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The word ‘laser’ is an acronym derived from ‘light amplification by the stimulated emission of radiation’. In the 1950's, Dr. Prokhorov and Dr. Nikolai G. Basov, physicists at the P. N. Lebedev Physical Institute in Moscow, discovered ways to nudge electrons around atoms into higher-energy states. Their findings led to the development of the maser, a device that emits microwave radiation of a single wavelength. (Maser is an acronym for microwave amplification by stimulated emission of radiation.)

When the maser techniques were adapted to the shorter wavelengths of light, the devices became known as lasers.

Dr. Prokhorov and Dr. Basov, along with Dr. Charles H. Townes, who independently developed a maser about the same time, received the Nobel Prize in Physics in 1964 for their research.

Although the first such device to be constructed was the ammonia maser in 1954 it is the lasers made subsequently which operate in the infrared, visible or ultraviolet regions of the spectrum which have made a greater impact.

Laser radiation is emittedentirely by the process of stimulated emission, unlike the more conventional sources of radiation, which emit through a spontaneous process. For induced emission from the upper energy level n of the two-level system there must be a population inversion between the two levels. The process by which such a population inversion is brought about is known as pumping. A system, which may be gaseous, solid or liquid and in which a population inversion has been created is referred to as an active medium. The active medium is capable of acting as an amplifier of radiation falling on it. For every photon entering the active medium, two photons are emitted from it.

To make an oscillator from an amplifier requires, in the language of electronics, positive feedback. In lasers this is provided by the active medium being between two mirrors, both of them highly reflecting but one rather less so in order to allow some of the stimulated radiation to leak out and form the laser beam. The region bounded by the mirrors is called the laser cavity. Various mirror systems are used but, consisting of two plane mirrors a distance d apart, is one of the simplest. The reflecting surfaces of the mirrors are specially coated, with alternate layers of high and low dielectric materials such as TiO2 and SiO, to give almost total reflection at the specific laser wavelength. The usual aluminium, silver or gold coatings are not sufficiently highly reflecting. One of the mirrors is coated so as to allow 1 to 10 per cent of the radiation to emerge as the laser beam.

Photons are generated initially in the cavity through spontaneous emission. Those that strike the cavity mirrors at 90 degrees are retained within the cavity causingthe photon flux to reach a level which is sufficiently high to cause stimulated emission to occur, and the active medium is said to lase.

Laser radiation has four very remarkable properties:

1. Directionality. The laser beam emerging from the output mirror of the cavity is highly parallel, which is a consequence of the strict requirements for the alignment of the cavity mirrors. Divergence of the beam is typically a few milliradians.