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A laser (acronym for Light Amplification by Stimulated Emission of Radiation) is a device which produces a narrow beam of intense radiation. The radiation is produced at a fixed frequency depending on the molecules in the cavity of the laser. The laser has reflecting surfaces at each end. The energy source triggers oscillation between the reflectors at each end and energy transfer corresponds to changes of atomic state in a specialized material in the cavity. The laser energy leaves the cavity through a hole in the centre of one reflector and is generally focused to a narrow beam with a very small angle of divergence. This results in the beam losing very little of its energy while travelling over long distances. The frequency of the laser and thus its colour corresponds to the material present in the cavity. Well known types are the CO2 laser, the neon, the neodymium YAG (Yttrium Aluminium Garnet), the ruby and the copper laser.
Lasers may function continuously or in a pulsed fashion. The pulsed laser may transmit what are apparently small bursts of energy, for example 10 joules, in times as short as 4 microseconds. This results in an average power level in each pulse of 2.5 million watts! Some of the uses of lasers in cutting or welding utilize these very high rates of power transmission to produce localized reactions or disruptions due to heating.
In medical practice, lasers are used for cutting tissue, and the localized heat which they generate coagulates blood issuing from small vessels and results in a ‘bloodless field’. By transmitting the laser through fibre optics the energy can be transmitted to the vessels of the heart from an incision in the thigh, the heat developed can be used to convert blood clots into gas or liquid and thus remove the obstruction from the vessel. In opthalmic surgery the small diameter of the laser beam allows the ‘welding’ of a detached retina to the base structure restoring the function of the eye.
In the same way as all other forms of energy, laser energy effects depend on the frequency of the wave form and the absorption of the energy by a target is governed by the colour and surface condition of the target. A rough black surface absorbs light and heat at almost all frequencies and a smooth silver surface absorbs very little. Laser energy can be targeted at a specific wavelength. For instance, laser energy at a wavelength of 577 nanometres can be used to heat small volumes of blood to near boiling point while the skin and other tissues through which the beam passes take up very little energy. Thus for a patient with a red ‘port wine’ birthmark the small blood vessels causing the colouration can be obliterated by laser treatment without the overlying skin being burnt.
In large-scale structural civil engineering work the non-divergent beam of the lasers is used for the precise alignment of parts separated by long distances, or in mechanical engineering for aligning the bearing of long rotating shafts such as propeller shafts in ships. At the other end of the scale in the confocal microscope small lasers can be focused precisely to points on a surface allowing microscopic investigation of the roughness and other characteristics of surfaces, even penetrating a few micrometres below the surface.
The introduction of the laser in 1960 in California symbolized modern technology in all its glory. Originally its invention was seen as a solution looking for a problem, as no one was sure what practical purposes it could be put to. But nowadays the laser has become an accepted part of science. Its uses are measurement and energy transfer in military, scientific, engineering and machine operations.
Another exciting development is the use of lasers in nuclear fusion. The idea behind this is to bombard a volume of hydrogen with an intense pulse of laser light and raise its temperature to over ten million degrees kelvin. At this temperature, fusion can take place releasing very large amounts of energy. The major drawback in devising this technique has been the development of sufficiently powerful lasers, and the difficulty of maintaining an intact volume of hydrogen.
It was originally envisaged that the laser would be used as a wonder weapon, and the introducing of the Strategic Defence Initiative, or ‘Star Wars’ project, by the Americans, has enhanced the laser\'s war-like image. The idea behind the project was to use high-energy beams to burn holes in ballistic missiles, so rendering them useless. This whole project has become highly controversial in terms of expenditure and technical problems that have arisen, and it seems that the project will be shelved indefinitely. AA
Further reading C. Lawrence, The Laser Book. |
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