Basic Principles of Lasers

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LASER is a source which emits a kind of light of unrivaled purity and intensity not found in any of previously known sources of electromagnetic radiation.

The word LASER stands for light amplification by stimulated emission of radiation. In a laser, the process of stimulated emission is used for amplifying light waves. Einstein (1917) first predicted the existence of two different kinds of processes by which an atom can emit radiations these are called spontaneous and stimulated emissions. Towns and scholar (USA) in 1958 and Basov and Prokhorov (USSR) worked out the principle of laser. And finally in 1960 Maiman demonstrated the first Laser. Laser action has been obtained with atoms, ions, and molecules in solids, gaseous, liquids, glasses, plastics, and semiconductors at wavelength spanning from ultraviolet to radio frequency region with power ranging from milliwatts to megawatts. The characteristics which distinguish the lasers from other light sources are () highly intense (i) highly monochromatic (ii) high degree of coherence and (iv) unidirectional.

The three main components of any laser are the amplifying medium, the pump, and the optical resonator.

Light Amplification:- Laser action works on the light amplification Principle of the quantum theory of radiation when an electron in an orbit absorbs energy it goes to an orbit of higher energy level. When the electron jumps back to stable orbit it emits radiation in the form of electromagnetic
waves. The energy of each photon has applications The spontaneous omission gives out radiation at random moments and in random directions thus, radiation in this h emits a kind case in a random mixture of quanta having intensity, not various wavelengths. Thus the radiation own sources are incoherent and have a broad spectrum f The remarkable feature of the stimulated for light emission is that it is coherent with the emission of stimulating incident radiation. It has the process of the same frequency and phase as the incident or amplifying radiation. The amplifying medíum consists first predicted of a collection of atoms molecules or ions ent kinds of which act as an amplifier for light waves m can emit In order to have amplification, it is spontaneous necessary to have population in version Towns and (between two atomic states) in which there Basov and is a large number of atoms in the higher the principle energy state as compared to that in the 60 Maiman lower energy state, when a wave passes Laser action through a collection of atoms which are s, ions and excited and in a state of population us, liquids, inversion the wave will induce more emissions. The radiation thus induced has Ultraviolet to the same direction as that of incoming chair) inductors at were ranging radiation. The same relates to the frequency phase and polarisation of the stimulated and stimulating emission. Consequently, the stimulating Modulations are strictly coherent and the resultant beam is amplified.

Laser Pumping:- The process by which the atoms are raised from the lower level to the upper level is called pumping. The pumping is the source of energy which maintains the medium in this Principle of when a population inverted state.

Diverse techniques of excitation are used in laser devices such as energy it goes I. When the used in a laser devices such as (i) Excitation by strong source of light flash or arc lamp — optical pumping (ii) Excitation by electron impact– electrical pumping (iii) Excitation by chemical pumping, (iv) Excitation by supersonic gas expansion– gas dynamic pumping,

The optical resonator which consists of a co Pair of mirrors facing each other provides optical cr feedback to the amplifier so that it can act as a is a source of radiation.

Various Types of Lasers:- (a) Solid state lasers ruby, Nd YAG, Nd: the glass (b) Gas Lasers He-Ne, argon ion, and c Co, (c) Liquid lasers, dyes (d) Excimer lasers p (e) Semiconductor Lasers.

There are different types of laser level f system -such as two level
the laser system, three si level laser system, Four-level laser system. Now we shall discuss a three-level laser system.

The pump excites the atoms from level E1 to E3 and laser action takes place between levels E2 and E1.

We consider a three-level system consisting of energy levels E1 E2 and E3, all of which assumed to be nondegenerate let N1, N2 N3, represent the population densities of the three levels. The pump is assumed to lift atoms from level 1 to level 3 from which they decay rapidly to level2 through some nonradiative process. Thus the pump effectively transfers atoms from the ground level 1 to the exciting level 2 which is now the upper laser level, the lower laser level being the ground state 1 if the relation from level 3 to level 2 is very fast, then the atoms will relax down to level 2 rather than to level 1. Since the upper level, 3 is not a laser level, it can be a broad level so that a broadband light source may efficiently be used as a pump source.

