The LASER (Light Amplification by Stimulated Emission of Radiation) is a triumph of modern optics. By exploiting a quantum mechanical effect called stimulated emission, lasers generate a coherent, nearly monochromatic beam of photons. Non-laser light sources typically generate incoherent, unfocused beams of light at a variety of wavelengths, prohibiting certain applications.
To create a laser, two components are necessary — a gain medium and a resonant optical cavity. For a gain medium, certain crystals, glasses, gasses, semiconductors and even dyed liquids may be used. The gain medium is stimulated by an energy pump source such as an electrical current or another laser. The medium absorbs the energy, exciting the states of the particles in the medium. After a certain threshold, called population inversion, is achieved, shining light through the medium causes more stimulated emission, or release of energy, than absorption.
A resonant optical cavity is a specially sized chamber with a mirror at one end and a semi-silvered mirror at the other. The two reflective surfaces cause light trapped inside to reflect back and forth through the gain medium, acquiring greater energy with each pass. When this effect levels off, the gain is said to be saturated and the light becomes true laser light. Different gain mediums give rise to lasers of different wavelengths.
Two varieties of laser are continuous and pulse. The continuous laser is more useful for most applications, but the energy in a pulse laser can be very large. The degree to which the beam diverges over time varies inversely with proportion to its diameter. Small beams diverge rapidly, while larger ones remain coherent.
When the laser was patented by Bell Labs in 1960, it could not immediately be given any applications, although spectrometry, interferometry, radar and nuclear fusion were discussed as potential areas of interest. Today, the laser is among the most versatile of technological wonders, with applications in data storage and retrieval, laser cutting, vision correction, surveying, measurements, holography and displays, and even nuclear fusion. Maximum achievable laser pulse intensity has increased exponentially since the mid-1980s. One day, lasers may be used to generate net energy-producing fusion reactions, providing energy for the entire human race. They also might be used to push solar sails into the depths of outer space.