Arguably the most significant invention of the twentieth century, the laser has been the focus of much misconception. While many people may associate lasers with death rays and hostile aliens, most do not realize that the word laser (an acronym for Light Amplification by the Stimulated Emission of Radiation) describes not the beam of light but the machine used to create that beam. The origin and invention of the laser is a subject of much debate. The elements necessary for the invention of the first laser had been around for quite some time; theories of coherence and stimulated emission were discussed in detail by Albert Einstein (1879-1955) in 1917, but they were presented only as hypotheses because, at that time, Einstein did not describe a device that could achieve stimulated emission. This was the case until 1954, when Charles Townes accomplished stimulated emission in microwaves. With proof that stimulated emission of light was indeed possible, scientists around the world raced to create a working laser.
In 1958 Townes and Arthur Schawlow (1921-) delivered a paper that explored the requirements for a laser radiator. At the same time, Gordon Gould designed what would be the working model of a laser, and it was he who coined the term laser; unfortunately, due to a misunderstanding at the patent office, Gould did not apply for a patent on his design. Townes and Schawlow eventually received the patent, but it was Theodore Harold Maiman who, in 1960, actually constructed the first working laser in the United States and received credit for its invention. Concurrent with these early studies in the United States, Soviet scientists F. A. Butayeva and V. A. Fabrikant had amplified light using laser technology. Failing to publish their work until years later, they received virtually no recognition in the Western world.
To understand how a laser works, we must understand how all light functions. Normal light, for example sunlight, is emitted from its source in all directions. No stimulus is required to generate this sort of light; the phenomenon is called spontaneous emission. Furthermore, the light is incoherent; that is to say, the lightwaves are disordered with respect to each other, like the groups of people walking through a shopping mall.
Laser light is generated within a medium. A large number of atoms in the medium must be excited into an energetic state, creating what is known as a population inversion3/4an avalanche of light, waiting to happen. To reach the excited state, the atoms must be energized, or pumped, generally by a beam of intense light. In Maiman's laser, a flash tube wrapped around a ruby rod generated this pump light.
Atoms can only remain in an excited state for a limited period of time. Once the atoms are excited, some of them will spontaneously emit their stored energy in the form of a lightwave, often at a different wavelength, or color, than the pump light used to excite the atoms initially. If this emitted lightwave travels to another excited atom, it will trigger that atom to also emit its stored energy as an identical lightwave. This is known as stimulated emission.
Specially aligned mirrors force the light to pass back and forth through the laser medium to extract light from as many atoms as possible, and force the lightwaves into a highly directional beam. Only a small amount of light is allowed to escape in the form of a tightly focused laser beam; most of the rest of the light continues to reflect back and forth, extracting light from the atoms, which are continually excited by the pump light. In this way, a continuous beam of light is formed.
Stimulated emission is responsible for one of the most important characteristics of laser light3/4its self-consistent, or coherent, nature. Because each lightwave emitted is identical to the lightwave that triggered its emission, all light in the beam is well ordered, like a platoon of soldiers marching in formation. It is this aspect of laser light that allows it to make high-precision distance measurements, create three-dimensional holograms, or beams energetic enough to cut through steel.
Maiman's laser used a flashlamp wound around a ruby rod to create a red beam of light. Commercial reaction to his invention was dramatic: within eighteen months of its debut, almost four hundred companies--as well as several government agencies--began their own research involving lasers. The intensity of lasers was found to be unhindered by distance, as had been predicted by Einstein nearly fifty years prior. In a 1962 experiment, a laser beam directed at the moon spanned a mere two miles upon reaching its destination--a remarkably controlled diffusion if compared to conventional light sources. When focused through a lens, a laser's powerful beam can cut through even diamond with unsurpassed precision. Not surprisingly, the government has maintained interest in laser beams as high power, long-range weapons.
Although the first laser demonstrated was a solid state ruby laser, for many years the most common commercial systems were gas lasers such as helium neon lasers and argon ion lasers, or lasers based on organic dyes. Helium neon lasers were frequently limited in output power, argon ion lasers required expensive, sophisticated power supplies and cooling sources, and the dyes used in dye lasers were messy and often toxic.
