Laser devices use light to store, transfer, or print images and text; they are also used in a wide range of other applications, including surgery and weaponry. The coherent radiation of the laser gives it special strength. The laser (Light Amplification by Stimulated Emission of Radiation) started life as an extension of the maser, or "Microwave Amplification by Stimulated Emission of Radiation." As its name indicates, the maser is an amplifier that was originally used for amplifying weak radio signals from space. Light waves are very much like radio waves but with a much shorter wavelength.
The laser generates light energy by converting the energy states of a material. The energy level of an atom is a function of its temperature. Its lowest energy level is called its "ground state." The application of additional heat, light, or electric field can raise its energy level. The familiar neon sign, which is glass tubing filled with neon gas, works on this principle. Two electrodes are inserted in the ends of the glass tubing and a high voltage is applied to the electrodes to raise the energy levels of the gas atoms.
Neon light results from the gas's natural endeavor to return to its lowest energy state, emitting photons of energy as it does so. A photon is an energy packet of electromagnetic waves. The energy of a photon is inversely proportional to the wavelength of the associated electromagnetic wave, so shorter wavelengths represent the higher energy photons. Small energy transitions emit photons with long wavelengths such as infrared light, while the larger energy transitions produce photons of visible light with blue light.
In the neon sign, the extra energy added is first stored in the atoms of neon gas in the tube by raising them to a higher energy state. As the neon atoms return to lower energy states, the atoms give up the excess energy as photons.
From Neon Signs to Lasers
One form of laser contains a gas, in which the energy level of the atoms in the medium can be increased above their ground state using a high voltage similar to a neon sign. The majority of atoms are forced to an enhanced energy state or a situation called population inversion . But, in order to make a laser, the elevated energy state must be metastable, as described in the following paragraph.
In the neon sign, the increased energy state is unstable. The gas returns spontaneously to a lower energy state and eventually to the ground state. A metastable state implies that the atoms will remain in an elevated energy state for a period of time, on the order of one one-thousandth of a second or so, and can be encouraged to change to the ground state by the application of a stimulus. Not all materials have this metastable state and those that do are suitable for lasers. Pure neon is used to generate light in a neon sign, but a mixture of helium and neon can be used to create a laser because the mix has a metastable state.
An atom that has emitted a photon stimulates other atoms to return to a lower energy state from the metastable state. The laser has mirrored ends so the photons bounce between the mirrors and cause other atoms to emit photons. Before long, a large number of photons are bouncing between the mirrors and a very large amount of light energy is generated. One of the mirrors is only half silvered so that some of the photons pass through to form a beam of light instead of being reflected.
In the laser, every atom that releases a photon from the metastable state produces exactly the same color or wavelength of light. Also, the waves associated with the photon are all "coherent" or in step with each other. This produces a light beam that is very pure, having a beam of only one color (that is, a monochromatic beam).
Laser light can be very intense. Even though a laser has a relatively low total power, the power is concentrated in a very small area. The common laser pointer has a power output of only one or two milliwatts , but the power is spread over an area of only about a millimeter in diameter. The intensity of the laser pointer is greater than the intensity of a projected image and is easily visible. Large lasers can produce a total power of more than a kilowatt with an intensity that can cut metal.
Semiconductors and Lasers
A number of materials have been developed that have the necessary characteristics for lasers. Among them are semiconductors, those materials, like silicon, whose electrical conductivity is between that of a conductor and that of an insulator. Many semiconductor diode lasers produce an infrared wavelength longer than the deepest red visible to the human eye. The current state of the art in semiconductor lasers limits the shortest wavelength to the visible red at about 630 nanometers . It is generally more difficult to generate shorter wavelengths using a laser of any type.
Semiconductor lasers are particularly useful because they are very inexpensive and are quite small. They are suited for connecting directly to thin optical fibers with little loss of light energy. Special laser diodes are made that produce wavelengths in the infrared that produce the lowest loss in glass fiber. Without the semiconductor laser, long-distance fiber optic communications would not be possible.
Harnessing Laser Technology
Just as important as the generation of pure light energy is the detection of light photons. A number of semiconductor devices, diodes, transistors, and integrated circuits are capable of sensing light energy. Some of these devices, particularly diodes called photo diodes, can handle very high-speed data and are the receiving end of fiber optic communications systems. The field of generating and detecting light energy is called electro-optics.
