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INTRODUCTION: LASER: No other scientific discovery of the 20th century has been demonstrated with so manyexciting applications as laser acronym for (Light Amplification by Stimulated Emission of Radiation). The basic concepts of laser were first given by an American scientist, Charles Hard Townes and two Soviet scientists, Alexander Mikhailovich Prokhorov and Nikolai Gennediyevich Basov who shared the coveted Nobel Prize (1964). However, TH Maiman of the Hughes Research Laboratory, California, was the first scientist who experimentally demonstrated laser by flashing light through a ruby crystal, in 1960. Laser is a powerful source of light having extraordinary properties which are not found in the normal light sources like tungsten lamps, mercury lamps, etc. The unique property of laser is that its light waves travel very long distances with e very little divergence. In case of a conventional e source of light, the light is emitted in a jumble of e separate waves that cancel each other at random (Fig. 1.1a) and hence can travel very short distances only. An analogy can be made with a situation where a large number of pebbles are thrown It into a pool at the same time. Each pebble generates a wave of its own. Since the pebbles are thrown at random, the waves generated by all the pebbles cancel each other and as a result they travel a very short distance only. On the other hand, if the pebbles are thrown into a pool one by one at the same place and also at constant intervals of time, the waves thus generated strengthen each other and travel long distances. In this case, the
waves are said to travel coherently. In laser, the light waves are exactly in step with each other and thus have a fixed phase relationship .

It is this coherency that makes all the difference to make the laser light so narrow, so powerful and so easy to focus on a given object. The light with such qualities is not found in nature.

A high degree of directionality and monochromatic is also associated with these light beams. Therefore, in a laser beam the light waves not only are in the same phase but also have the same color (wavelength) throughout their journey. The beam of the ordinary light spreads out very quickly. On the other hand, the laser beam is highly collimated and spreads very little as it 'Fig. travels through space; even after traveling to the , surface of the moon the spread of laser light has been found to be only about 3 km across. Hypothetically, if ordinary light was able to travel to the so moon, its beam would have fanned out to such an extent leading to a diameter of the light on . the moon as much as 40, 000 km.
Another remarkable feature of laser is the concentration of its energy to extremely high intensities, the intensity remaining almost constant over long distances because of low divergence. If a laser beam with a power of a few megawatts (106 W) is focused by a lens at a spot with a diameter of 1/1000th of a centimeter, the beam intensity increases to a few hundred billion watts per sq. cm. This concentrated energy is so intense that it easily ionizes the atmospheric air to create sparks. With the beam focused from a high power laser, even the hardest material like "diamond can be melted in a fraction of a second.

These unique characteristics of laser have made it an important tool in various applications. The initial notable application of laser was made c on the lunar ranging experiment of Apollo II Mission of 1969, when an array of retro reflectors was mounted on the surface of the moon and pulses from a ruby laser were sent from the earth. The reflected beams were received by suitable detectors and by measuring the time taken by the pulses in going from the earth to the moon and back, the distance of the moon from the earth was calculated to an accuracy of 15 cm.

After the first demonstration of laser in 1960, new applications of lasers in the various field are announced almost every day. Laser finds applications In the fields of
communication, Industry, medicine, military operations, scientific research, etc. Besides, laser has already brought great benefits in surgery, photography, holography, engineering and data storage. Though it is not possible to illustrate all the laser applications reported so far in this small book, the more important ones are covered in the Chapters on Laser Applications.

The word Laser is an abbreviation for ‘Light Amplification by Stimulated Emission of Radiation’. It is one of the most important discovery of 20th century. The word Laser is also used for a device that emits a narrow intense beam of light which differs from ordinary light and has very special applications. Output of a laser can either be a continuous beam of low to medium power or pulses of intense radiation. The first laser was built in 1960 by Theodore Maiman and other scientists in California (U.S.A.), although the essential ingredients for lasers were provided by Einstein in 1917. At present various types of lasers have been developed using liquids and solids. Before describing the operation of a particular laser, it is important to discuss the following three transition phenomenon given by Einstein.