More than anything else, it is this ability of laser light to focus into a point with an immense power concentration that makes the laser a wonderful tool for many applications.
The rays of laser light will go through the pupil of the human eye without harming it, but when they meet at one point of the retina (the screen at the back of the eye that signals the visual image to the brain), they become a surgeon's scalpel operating with a delicate sharpness that no metal scalpel can duplicate.
But even without being focused to a point, laser light with its vast "n^2" power can be made to propagate in a strictly parallel beam, whereas "hash" light beams, such as those of searchlights, must diverge to some extent, so that their power per unit area weakens with distance from the source. Such a beam can shine continuously, or its power can be packed into periodic pulses with a commensurate increase in peak power. This is to some extent analogous to pressing a nail into wood with a continuous force as against hitting it with powerful blows at certain moments, separated by intervals when no power is applied. It is these hammer blows that could destroy Soviet missiles before they reach the US, and there is no real technical defense against them: decoys, armor, spinning and other methods have been shown ineffective. Alas, the Soviets do have an effective countermeasure: the non-interventionists, the mass media, and their flunkies in Congress.
One of the applications that is close to the subject matter of this newsletter is the use of laser light to probe the atmosphere for pollutants by using the radar principle. The word radar originated in WWII as an acronym for RAdio Detection and Ranging, and the same technique using laser LIght has now acquired the name of "lidar." The principle is examining the echo from a target. The delay (since emission of the probing pulse) and direction of arrival give the location of the target, and that part is elementary. But then comes the deeper part of the analysis, often facilitated by the type of laser used for a special purpose.
For example, a moving target will return a changed frequency (the Doppler effect, used by highway patrols to measure the speed of automobiles
¾by radio, not light). By the echo from molecules and aerosols in the atmosphere, Doppler lidars can measure the speed of small air masses at ranges up to 7.5 miles with an accuracy of about 2 ft/sec. This is important for measuring wind shear at airports. Wind shear is the motion of two adjacent air masses in opposite directions; if a plane gets caught between the two, the result, especially during landing, can be disastrous.Other techniques include the determination of chemicals in the investigated air mass
¾for example, in the effluent plume of an industrial plant, in the motion of air masses near urban centers, and in the search for trace elements in the stratosphere. This can be done by looking for the characteristic backscatter and absorption by certain molecules, or even by "fluorescence" lidars, which excite the desired species on one of its own absorption spectral lines, so that the return echo is not merely backscatter, but active radiation by the target itself.[More: D.K. Killinger and N. Menyuk, "Laser remote sensing of the atmosphere," Science 2 Jan. 87, pp.37-45.]
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Vol. 14, No. 6
Newsletter: Access to Energy Newsletter Archive Volume: Issues Issue/No.: Vol. 14, No. 6 Date: November 30, 2004 08:41 AM Title: "The same thing"
Copyright © 2004 - Access to Energy Newsletter Archive
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