As the laser industry has matured, the technology trend has been toward semiconductor and solid state lasers, primarily as a result of their robustness, ease of use, and reliability. Semiconductor, or diode, lasers are based on a multiple-layer semiconductor material structure which defines the output wavelength. Diode lasers are usually pumped by electrical current, often from a power supply running off of a wall plug. Although the lasers require thermal regulation for optimal performance, this is often supplied by attaching the laser chip to a metal heatsink, in comparison to the bulky, power-hungry water-based cooling units required by many full-scale lasers. Produced by the same batch processing techniques used to fabricate computer chips, diode lasers are economical, and when configured in arrays known as diode bars, the lasers can produce tens of watts of output.
As of 1998, diode lasers operating from mid-infrared wavelengths to green wavelengths are commercially available. Several companies and research laboratories are working to make a commercially viable blue diode laser; though a number have been demonstrated, the material systems are vulnerable to small amounts of impurities which cause early failure. Common applications for diode lasers include telecommunications transceivers, CD players, and laser pointers.
Solid state lasers have gained as strong a foothold in the commercial market as diode lasers. A solid state laser is a rod or slab of laser crystal or laser glass, fitted with mirrors and a pump source. Certain types of solid state crystals, for example Nd:YAG, can be pumped by diode lasers instead of by other lasers or by flashlamps, which is often the case. Such diode-pumped, solid state systems are reliable, economical, and easy to operate--in fact, many commercial systems are turnkey, needing only to be plugged in and turned on to operate. Solid state lasers are available from mid-infrared to ultraviolet wavelengths, and at a variety of output powers. Many solid state lasers are tunable, providing output across a range of wavelengths. Uses include medical applications, materials processing, and remote sensing.
In 1962 research began on the use of the laser in medicine, specifically its ability to perform very delicate surgery upon the eye. Initial results were promising, but the early ruby lasers did not produce a beam of the correct color or intensity for surgical purposes. It was not until the creation of the argon gas laser that laser surgery was considered viable. Now, in a relatively short procedure that requires no anesthetic, lasers are routinely used to correct myopia and astigmatism, as well to remove birthmarks, tattoos, and small varicose veins from the skin, cut tissue and seal blood vessels during surgery, or resurface skin to minimize wrinkles. Cutting-edge researchers are using visible and infrared lasers to increase bloodflow to oxygen-starved regions of the heart, to visually detect cancer cells on the skin's surface, or to image through the skin to monitor bloodflow or detect cancerous regions.
More modern applications of the laser include communications, wherein a strand of fiber optic material can be used to transmit a stream of high-speed, infrared light pulses encoded with voice discussions or computer data. Because of its coherent nature, laser light passes down the fiber with minimal loss, and multiple beams of different colors can pass down the same fiber simultaneously without interfering with each other. Thus, fiber optic lines can carry many times more information than copper wires, making them more practical for today's high-volume communications demands.
In factories, laser systems are used to measure parts, inspect them for quality, and label, cut, or resurface materials ranging from plastic film to sheet steel a quarter of an inch thick. Lasers form the basis of precision-measuring tools called interferometers that can measure distances less than 1/100th the thickness of a human hair, and are as useful on construction sites as in laboratories. Such instruments can be scanned over objects to create images, and are used on highways to identify vehicles automatically, or on NASA spacecraft to map the surface of the moon and asteroids. In semiconductor manufacturing, ultraviolet lasers provide the source for optical lithography, a technique used to produce computer chips with features as small as one hundred thousandth of an inch (0.25 microns).
Lasers have also found a niche in the entertainment world: concerts frequently include laser lights in a synchronized "dance" to the music. Laser light can also be used to form a hologram. In a complex process, laser light is reflected off a subject and onto a piece of film. Because of the unique properties of coherent light, the subject on film is given depth, an illusion that persists even when the holograph is viewed from many angles.
Perhaps the most familiar use of the laser is that associated with laser disk players. Disks of aluminum containing audio or visual information are encased in clear plastic; the laser beam then reads the information through the plastic without ever contacting the surface, thus providing a nearly infinite lifespan for the information.
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