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Content Laser technology History of Lasers The principle of laser Working of laser Amplification Population inversion Optical Resonators /laser Resonators Laser Design Laser radiations Types of Lasers and their Characteristics
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Sr.no Content Page. no. 1 Laser technology 3 2 History of Lasers 4 3 The principle of laser 5- 4 Working of laser 6- 5 Amplification 8- 6 Population inversion 9- 7 Optical Resonators /laser Resonators 11- 8 Laser Design 13- 9 Laser radiations 14- 10 Types of Lasers and their Characteristics 16-
Laser has extra ordinary properties which are not present in other sources of light and it is a powerful source of light. The most specific property of laser light is that it travels in long distances with small divergence. When the source of light is conventional the light emits when mixing of separate waves cancel each other and travel in a very short distances only.
The laser word itself is an acronym for light amplification by stimulated emission of radiation. Laser is device which amplifies or produces coherent radiation at frequencies in the range of visible or ultraviolet or infrared of electromagnetic spectrum. Laser light is different in properties from regular light. These properties are following which make laser different from other light sources. Laser is collimated mean in the form of narrow beam it can travels for long distances than distribute in many directions as regular light. Intense beams of light produced by laser devices which are monochromatic coherent and collimated. When compared with other sources of light its wavelength is extremely pure (monochromatic) mean pure in color. Laser beam has coherence with one another as compared to other light sources. Laser light has very low divergence. It can be focus on a very small point. Due to above properties lasers has large number of applications in all field of life. History of Lasers.
The principle of laser.
According to quantum theory each atom has energies in the form of certain discrete energy states. Commonly the atoms are present in or ground state E 1 when exposed to the light like a mercury arc and flash lamp. The atoms are excited from ground energy E 1 level to the higher energy E 2 levels is called absorption.
For a much short duration of time
(c)-Stimulated emission. The atomic state will affect by the interaction of a photon. The small electric dipole in the atom is produced oscillate in the presence of external field. Fig: The phenomenon of stimulated emission. Thus an electron is decay to the lower energy level due to the presence of another photon release a second photon which in the same phase as first photon. The second photon have same direction and energy and polarization as like first photon. These two photons are coherent totally. N 2 = the number of atoms in the excited state The rate of stimulated emission is given as B 21 = proportionality constant for this transition and is also called the Einstein B 21 coeffcient. = is the radiation density of photons of the frequency. Working of laser.
Fig: Principle of a laser Fig. Boltzmann statistics the population distribution shows an exponential curve in thermal equilibrium.(N 2 < N 1 ) no of higher energy excited atoms is always less then lower energy levels Population inversion is the opposite (N 2 > N 1 ). Amplification First atom interacts with incoming or stimulating photon and a coherent photon produced stimulated emission. The next two atoms interact with these two photons as a result four coherent photons produced in the lower line.
Eleven coherent photons generate at the end of process. These all have same phases and direction. A factor of eleven amplified the initial photon. Through some external energy source energy is provided the atoms for jump in the excited states. This external source is known as pump source. Boltzmann’s principle states as collection of atoms in thermal equilibrium the relative population of atoms in two energy levels given as N 2 = populations of the upper energy state N 1 = populations of lower energy states T = equilibrium temperature K= Boltzmann’s constant. The probability of finding atom to absorb a photon = The probability of finding an excited atom to emit a photon
A population inversion would take place between E 2 and E 3 at equilibrium and with pumping process. When photon is coherently amplified when it enters the population of atoms. Fig: 4-level laser pumping system Optical Resonators/laserResonators Sometimes excited atoms spontaneously emitted and do not interact to the overall output. A positive feed-back mechanism is required to return the system in laser. A positive feed-back mechanism responsible to generate the coherent output. This system is called resonator consists of mirrors which reflects off-axis or undesirable photons from the system.
