequency, phase and polarization

Absorption Spontaneous Emission

Stimulated Emission

Avalanche Multiplication of photons

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

Gain MediumThe 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.Optical ResonatorProvides an optical path which allows emitted photons to oscillate and interfere constructively as energy is added or pumped into the system .Two parallel mirrors placed around the gain medium.Light is reflected by the mirrors back into the medium and is amplified .One end of the resonator is highly reflecting mirror or other reflecting devices to feed photons back into the gain medium and the other end has a partially reflecting mirror (i.e. output coupler ) to allow laser radiation to come out.

Laser Action 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 occurThe most common methods of pumping commonly employed in laser systems are

a). optical pumping b). Electrical pumping (Direct electron excitation)c). Collisional pumping (In elastic atom- atom collisions)d). Direct conversion (electrical pumping)e). Chemical pumping ( chemical reactions)Energy Transitions leading to Laser actionThe 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

Three-level Laser System

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.

Four Level Laser Systems Working of Ruby Laser

HeNeon Laser

Energy level Diagram

DIRECT BAND GAP SC & INDIRECT BAND GAP SC

Semiconductor Diode LaserIn 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.

Heavily Doped PN Junction under No BiasHeavily Doped PN Junction Under Forward Bias

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