LASER

What Is a Laser?

The word laser is an acronym for light amplification by stimulated emission of radiation, although common usage today is to use the word as a noun - laser - rather than as an acronym - LASER.

A laser is a device that creates and amplifies a narrow, intense beam of coherent light.

Atoms emit radiation. We see it every day when the "excited" neon atoms in a neon sign emit light. Normally, they radiate their light in random directions at random times. The result is incoherent light - a technical term for what you would consider a jumble of photons going in all directions.

The trick in generating coherent light - of a single or just a few frequencies going in one precise direction - is to find the right atoms with the right internal storage mechanisms and create an environment in which they can all cooperate - to give up their light at the right time and all in the same direction.

Exciting atoms or molecules

In a laser, the atoms or molecules of a crystal, such as ruby or garnet - or of a gas, liquid, or other substance - are excited in what is called the laser cavity so that more of them are at higher energy levels than are at lower energy levels. Reflective surfaces at both ends of the cavity permit energy to reflect back and forth, building up in each passage.

If a photon whose frequency corresponds to the energy difference between the excited and ground states strikes an excited atom, the atom is stimulated as it falls back to a lower energy state to emit a second photon of the same (or a proportional) frequency, in phase with and in the same direction as the bombarding photon.

This process is called stimulated emission. The bombarding photon and the emitted photon may then each strike other excited atoms, stimulating further emission of photons, all of the same frequency and phase. This process produces a sudden burst of coherent radiation as all the atoms discharge in a rapid chain reaction.

Wide range of sizes and uses

First built in 1960, lasers now range in size from semiconductor lasers as small as a grain of salt to solid-state and gas lasers as large as a storage building. The light beam produced by most lasers is pencil-thin and maintains its size and direction over very large distances.

Lasers are widely used in industry for cutting and boring metals and other materials, in medicine for surgery, and in communications, scientific research, and holography. They are an integral part of such familiar devices as bar-code scanners used in supermarkets, scanners, laser printers, and compact disk players.

Why Lasers Are Important Today

The laser has contributed to humanity as a powerful scientific tool for expanding human knowledge and in its many applications that help people directly. In the 40 years since Arthur L. Schawlow and Charles H. Townes published their technical paper on the principles of the laser in 1958, the device has been put to work in a vast range of applications and has assumed many forms.

Their paper caused an explosion of research by scientists at Bell Labs and at universities and industrial laboratories around the world that is unabated today at Bell Labs and elsewhere.

In communications, engineers recognized the potential of the laser to replace electrical transmission over copper wires, but how to transmit the pulses presented enormous problems. In 1960, Schawlow, D.F. Nelson, R.J. Collins and others transmitted pulses of light between Bell Labs facilities in Murray Hill, N.J., and Crawford Hill, N.J., a distance of 25 miles. Then called an optical maser, Townes' preferred name for the device, the laser produced an intense and extremely narrow beam of light that was more than a million times brighter than the sun.

Lasers in search of a medium

Unfortunately, laser beams could easily be adversely affected by atmospheric conditions, such as rain, fog, low clouds, and objects in the air, such a birds. Scientists and engineers suggested a number of novel schemes to protect the light from interference, including shielding it in metal tubes and using specially designed mirrors and thermal gas lenses to navigate around bends.

It took another major innovation, the development in the early 1970s of hair-thin strands of encased glass, called fiber optic waveguides, before the laser could transmit telephone signals. Since then, optical fiber has increasingly become the medium of choice for telecommunications companies to transmit voice, data, and video.

Telecommunications, once largely electronic, today relies on photons, as tiny semiconductor lasers routinely transmit light pulses carrying billions of bits of information per second over glass fibers. Wavelength division multiplexing technology uses various wavelengths, or colors, of light to transmit trillions of bits simultaneously over a single fiber.