Laser
From Scholarpedia
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Author: Dr. Jeff Hecht, --
Author: Dr. Charles H. Townes, Astrophysics, University of California, Berkeley, CA
Dr. Jeff Hecht accepted the invitation on 13 April 2009 (self-imposed deadline: 15 September 2010).
this is an unfinished draft
The word laser is an acronym for light amplification by stimulated emission of radiation and the maser, which preceded it, is for microwave amplification by stimulated emission of radiation.
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History
The general idea for lasers and masers grew out of a combination of microwave spectroscopy and engineering, and originated independently from three sources: Weber of the University of Maryland, Basov and Prokhorov of the Russian Academy of Sciences, and Townes at Columbia University – all groups working then in microwave spectroscopy. By 1951, Townes had for several years been puzzling over how to amplify waves shorter than they could be amplified by electronic amplifiers and suddenly recognized that atoms and molecules, if they were in a non-thermodynamic state with more in upper than in lower energy levels, could provide the desired amplification and oscillation at short wavelengths.
The first such system was made to work by Gordon, Zeiger, and Townes at Columbia University in 1954 at a wavelength of about 1 centimeter, and they named it the maser. The field of masers became very popular, but almost no one thought such devices could be made to produce wavelengths as short as infrared or visible light. In 1957, Townes sat down to see how much shorter waves could be produced and suddenly realize the technique could even produce light waves. He and Arthur Schawlow wrote a paper, published in 1958, showing how this could be done, and many individuals then started working in this direction. Masers and lasers are really basically the same, but what are called masers are defined as producing wavelengths longer than one millimeter and lasers wavelengths shorter than one millimeter.
The first working laser was produced in 1960 by Theodore Maiman, at the Hughes Research Laboratories, who optically pumped a small rod of synthetic ruby, and shortly after that many others were made to work. Javan, Bennett, and Herriott at Bell Labs made the first gas discharge laser in December 1960, and in 1962 Hall at General Electric the first semiconductor diode laser. Since then many contributors have developed many other lasers, most of which fall into the three broad categories of optically pumped solids, gases usually excited by an electric discharge, and semiconductor diodes. These developments have had broad and important impacts on both science and technology.
Properties
Lasers provide radiation of very pure frequencies and with simple coherent wave-fronts so that light can be focused to a small point, limited only by diffraction to a size of about one wavelength. They provide ideal amplification, limited only by quantum phenomena. And the power of lasers, especially the spatial concentration of power, can be enormous. The smallest, single-atom lasers emit only about 10-16 watts. But the most powerful pulsed lasers emit 1016 watts (ten million billion), more than any other source. Such power lasts only a short time, but its intensity allows us to study and understand new states of matter. When focused, this can become 1023 watts per square centimeter.
In addition to high power intensity and temperatures, lasers have also produced the coldest things we have every known – below one millionth of a degree absolute temperature.
Masers and lasers measure time very precisely, to a precision of about 10-15, and very short times – as short as 10-15 seconds. Lasers measure length very accurately and conveniently. We can even send a laser beam to the moon and measure its distance to one or two centimeters. Lasers and masers are great scientific tools, and have been used in research which has already produced a dozen Nobel Prizes.
Applications
Lasers have an enormous variety of technical and commercial applications, and now have an important commercial impact. Applications include communications with speeds up to a million billion bits per second, recording and reading, medical work, cutting and welding, as well as much measuring and pointing. Some lasers are small and inexpensive, a few large enough to require a building to contain them.
Lasers are still young and probably still have a long way to go. X-ray lasers will probably have a big impact in the future, as well as many types and applications not presently envisioned.
