Essentially, a laser is composed of light. Usually, a light source "sprays" photons out in all directions, in a wide wavelength spectrum. In the case of a laser, the light is created, collumated so that it is all going in the same direction, and released through a filter. Instead of a cone/spray of incoherant photons, a laser produces a beam of photons which all have the same approximate wavelength, which are all headed in the same direction. No laser is perfect, and over distance the diameter of the beam will spread out a certain amount. Also, particles in the air will reflect/refract some of the photons in other directions. This is what you see when you go to a laser light show. What you are actually seeing is the light that has been diverted by particles in the air, not the actual beam itself, which would have to be aimed directly at your eye(s) for you to see it. Essentially what this means is that over distance, a laser loses coherancy and power.
Maser is similar, but with microwave radiation instead of visible light.
A laser can function either as an amplifier or as a light source. As an amplifier, the amplified signal is usually very similar to the input in terms of wavelength, phase, polarisation and direction. As a source, lasers are able to produce a very "pure" light in that the output can have a close approximation to a single wavelength, phase (ie [coherent light]?), polarisation, and direction.
Because of the focused nature of the light, and the frequencies used, lasers pose an inherant risk to eyesight. The more powerful a laser is, the further it will travel, but the more dangerous it is to personal exposed to it. Shining a laser onto a person's eyes is equivalent to using a magnifying glass to focus sunlight onto a leaf, with similar results. Lasers are classified Class I through Class IV, with Class IV being the most dangerous, and Class I being "safe" as determined by scientific research. Class IV lasers include Neodymium YAG lasers, which is the type of filter used. The primary factor that makes these lasers dangerous is the wavelength that they operate in. It is not in the visible spectrum, and will effectively cauterize the tissue in the eye upon contact. This will happen almost instantly. Also, these lasers are typically high powered, meaning that they will travel further (many miles). This is a common laser type used in military targetting applications.
The basic physics of lasers centres around the idea of population inversion, also called "negative absolute temperature" in some accounts. A great deal of Quantum Mechanics and thermodynamics theory can be applied to laser action, though in fact most of the more useful laser types were discovered by accident or trial and error.
Population inversion is also the concept behind the maser, which is similar in principle to a laser but works with microwaves. The first maser was built by Charles H. Townes in 1953, who later worked with Arthur L. Schawlow to describe the principle of the laser. The first working laser was made by Theodore H. Maiman in 1960 at Hughes Research Laboratories. Maiman used a flashlamp?-pumped ruby crystal to produce red laser light at 694 nanometeres wavelength. (See parts of a laser).
Even earlier was a similar device working at radio frequencies. These were first developed in the 1930s, and further developed in the early 1950s. They don't seem to have a name of their own, possibly because they don't have any obvious practical uses. (Ordinary electronic radio amplifiers and oscillators perform better for less effort.) Some of the basic theory of lasers was in fact developed to explain these earlier devices.
Types of lasers include:
See also Laser applications, Parts of a laser, Laser science.
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