This resonator reflects on-axis or desirable photons into the excited state where they amplified. A population inversion is generated through continuously lasing pump. When the excited atoms decay they spontaneously emitted photons in all direction. Photons move along the axis of lasing medium but some of the photons are move outer direction. Photons return in the lasing medium after reflection and excited the other atoms. These photons move parallel to the axis. Back and forth reflection of photons on axis interact with more and more atoms decrease the spontaneous emission and enhance the stimulated emission with axis and prodused a laser Fig: diagram of a basic laser FabryPerot Resonator
Fig: Different unstable and stable resonator confgurations. Laser Design A laser system has three components for its operation
It may be a solid crystal glass. Organic solvents, liquid dyes, semiconductors gas helium, CO 2. Pump or energy source It may be chemical reaction or high voltage discharge or flash lamp or another laser diode.
This cavity consist of active medium which acting as storage of emitted radiations and balance of radiation coherence in order to supply
feedback mechanism. This cavity has 2 parallel mirrors one is partially reflective and other is highly reflective. These are set in such a way that they leave the cavity to generate output beam is called output coupler. Fig: Designing of laser Those materials are used to produce radiation of laser. Those materials can pump for gaining population inversion. The energy levels have longer life time pile up in the excited state. The materials gives sharp lines in the spectrum. Laser Radiation Parallelism is the characteristic of laser radiation temporal coherence spatial monochromatism divergenc epolarization pulsed mode. A bell shaped Gaussian function is the intensity distribution across the Gaussian beam. Lasers in TEM00 mode has better quality. This beam has better quality and focus at a small spot. Its intensity depends on the distance z from the beam radius w intensity I(r z) given in the formula. I 0 given in the laser power P by the formula
If laser beam waist is equal to the Gaussian beam given as Fig: Linear polarized light produced by laser Brewster resonator light.
There are following types of laser Pumped Solid-State Lasers Gas Lasers Liquid Lasers X-ray Lasers Chemical Lasers Free Electron Lasers Solid-State Lasers and their types. (1)-Ruby Laser (Cr 3+ Al 2 O 3 )
It was proposed by Maiman in 1961. Its composition is consists of flash tube of ruby rod xenon. It also has cavity reflection of light to ruby rod. High voltage is supply to generate electrical energy travel to the flash tube. The diameter of ruby 1/2 cm and has 4 cm in length. The ends of rod become flat and parallelism. One end of rod is partially silvered and other end is complete silvered for making mirror. Fig: Ruby Laser ( 2)-Rare Earth Ion Lasers
It is has high energy applications produces high homogeneity and its shape and size are in flexible. Oxides non-metal oxides P 2 O 5 , Si 2 O 3 and boron oxide are compound of glass. They used for military as well as material process. Fig : (a) Nd-Glass Lasers Fig : (b) Military surplus, Nd-Glass Lasers Liquid lasers These are same to the solid state laser. Its composition is that alcohol as solvent dye arhodamines coumarins are dissolved. Liquid shows process of fluorescence in the presence of light. Its applications are spectroscopy mode locking pollution monitoring isotope separation photochemistry. Fig: Liquid laser Gas Lasers Mixture of gases are using in gas lasers and lightest amplifying substance. Examples are followings (a) Helium-neon lasers (b) Argon ion lasers (c) Carbon dioxide lasers.
(a)-Helium-Neon Laser It is glass laser type contains helium and neon mixture in the ratio 10:1. It has small capillary tube bore excites by DC discharge. Pressure of Hg in tube is 1 mm. It used in industries and scientific purpose. Fig: Helium-Neon Laser (b)-Carbon Dioxide laser. The discovery of carbon dioxide laser takes place in 1963 1964 by CKN Patel. It is used as electrical discharge has far infrared range of 10600 nm. Carbon dioxide laser uses in cutting coagulation and skin resurfacing and in laser scalpel. Fig. Co 2 laser for medical treatment (c)-Excimer Laser The pulsed gas is excimer lasers. Mixture of halogen and rare gas used as active medium. The molecules of mixture exist only in excited state. It means that
