carried by laser beams traveling through optical fibers thinner than a human hair. Lasers have become indispensable weapons in the arsenal of medical therapeutic and diagnostic procedures, frequently providing attractive lower-cost alternatives to conventional surgery. Low-cost lasers have been developed, allowing their application in a wide variety of consumer products, including compact-disk (CD) players and laser printers. The total annual revenue of U.S. laser manufacturers is now approximately $1.1B (D. Kales, "Review and Forecast of Laser Markets: 1993," Laser Focus World 29 (January), 70-88, 1993), but the true economic impact of the laser is at least 100 times greater because lasers are a critical component of many consumer products and services.

The widespread application of lasers is a result of the unique characteristics of the radiation they provide. The output beam from a laser can be ultradirectional. Thus small objects can be selectively illuminated, even at large distances. This is key to the development of laser rangefinders, now widely used in surveying and in military applications, and of laser target designators. The pinpoint accuracy of "smart" weapons guided to their targets by scattered laser light was graphically demonstrated during Desert Storm. Laser beams may also be focused to extremely small spots, resulting in high energy densities sufficient to melt or vaporize many materials. This capability has resulted in numerous applications in, for example, industrial processing. Today lasers are used to heat, cut, or weld a wide variety of materials, including metals, ceramics, plastics, wood, and cloth. Radiation from a laser also can be highly monochromatic; that is, it encompasses a narrow range of frequencies and wavelengths. The output wavelengths available and their range of tunability vary with laser type. This wavelength choice makes possible spectroscopic analysis of atomic and molecular species and is the basis of many techniques used in remote sensing of atmospheric pollutants and of species present in environments such as combustion chambers.

Since the initial invention of the laser, research and development activities have resulted in the discovery of many different classes of lasers that, taken together, provide an enormous range of output wavelengths, pulse lengths, and power levels. A variety of lasers based on electrical discharges in gases have been developed, a good example of which is the helium-neon laser that provides the red beam observed in the point-of-sale scanners at the checkout stands in supermarkets and other stores (Figure 1.1). Argon and krypton ion lasers are widely used in ophthalmology and laser light shows, and carbon dioxide lasers find application in laser surgery. Many laser systems employ solid-state gain media including the ruby laser, which was the first system to demonstrate successful lasing. Recently, solid-state laser systems have been developed at infrared wavelengths that are "eye-safe," that is, that are absorbed by the fluid in the eye without damage to the retina, and these are finding applications in, for example, rangefinders and remote sensing. Laser systems based on organic dyes also provide tunable radiation but at wavelengths extending down into the ultraviolet.

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