Laser light proves to have many uses in business, health, communications

By HOW AND WHY
Tuesday, January 19, 2010

This year marks the 50th anniversary of one of the most portentous events in the history of science: the creation of the laser. Like many a transformative development, it was met initially with thunderous public indifference, although there were a few mutterings about "death rays." A number of techno-pundits regarded the upstart gizmo as basically a glorified parlor trick, a "solution looking for a problem," as Charles Townes, who won the Nobel Prize for pioneering the idea, later wrote.

Half a century later, lasers check out our groceries, read and write CDs and DVDs, guide commercial aircraft, enable eye surgery and dental repairs, target weapons, provide worldwide communications, survey the planet, print documents, cut fabric for clothing and metal for tools, make powerful pointers for PowerPoint slides and are now poised to ignite nuclear fusion, among scores of other uses.

Who knew? Certainly not Albert Einstein, who had predicted the laser effect way back in 1917. By then, physicists understood that virtually all the light you see is produced by a process called spontaneous emission. Zap a few atoms with the right amount of energy -- including energy from light itself -- and their electrons will absorb the energy and jump up to excited levels, the original "quantum leap."

But they won't stay there. That's because, as the parent of any teenager can tell you, it is the natural tendency of things in this universe to preferentially seek the lowest energy condition, which is why water always flows downhill, shoelaces never re-tie themselves and your check is still in the mail. So the excited electrons soon drop back to lower levels; in the process, they spontaneously shed the surplus energy in the form of photons, the smallest individual units, or quanta, of light. The size of the drop determines the wavelength of the emitted photon. That's how light emerges from a flickering campfire, the surface of the sun, the bulb in a lamp or the screen of your TV.

Einstein's idea was that there could be a second, very different kind of emission in certain types of materials -- not spontaneous, but stimulated. It would work like this: Suppose that you had already excited the electrons in the material's atoms until they were trembling on the brink of emitting photons. But before they did so, what would happen if you whacked those atoms with incoming photons from another source that were exactly equal in energy to the ones the electrons were about to emit? In that case, Einstein figured, instead of absorbing the incoming photon, each atom would be stimulated to give off a photon identical in every way to the incoming photon while leaving the original unchanged.

Not only would the emitted photon be precisely the same wavelength as the one that stimulated it, but its wave peaks and troughs would be perfectly in phase (a condition called coherence) and it would be traveling in the same direction. Those twin photons would then fly off to stimulate more nearby atoms, which would emit yet more photons in a glowing profusion of luminous clones.

Hence the term "laser": Light Amplification by Stimulated Emission of Radiation.

By the mid-1950s, scientists had identified several excellent materials and had recognized that putting a mirror on each side of the laser medium would drastically increase the output, reflecting the photons back and forth, and producing more stimulus and more emissions on each transit. If one of the mirrors was partially transparent, a stream of photons would emerge from that end -- the now familiar laser beam. Finally in May 1960, Theodore Maiman, a physicist at Hughes Research Laboratories, constructed the first laser that emitted light in the visible range.

Today there are dozens of designs exploiting all three of the distinctive properties of laser light. The narrowly defined wavelength allows a laser scanner at the grocery store to bounce its beam off a bar code and read the result when the store lights are on. Indeed, the outputs of different lasers are so sharply differentiated that you can run signals from a bunch of them through the same fiber optic cable simultaneously and still separate them easily at the end. The unidirectional tightness of laser light makes it ideal for surveying because, unlike a flashlight beam, it doesn't diverge much over distances. Even really long distances: Laser light from the surface of the Earth, bouncing off a reflector placed on the lunar surface by Apollo astronauts, has revealed that the moon is receding from our planet by about an inch and a half a year.

Finally, the beam's coherence makes it stunningly powerful. A laser drawing a couple of kilowatts (slightly more than your home hair dryer) can cut through an inch of carbon steel. But that's nothing compared with another device you own, along with every other American taxpayer. Out in California, researchers at the Department of Energy's National Ignition Facility are about to concentrate 192 laser beams totaling 500 trillion watts on a capsule of hydrogen the size of a pencil eraser. If it works, the power of the lasers will shove the hydrogen atoms together hard enough to ignite nuclear fusion, creating (if you could see it) a microscopic star -- and with it, some people believe, the prospect of limitless energy for society using the same energy source that fires up the sun. One of these days. Perhaps.

But not this year, which will leave you plenty of time to take advantage of the 50th birthday activities surrounding LaserFest (see http://www.laserfest.org), the worldwide celebration of civilization's leading light.


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