RUBY LASER: It is a three level laser The first laser constructed in 1960 by Maiman was a ruby laser. It consists of three main parts (i)An active working material; a rod of ruby crystal l (ii) A resonant cavity (iii) Exciting system.

The working medium in such a laser is a crystal of aluminium oxide Al2O3 (corundum) a containing cr,o as an impurity introduced during l crystal growth. The red colour of the ruby erystal a is due to the crt3 ion. The cr+3 ion replaces Alt3 ion in the lattice of AlO, crystal. It is the energy levels of the chromium ions which take part in : the lasting action. Typical concentration of chromium ions are ~ .05 % by weight . The pumping of the curious is performed with the help of flash lamp (a xenon or krypton flashlamp). One end of the ruby is highly e silvered and the other is semitransparent. The w flash tube is called pump and it surrounds the ruby rod in the form of spiral.

The cr ions in the ground state absorb radiation around wavelengths of 5500 Ao and 4000 A° and are excited to the levels marked E1 and E2. The cr ions excited to these levels relax rapidly through a non radiative transition (in a time 103- 10 s) to the level marked M which is the upper laser level. The level M is a metastable level with a lifetime of-3 ms. Laser emission occurs between level M and ground state G at an output wavelength of 6943A .

The electrons are excited to higher energy levels and they are highly unstable and give up their extra energy. When these electrons in the excited state are exposed to the radiations of the same frequency which they are about to emit. The emission process is triggered instant nuously. These radiations are exactly in phase with the exposed radiations and reinforce with sone another these radiations are allowed to pass back and forth million of times in the ruby with the help of the mirror at the two ends. A chain reaction is started and powerful coherent bearm of red light is obtained. The powerful beam of red light emerges out of the semi transparent mirror at one end.

The main drawback of ruby laser is that the out put beam is not continuous though very intense. For the continuous laser beam, gas lasers are used. The light from the gas lasers as compared to that from the solid state laser is found to be more directional and much more monochromatic. This is due to the various imperfections present in the solids and also heating caused by the flash lamp. The gas lasers are also capable of supplying a continuous laser beam without a need for elaborate cooling .

Hence gas lasers have wide applications in various branches of science engineering particularly in communications.

The CO2, lasers are much more efficient as compared to other gas lasers. The lower levels of CO2 are very close to the ground level and hence the large portion of the input energy is converted into the out put laser energy resulting in very high efficiency out put Powers of several watts to several kwatts can be obtained from Co, lasers. High power Co, laser find applications in materials processing, welding, hole drilling, cutting etc. because of their very high output power. In addition the atmosphere attenuation is low at 10.6 mm which leads to some applications of Co2 lasers in open air communication.

Some important uses : The lasers have wide applications in different branches of science () Due to the narrow band width, levers are used in microwave communication. By the use of lasers, more channels of message (80 million) can be accommodated because the band width is very small.

(i) Due to narrow, angular spread, the laser beams have become a means of communication on between earth and moon or other satellites. The earth moon distance has continuous been measured with use of lasers. (ii) The light storage capacity for information in computer is greatly improved due to narrowness of band width by the use of lasers. (iv) The lasers have wide industrial applications. They can blast holes in diamonds and hard steels. It is useful in welding microscope wires and vaporizing any known material. (v) Lasers have wide medical applications. have been used in the treatment of detached retinas Microsurgery is also possible because laser beam can be focused on very small areas with out spreading. Preliminary success had also been obtained to treat the human and animal cancer. (vi) Laser can serve as awar weapon. It has been predicted that they become legendary death ray which could anhilate anything near or far (vii) Lasers can initiate or hasten certain chemical reactions which could not be possible in the absence of suitable photons. They can be used for investigating structure of molecules. The Raman spectrum can be obtained for much smaller samples and faster too.

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