The devices known as masers and lasers serve as amplifiers and generators of radiation. Their common characteristic is that they make use of the conversion of atomic or molecular energy to electromagnetic radiation by means of the process known as stimulated emission of radiation. When the wavelength of the emitted radiation is in the vicinity of 1 cm we speak of microwave amplifiers or masers. Instruments which generate or amplify visible or nearly visible radiation are called optical masers or lasers.
Albert Einstein recognized the existence of stimulated emission in 1917, but it was not until the 1950s when the first device was demonstrated.
The maser period begins with the publication of an article by the Russian scientists Basov and Prokhorov and the construction of the first operating maser by Townes, Gordon and Zeiger (from the USA). Basov and Prokhorov gave a detailed theoretical exploration of the use of molecular beams in microwave spectroscopy. The article of Basov and Prokhorov contained detailed calculations pertaining to the role of the relevant physical parameters, the effects of line-width, cavity dimensions, and the like. Thus the quantitative conditions for the operation of a microwave amplifier and generator were found.
In 1954 at Columbia University Charles Townes and two of his students announced the construction and operation of a device that may be used as a high-resolution microwave spectrometer, a microwave amplification, or a very stable oscillator. They named the device a “maser” – an acronym for microwave application by stimulated emission of radiation.
From 1958 on, many masers were constructed for applications in radio astronomy and as components of radar receivers. These masers were mostly of the ruby type. Their design became a part of the engineering art and research interest turned toward the extension of stimulated emission techniques in the visible and infrared regions.
Arthur Schawlow of Bell Laboratories and Charles Townes proposed extending the maser concept to the optical frequency range in 1958.
The maser period extends from 1954 to 1960.
The laser period opens with the achievement of the ruby laser. The acronym l.a.s.e.r. stands for light amplification by stimulated emission of radiation.
Physicist Theodore Harold Maiman invented the first operable laser in 1960. He developed, demonstrated, and patented a laser using a pink ruby medium, for which he gained worldwide recognition. In 1962 Maiman founded his own company, Korad Corporation, devoted to the research, development, and manufacture of lasers.
Early in 1961 the first continuously operating laser was announced by Ali Javan and his coworkers at Bell Laboratories. This laser was the first to use a gas, a mixture of helium and neon, for the light emitting material. At the same years scientists from American Optical Company made the first neodymium-doped glass laser. In 1962 scientists at General Electric and International Business Machines (IBM) almost simultaneously demonstrated the first semiconductor junction laser.
In 1962 Basov and Oraevskii proposed that rapid cooling could produce population inversions in molecular systems. And in 1966, the first gas-dynamic laser was successfully operated at the Avco Everett Research Laboratory.
The 1970s years became the time of discovery of free electron laser.
The 1964 Nobel Prize in physics was awarded to Charles Townes and to the Russian scientists Nikolai Basov and Alexander Prokhorov “for fundamental work in the field of quantum electronics, which has led to the construction of oscillators and amplifiers based in the maser-laser principle”.
Laser applications have also increased in variety. Experiments requiring really high intensities in narrow spectral regions can only be done with lasers. Outside the field of scientific experimentation many applications were found in medicine, communications, geophysical and space exploration, military and metals technology. The potential importance of these applications continues to stimulate new developments in the laser field.
Now lasers are everywhere. In your computer CD-ROM, your CD рlaуег, at supermarket and in laser light shows. Asfar as technologies go, they have been one of the inventions most quickly absorbed into society.
The future of lasers is a promising one. Judging from the quick development of lasers in the past and continuing laser research, there does not appear to be a slowing of laser research in the near future. As time progresses, there will doubtless be new scientists with new ideas and new inventions.
Types of Lasers
According to the laser medium used, lasers are generally classified as solid state, gas, semiconductor, free-electron, liquid, chemical lasers and others.
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. Certain bursts have been achieved, which are useful in studying physical phenomena of very brief duration. Pumping is achieved with light from xenon flash tubes, arc lamps or metal-vapour lamps. The frequency range has been expanded from infrared (IR) to ultraviolet (UV).
The laser medium of a gas laser can be a pure gas, a mixture of gases, or even metal vapour 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.
The most compact of lasers, the semiconductor laser usually consists of a junction between layers of semiconductors with different electrical conducting properties. The laser cavity is confined to the junction region by means of two reflective boundaries. Gallium arsenide is the semiconductor most commonly used. Semiconductor lasers are pumped by the direct application of electrical current across the junction, and they can be operated in the CW mode with better than 50 per cent efficiency. A method that permits even more efficient use of energy has been devised. It involves mounting tiny lasers vertically in such circuits, to a density of more than a million per square centimetre. Common uses for semiconductor lasers include CD players and laser printers.
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