When laser light is used for communications, it is necessary to "modulate" or change the light in some way so that it can be used as a carrier of information. The same is true for radio communications. While there are two basic ways to modulate a radio carrier— amplitude or angle—in 2001 there were no practical methods of angle modulating a laser, or if angle modulation could be done, there were no practical techniques to gain the advantages that angle modulation produces. This is an area of intense research as many advantages can be gained from angle modulation.
Amplitude modulation can be achieved by changing the electrical power delivered to the laser or by the use of special light modulators. The disadvantage of modulating the electrical power applied to the laser diode is that it causes changes in the wavelength of the light. External light modulators do not cause this change. These modulators are made of non-linear material and are used for very short pulses of light of less than one nanosecond .
When laser light is used for producing a display or for scanning an object such as in a checkout counter bar code reader, the laser beam is deflected using mirrors. The mirrors are very small so that they can be deflected without the application of a large force. Special electric motors that turn only a few degrees are used. Deflected laser beams are used to generate images and can generate spectacular light shows. The most intriguing display produced by laser light is the hologram, a true three-dimensional image.
Laser light is used for reading optical discs as well as for scanning bar codes, measuring distances, and detecting objects. The "laser printer" alters the charge on a photoconductive drum to which charged ink particles, called "toner," are applied. High-power lasers can be used for cutting, burning, surgery, and even as weapons of war. Lasers are used in any application where an intense, monochromatic, and coherent light source is needed.
see also Internet; Telecommunications; Telephony.
Albert D. Helfrick
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LASER TECHNOLOGY. The word "laser" is an acronym for light amplification by stimulated emission of radiation and refers to devices that generate or amplify light through that principle. Lasers are used whenever a directed, sometimes very intense, beam of monochromatic (single wavelength) light is required. For a laser to function, the gas or solid of which it is composed must be excited into a non-equilibrium condition wherein the number of atoms or molecules in a highly energetic state is greater than the number in a less energetic state, a so-called inverted population. If a photon of light with energy equal to the difference between the two energy levels is introduced into the excited substance, either from the outside or by spontaneous decay of one of its own excited atoms or molecules, it can stimulate other excited atoms or molecules to decay to the less energetic state with release of additional photons. Emitted photons travel in the same direction, at the same wavelength, and in phase with the original photon, producing coherent radiation. Often mirrors (one fully reflecting and the other partially Reflecting) are placed parallel to each other on opposite sides of the laser material so that each photon is likely to make many transits of the laser, stimulating release of other photons, before escaping. Lasers may operate in the continuous wave mode or in the pulsed mode, in which they store energy and suddenly release a burst of photons. Since the first lasers were reported in the early 1960s, many thousands of different atoms, ions, and molecules, pure or in combinations, have been used. Each generates light at its characteristic wavelengths, which may lie in the energetic X-ray region of the spectrum; the ultraviolet, visible, and infrared regions; or the low-energy microwave region (in which case it becomes a maser).
Applications of lasers increased rapidly. The unique properties of laser beams allow them to be focused into tiny spots, travel long distances without diverging, and be turned on and off rapidly, making them ideal for many uses, including rapid transmission of digital information. In the 1990s lasers were used regularly in scientific research, military weapons, medical diagnosis and surgery, communications, air quality analysis, surveying and seismology, barcode scanners, CD and video disc scanners, printers, welding, etching, and micromachining. Chemists explored the use of lasers to trigger parts of molecules to react while other normally more reactive sites are unaffected, which may allow inexpensive commercial production of molecules that are difficult or impossible to synthesize with other processes. George W. Bush's election to the presidency in 2000 revived Ronald Reagan's dream of a Strategic Defense Initiative (popularly dubbed "Star Wars"), renewing the debate about the technical feasibility using airborne or space-based laser systems to shoot down enemy missiles. Finally, intense laser beams were also being used to heat molecules to the extremely high temperatures needed to initiate nuclear fusion. If achieved, controlled nuclear fusion could create virtually limitless energy, reducing or eliminating dependence on nonrenewable fossil fuels and nuclear fission. A more likely outcome is suggested by a report in March 2001 that petroleum industry researchers had begun exploring the use of high-energy lasers to explore and drill for more oil.
Bromberg, Joan L. The Laser in America, 1950–1970. Cambridge, Mass.: MIT Press, 1991.
Perin, Monica. "Drilling with Lasers." Houston Business Journal (March 2, 2001).
David K.Lewis/a. r.