The laser is a unique source of optical energy. This uniqueness lies in the nature of laser light, which has fundamental differences from light produced by the sun or by a light bulb. The light from the laser is said to be coherent and exhibits a very ordered nature, where all the light waves produced have the same frequency and phase. This means that laser light is essentially monochromatic and extremely directional. The coherent properties of the laser beam are particularly important for material processing applications as these provide a capability to focus all the laser's output energy into a very small spot. Thus power densities which are unattainable with any other source of light can be obtained.
The elements of all lasers include: a lasing medium, which can be of either solid, liquid or gaseous form; a pumping process to initiate the release of light and a feedback system (or resonator) to regenerate and maintain the light output.
Energy, when applied to some particular materials has the effect of exciting atoms to randomly produce light waves. The source of energy (the pump) might be an electric discharge, a very bright flash-lamp or another laser, and the material might be a mixture of gases, or a crystal (the lasing medium). These light waves travel in all directions with respect to the axis of the laser and some of them collide with the excited atoms causing further release of more light waves. In this case the incident and released light waves are exactly of the same wavelength and are exactly in phase with each other. When one of these light waves collides with another excited atom, this atom also releases a light wave identical to the others. As there are billions of atoms available, this process quickly builds up a stream of light waves with identical properties of increasing density. Light waves which do not travel parallel to the axis of the laser are lost from the system. Those light waves which do travel parallel to the laser have their path length (and thereby their ability to produce more identical light waves) increased, by fitting mirrors (the feedback) at each end of the laser, which continuously reflect the light waves back into the array of excited atoms. One of these mirrors is only partially reflecting and a proportion of the light is transmitted through it to provide the highly collimated coherent light beam that makes the laser so useful.