Laser
I INTRODUCTION
Laser, a device that produces and amplifies light. The word laser is an acronym for Light Amplification by Stimulated Emission of Radiation. Laser light is very pure in color, can be extremely intense, and can be directed with great accuracy. Lasers are used in many modern technological devices including bar code readers, compact disc (CD) players, and laser printers. Lasers can generate light beyond the range visible to the human eye, from the infrared through the X-ray range. Masers are similar devices that produce and amplify microwaves.

II PRINCIPLES OF OPERATION

Lasers generate light by storing energy in particles called electrons inside atoms and then inducing the electrons to emit the absorbed energy as light. Atoms are the building blocks of all matter on Earth and are a thousand times smaller than viruses. Electrons are the underlying source of almost all light.

Light is composed of tiny packets of energy called photons. Lasers produce coherent light: light that is monochromatic (one color) and whose photons are “in step” with one another.

A Excited Atoms

At the heart of an atom is a tightly bound cluster of particles called the nucleus. This cluster is made up of two types of particles: protons, which have a positive charge, and neutrons, which have no charge. The nucleus makes up more than 99.9 percent of the atom’s mass but occupies only a tiny part of the atom’s space. Enlarge an atom up to the size of Yankee Stadium and the equally magnified nucleus is only the size of a baseball.

Electrons, tiny particles that have a negative charge, whirl through the rest of the space inside atoms. Electrons travel in complex orbits and exist only in certain specific energy states or levels (see Quantum Theory). Electrons can move from a low to a high energy level by absorbing energy. An atom with at least one electron that occupies a higher energy level than it normally would is said to be excited. An atom can become excited by absorbing a photon whose energy equals the difference between the two energy levels. A photon’s energy, color, frequency, and wavelength are directly related: All photons of a given energy are the same color and have the same frequency and wavelength.

Usually, electrons quickly jump back to the low energy level, giving off the extra energy as light (see Photoelectric Effect). Neon signs and fluorescent lamps glow with this kind of light as many electrons independently emit photons of different colors in all directions.

B Stimulated Emission

Lasers are different from more familiar sources of light. Excited atoms in lasers collectively emit photons of a single color, all traveling in the same direction and all in step with one another. When two photons are in step, the peaks and troughs of their waves line up. The electrons in the atoms of a laser are first pumped, or energized, to an excited state by an energy source. An excited atom can then be “stimulated” by a photon of exactly the same color (or, equivalently, the same wavelength) as the photon this atom is about to emit spontaneously. If the photon approaches closely enough, the photon can stimulate the excited atom to immediately emit light that has the same wavelength and is in step with the photon that interacted with it. This stimulated emission is the key to laser operation. The new light adds to the existing light, and the two photons go on to stimulate other excited atoms to give up their extra energy, again in step. The phenomenon snowballs into an amplified, coherent beam of light: laser light.

In a gas laser, for example, the photons usually zip back and forth in a gas-filled tube with highly reflective mirrors facing inward at each end. As the photons bounce between the two parallel mirrors, they trigger further stimulated emissions and the light gets brighter and brighter with each pass through the excited atoms. One of the mirrors is only partially silvered, allowing a small amount of light to pass through rather than reflecting it all. The intense, directional, and single-colored laser light finally escapes through this slightly transparent mirror. The escaped light forms the laser beam.

Albert Einstein first proposed stimulated emission, the underlying process for laser action, in 1917. Translating the idea of stimulated emission into a working model, however, required more than four decades. The working principles of lasers were outlined by the American physicists Charles Hard Townes and Arthur Leonard Schawlow in a 1958 patent application. (Both men won Nobel Prizes in physics for their work, Townes in 1964 and Schawlow in 1981). The patent for the laser was granted to Townes and Schawlow, but it was later challenged by the American physicist and engineer Gordon Gould, who had written down some ideas and coined the word laser in 1957.

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