Lasers
Authors: Sliney David H. 
Published: August 2012

AbstractLasers are sources of light, a form of electromagnetic radiation that propagates at a velocity of 3 × 1010 cm/s and is characterized by an oscillating electric field. For visible light, the frequency of oscillation, denoted ν, is on the order of 1015 Hz. The distance between peak values of the electric field is the wavelength. The wavelike nature of light is shown clearly by phenomena such as optical interference and diffraction. However, in some experiments, such as photoelectric emission, light has particlelike characteristics, behaving as if it consisted of discrete bundles of energy, called photons. The energy of a photon,Ep, is equal tohν, wherehis Planck's constant, numerically equal to 6.63 × 10−34 J s. Because light sometimes exhibits wavelike and sometimes particlelike properties, it is said to have a dual nature, referred to as the wave–particle duality of light.The many types of lasers produce light at different wavelengths in the visible, infrared, and ultraviolet regions of the spectrum. Light from lasers has many properties different from those of light from conventional light sources. Laser light can be highly monochromatic, with very high radiance (brightness), highly collimated, coherent, and in some cases can have extremely high power. These unusual properties lead to a wide variety of applications for lasers in science, engineering, and industry.There are many types of lasers, having a wide variety of methods of construction and based on many different classes of materials. The properties of some commercially available lasers are summarized along with typically available characteristics in Table 1. In addition to the lasers listed, there are also chemical and free‐electron lasers that are only found in a few specialized laboratories.The three requirements for a laser are a material that possesses an appropriate set of energy levels (the active medium), some means for excitation or pumping the atoms or molecules to excited upper energy levels while at the same time leaving lower lying energy levels, and some means of resonant feedback to allow the light to pass back and forth through the active medium. During these passes, the light is amplified by the stimulated emission process and increases in intensity.The widespread interest in lasers is based on practical applications. Lasers are used to perform many useful functions in science, engineering, industry, and education.Most applications for which lasers are used were originally demonstrated using conventional light sources. In many cases, the application was only marginally successful using conventional sources and required the development of laser light sources to be practical.Laser radiation can pose significant risks to vision when the eye is located within the beam (intrabeam viewing), and occupational exposure limits have been developed for these conditions. Lasers pose a more significant optical radiation hazard than the conventional light sources because lasers have a far greater brightness (radiance) and because a typical laser has a collimated beam, which can present a hazard to the eye at a significant distance.The adverse effects associated with viewing lasers or other bright light sources such as the sun, arc lamps, and welding arcs have been studied for decades. Injury thresholds for acute injury in experimental animals for corneal, lenticular, and retinal effects have been corroborated for the human eye from accident data.<Abstract>