Laser (Edited from Wikipedia)

SUMMARY

A laser is a machine that makes an amplified, single-color source of light. The beam of light from the laser does not get wider or weaker as most sources of light do. It uses special gases or crystals to make the light with only a single color. Then mirrors are used to amplify (make stronger) that color of light and to make all the light travel in one direction, so it stays as a narrow beam, sometimes called a collimated beam.

When pointed at something, this narrow beam makes a single point of light. All of the energy of the light stays in that one narrow beam instead of spreading out like a flashlight (electric torch). The word "laser" is an acronym for "light amplification by stimulated emission of radiation".

A laser creates light by special actions involving a material called an "optical gain medium". Energy is put into this material using an 'energy pump'. This can be electricity, another light source, or some other source of energy. The energy makes the material go into what is called an excited state. This means the electrons in the material have extra energy, and after a bit of time they will lose that energy. When they lose the energy they will release a photon (a particle of light). The type of optical gain medium used (such as ruby) will change what color (wavelength) will be produced. Releasing photons is the "stimulated emission of radiation" part of laser.

Many things can radiate light, like a light bulb, but the light will not be organized in one direction and phase. By using an electric field to control how the light is created, this light will now be one kind, going in one direction. This is called "coherent radiation".

At this point, the light is still weak. The mirrors on either side bounce the light back and forth, and this hits other parts of the optical gain medium, causing those parts to also release photons, generating more light ("light amplification"). When all of the optical gain medium is producing light, this is called saturation and creates a very strong beam of light at a very narrow wavelength, which we would call a laser beam.

The light moves through the medium between the two mirrors that reflect the light back and forth between them. One of the mirrors, however, only partially reflects the light, allowing some to escape. The escaping light makes up the laser beam.

1 This is a simple design; the type of optical gain medium used usually defines the type of laser. It can be a stone like ruby or garnet. Gases can also be used for lasers by using helium, nitrogen, carbon dioxide, neon or others. Finally, the smallest lasers use semiconductor diodes to produce the light. These are used in electronics.

HISTORY

Albert Einstein was the first to have the idea of stimulated emission that could produce a laser. From that point many years were spent to see if the idea worked.

In 1953, Charles Hard Townes and graduate students James P. Gordon and Herbert J. Zeiger produced the first microwave amplifier, a device operating on similar principles to the laser, but amplifying microwave radiation rather than infrared or visible radiation.

Meanwhile, in the Soviet Union, Nikolay Basov and Aleksandr Prokhorov were independently working on the quantum oscillator and solved the problem of continuous-output systems by using more than two energy levels. These gain media could release stimulated emissions between an excited state and a lower excited state, not the ground state, facilitating the maintenance of a population inversion.

In 1955, Prokhorov and Basov suggested optical pumping of a multi-level system as a method for obtaining the population inversion, later a main method of laser pumping.

In 1957, Charles Hard Townes and Arthur Leonard Schawlow, then at Bell Labs, began a serious study of the infrared laser. As ideas developed, they abandoned infrared radiation to instead concentrate upon visible light. The concept originally was called an "optical maser". In 1958, Bell Labs filed a patent application for their proposed optical maser; and Schawlow and Townes submitted a manuscript of their theoretical calculations to the Physical Review, published that year in Volume 112, Issue No. 6.

Simultaneously, at Columbia University, graduate student Gordon Gould was working on a doctoral thesis about the energy levels of excited thallium. When Gould and Townes met, they spoke of radiation emission, as a general subject; afterwards, in November 1957, Gould noted his ideas for a "laser", including using an open resonator (later an essential laser-device component).

Moreover, in 1958, Prokhorov independently proposed using an open resonator, the first published appearance (in the USSR) of this idea. Elsewhere, in the U.S., Schawlow and Townes had agreed to an open-resonator laser design – apparently unaware of Prokhorov's publications and Gould's unpublished laser work.

2

At a conference in 1959, Gordon Gould published the term LASER in the paper “The LASER, Light Amplification by Stimulated Emission of Radiation.” Gould's linguistic intention was using the "-aser" word particle as a suffix – to accurately denote the spectrum of the light emitted by the LASER device; thus x-rays: xaser, ultraviolet: uvaser, et cetera; none established itself as a discrete term, although "raser" was briefly popular for denoting radio-frequency-emitting devices.

In 1964 Charles H. Townes, Nikolay Basov, and Aleksandr Prokhorov shared the Nobel Prize in Physics, "for fundamental work in the field of quantum electronics, which has led to the construction of oscillators and amplifiers based on the maser–laser principle".

3