Technical : Online Tutorial on Lasers



Laser is one of the most important invention in the field of science. It changed the traditional thinking about light. Lasers are useful in medical and engineering fields - it is useful to people having dental problems , cancer , thyroid problems or any other disease. Lasers are widely used in engineering field . We discuss this topic under following headings 1. Introduction to laser 2. Quantum transitions 3. Population inversion 4. Pumping 5. Ruby lasers 6. He-ne lasers 7. Semiconductor lasers 8. Properties of lasers 9. Engg. application of lasers 1. Introduction To Laser Lasers are the conventional source which have very high monochromaticity, intensity and directionality. It fulfils the requirements of modern science , technology about that source of light which is highly monochromatic and have very high intensity. First successful laser was built by T. H. Maiman in 1960 . 2. Quantum Transitions Transition of an atom or molecule from one energy level to another is called the quantum transitions. Usually three types of quantum transition. (a) Absorption :- In this type of quantum transition atom in lower energy state after absorption of energy get excited to higher energy state. If atom in E1 energy state after absorption of energy hv get excited to energy state E2 then, E2 - E1 = hv (v is frequency nu) * The number transitions at any instant depend on number of atoms in state E1. (b) Spontaneous emission :- In this type atom in excited state drops down to lower energy state with out using any external agency(force,energy).During this there is emission of energy equal to nhv (E2 - E1).This is a random and uncontrollable process.The no. of emissions depend on the no. of atoms present in the energy level E2 (i.e.N2). This transition occur only when N2>N1. (c) Stimulated emission :- In this there is drop down of atom from higher energy to lower energy state with the help energy supplied In this no. transitions depend on no. of atoms in higher energy state. This process can be controlled and it is directional process. In this transition no. of photons get doubled. This occurs only when N2>N1. This type of transition is used in lasers. 3. Population Inversion From the above processes, stimulated emission and spontaneous emission occur only when N2>N1 . But in ordinary condition (normal temperature & pressure) N2 cannot greater than N1.(it is possible only when temperature is negative and negative temperature cannot be obtained). * When population of higher energy state is higher than the lower egergy state then that situation is called population inversion. Population inversion can also be called as negative temperature state. It can be achieved by pumping. 4. Pumping The process of supplying energy to the medium in order to transfer it into the state of population inversion is called pumping. There are various methods of pumping, following are some of those - (a) Optical pumping:- Higher population of upper energy level is achieved by supplying light energy.This pumping can be used to any laser medium(solid,liquid,gaseous). (b) Electrical pumping :- In electrical pumping population inversion is achieved by means of sufficiently intense discharge in the medium. Electrical pumping can only be used with lasor meterial that can conduct electricity without destroying laser action. (c) Chemical pumping :- When the population inversion is achieved by suitable exothermic chimical reaction in the active material is known as chemical pumping. (d) Heat pumping :- Population inversion is achived by means of increaing the temperature. On the basis of no. of levels pumping is again classified, 1).Three level pumping, 2).Four level pumping. The life-time of excited atom is less then 10 nano sec.However there exists some states in which the life time is greater than 10 nano sec.These states are known as metastable state. 5. Ruby Lasers In 1960 Miaman built the first working laser.That was ruby laser. It is solid state laser. Ruby is an aluminium oxide crystal, containing about 0.05%(by weight)of chromium oxide.The chromium ions constitute the active centers. Chromium ions give pink colour to ruby rod.The ruby rod is cut and polished such that the end faces are optically flat and exactly parallel to each other and are strictly perpendicular to the axis of the rod. The faces are coated with a highly reflecting material.Ruby rod is surrounded by a spiral flash tube.Chromium ions are excited by absorption of photons from flash tube.The resulting emission occurs in a releative broad spectral range centered about 696.3 nano meter.The beam is of red colour having wavelength 696.3 nano meter and a line width of about 0.0001 nano meter. It lastes for about 7 meters. 6. He-Ne Lasers In 1960 Ali Javan and his co-workers at Bell Telephone laboratories fabricated first helium-neon laser.In He-ne lasor the mixture of helium neon is in the ratio 10:1 placed in long narrow discharged tube. The pressure is kept at about 1mm of Hg. Pumping is accompanied by electric discharge.Free electrons and ions are accelarated by the applied electric field and excite the gaseous medium through collisions.The dominant laser transitions correspond to 1152.3 nano meter and 3391.2 nano meter in infrared region. 7. Semiconductor Lasers In semiconductor laser a very highly doped pn junction in forword bais is used to obtain laser action. When recombination of electrons & holes takes place near junction energy is released in the form of heat in case of Ge & Si semiconductor, but in case of Gallium arsenide(GaAs) this energy is released in the form of light. This property is used in semiconductor laser. 8. Properties Of Lasers Laser is differ from light in many properties, following are some of those properties (a) Monochromaticity :- Laser light is much more monochromatic than any other light sources (it is highly monochromatic) (b) Lasers are characterised by very high degree of coherence. (c) Laser light is emitted only in one direction not in all directions like other light sources. (d) Lasers have very high intensity & are brighter than any other sources of light. 9. Engineering Application Of Lasers Lasers have many applications in the fields of medical & engineering; Following are some of engineering applications, (a) Laser can be used for drilling very soft as well as hard material. (b) Laser can be ued for the cutting, it can act as knife. (c) Laser can be used for welding work.