LASER, acronym for light amplification by stimulated emission of radiation. Lasers are devices that amplify light and produce coherent light beams, ranging from infrared to ultraviolet. A light beam is coherent when its waves, or photons, propagate in step with one another. Laser light, therefore, can be made extremely intense, highly directional, and very pure in colour (frequency). Laser devices now extend into the X-ray frequency range. Masers are similar devices for microwaves .
            Light consists of electromagnetic waves, and the colour of light is determined by its wavelength (distance from one peak of a wave to the next).  Ordinary light consists of waves of many wavelengths--and colours.  When all these waves are seen together at the same time, their colours appear white--like those from a light bulb.  But light produced by most lasers consists of waves with a very narrow range of wavelengths.  Because this range is so narrow, laser light appears to consist of a single colour.  Some lasers can produce beams with several different colours, but each colour band will be narrow.  Some lasers produce an invisible beam.  These beams consist of such forms of radiation as ultraviolet or infrared rays. 

            Laser light is highly organized, or coherent.  The waves of a laser beam move in phase--that is, all the peaks move in step with one another.  These waves travel in a narrow path and move in one direction.  Thus, coherent light is like a line of marchers in a parade moving with the same strides in the same direction.  The waves of ordinary light, on the other hand, spread rapidly and travel in different directions.  Ordinary light is known as incoherent light.  Incoherent light acts much like the way people usually travel along a street--with different strides and in many directions.  A laser beam's coherence allows it to travel long distances without losing its intensity.

Principles of Operation

            Lasers harness atoms to store and emit light in a coherent fashion. A typical laser has three main parts.  These parts are (1) an energy source, (2) a substance called an active medium, and (3) a structure enclosing the active medium known as an optical cavity.  The energy source supplies an electric current, light, or other form of energy. The electrons in the atoms of a laser medium are first pumped, or energized, to an excited state by an energy source. They are then "stimulated" by external photons to emit the stored energy in the form of photons, a process known as stimulated emission. The photons emitted have a frequency characteristic of the atoms and travel in step with the stimulating photons. These photons in turn impinge on other excited atoms to release more photons. Light amplification is achieved as the photons move back and forth between two parallel mirrors, triggering further stimulated emissions. The intense, directional, and monochromatic laser light finally leaves through one of the mirrors, which is only partially silvered.

            Stimulated emission, which is the underlying process for laser action, was first proposed by Albert Einstein in 1917. The working principles of lasers were outlined by the American physicists Arthur Schawlow and Charles Hard Townes in their 1958 patent application. The patent was granted; however, it was later challenged by the American physicist and engineer Gordon Gould. In 1960 the American physicist Theodore Maiman observed the first laser action in solid ruby. .  At first, lasers had few uses, and scientists often thought of them as "a solution looking for a problem."   A year later a helium-neon gas laser was constructed by the Iranian-born American physicist Ali Javan. Then in 1966 a liquid laser was constructed by the American physicist Peter Sorokin. The U.S. Patent Office court in 1977 affirmed one of Gould’s claims over the working principles of the laser.

Characteristics of laser light.

            Laser light differs from ordinary light in two major ways. (1) It has low divergence (spreading). (2) It is monochromatic (single-coloured).  Light with these two characteristics is known as coherent light. 

Types of Lasers

                  Based on the laser medium used, lasers are generally classified as solid state, gas, semiconductor, or liquid.

Solid-state lasers

            The most common solid laser media are rods of ruby crystals and neodymium-doped glasses and crystals. The ends of the rod are fashioned into two parallel surfaces coated with a highly reflecting nonmetallic film. Solid-state lasers offer the highest power output. They are usually operated in a pulsed manner to generate a burst of light over a short time. Bursts as short as 12 X 1015 sec have been achieved, useful in studying physical phenomena of very brief duration. Pumping is achieved with light from xenon flash tubes, arc lamps, or metal-vapor lamps. The frequency range has been expanded from infrared (IR) to ultraviolet (UV) by multiplying the original laser frequency with crystallike potassium dihydrogen phosphate, and X-ray wavelengths have been achieved by aiming laser beams at an yttrium target. .  The most common crystal laser contains a small amount of the element neodymium (chemical symbol Nd) in an yttrium aluminium garnet (YAG) crystal.  It is called an Nd:YAG laser.  In some lasers, the neodymium is dissolved in glass.  Flash lamps are generally used to pump the active media of solid-state lasers.   

Gas lasers

            The laser medium of a gas laser can be a pure gas, a mixture of gases, or even metal vapor, and is usually contained in a cylindrical glass or quartz tube. Two mirrors are located outside the ends of the tube to form the laser cavity. Gas lasers are pumped by ultraviolet light, electron beams, electric current, or chemical reactions. The helium-neon laser is known for its high frequency stability, color purity, and minimal beam spread. Carbon dioxide lasers are very efficient, and consequently they are the most powerful continuous wave (CW) lasers.