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| Applications of Lasers |
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| The development of the laser has opened many avenues of investigation and improved many others. The important characteristics that have contributed to its usefulness in these many areas are tile coherence, monochromaticity and great intensity of its radiation. |
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| (i) The smallest lasers used for telephone communication over optical fibres have as their active medium a semiconducting gallium arsenide crystal about the size of a pin-head. |
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| (ii) The lasers are used for laser fusion research. These lasers are tile largest. They can fill a large building. They can generate pulses of laser light of 10-10s duration, which have a power level of 1014W during tile pulse. This is about 100 times the total power-generating capacity of all the electric power stations on Earth. |
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| To obtain energy from a controlled fusion process, a temperature of the order of 107 K is needed. Because of the ability to obtain extremely high energy densities with lasers they are being used in the attempt to obtain these very high temperatures. |
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| (iii) The lasers are used for drilling tiny holes in diamonds for drawing fine wires. |
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| (iv) They are used in precision surveying. |
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| Because of its high intensity and exceptional directionality, the laser is an excellent tool for surveying. In order to be certain that a bridge or tunnel is being constructed along the proper line, it is only necessary to set up a laser and have it pointed in the proper direction. Then, at each point along the bridge or tunnel, the engineers can measure very accurately using the laser beam as a reference. |
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| (v) They are used for cutting cloth (50 layers at a time, with no frayed edges) in the garment industry. |
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| (vi) They are used in precise fluid-flow velocity measurements using the Doppler effect. |
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| (vii) They are used in precise length measurements by interferometery. |
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| (viii) They are used in the generation of holograms. |
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(ix) They are used to measure the x, y and z co-ordinates of a point by laser interference techniques with a precision  |
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| It is used for measuring the dimensions of special three-dimensional gauges, which, in turn are used in industry to check the dimensional accuracy of complicated machined parts. |
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| (x) Medical applications. Lasers have been used successfully in the treatment of detached retinas and cancer. The laser beam, which lasts a thousandth of a second, can be precisely aimed in the right location and hardly more than a single pulse is needed for welding the retina. |
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| The pump |
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| It is an external source, which supplies energy to obtain population inversion. The pump can be optical, electrical or thermal. In the ruby laser, we use optical pumping. In the He-Ne laser, we use electric discharge pumping. |
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| The energy supplied by the pump excites the atoms to higher energy levels and through spontaneous emission or through non-radioactive processes the population inversion occurs. |
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| The laser medium |
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| It is material in which the laser action is made to take place. It may be solid, liquid or gas. Many lasers are named after the material used e.g. ruby laser, He-Ne laser, CO2 laser, Nd: YAG laser etc. |
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| The most important characteristic requirement for the laser medium is that we should be able to obtain the population inversion in it. According to the Boltzmann condition if N1 and N2 be the number of atoms in the energy states E1 and E2, then |
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| where hn = E2 - E1 Therefore, N2 is in general less than N1. Because of this reason, rigorous pumping may be required for sustaining the population inversion and so only certain pairs of energy levels with appropriate life times can be inverted. |
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| The resonator |
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| It consists of a pair of plane or spherical mirrors having common principal axis. The reflection coefficient of one of the mirrors is very near to 1 and that of the other is kept somewhat less than 1. It enables a part of the internally reflected beam to escape out as a laser beam. The resonator is basically a feedback device that directs the photons back and forth through the laser medium and in the process, the number of photons is multiplied due to stimulated emission. |
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