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August 22, 2006 > How does a laser work?

How does a laser work?

Invented less than 50 years ago, lasers have become ubiquitous. They are used in store checkout scanners; in CD players; as surgical instruments; for industrial welding and cutting; to transport data through optical fibers; in precision measuring devices; and in presentation pointers, to name just a few common applications. There is a laser in the computer mouse used to write this column. The military uses laser-guided bombs and is developing laser weapons.

There are many different kinds of lasers, but they all operate on the same general principles. Laser is an acronym: Light Amplification by the Stimulated Emission of Radiation. This is a very descriptive name; a laser works by stimulating a material to emit a very specific type of light. For a more in-depth answer, we will need a very basic foundation in atomic physics. The following is an extremely simplified discussion, but is adequate to our needs.

All things-air, water, steel, wood, plastic, and your Uncle Jerry-are made of tiny particles called atoms. There are only about 100 kinds of atoms, or elements; these combine in different ways to create different materials. An atom is composed of a relatively dense, positively-charged core, called a nucleus, surrounded by a cloud of negatively charged particles called electrons. These electrons are bound to the nucleus in different energy states. The total number of electrons and the energies associated with their various states are specific to the type of atom.

When stimulated with an external source of energy-electricity, heat, or light for example--an electron may transition from a lower energy state to a higher energy state. This results in an "excited" atom. An excited atom is inherently unstable, and the electron quickly drops back to a less energetic state, releasing some of the energy it absorbed. When this transition occurs, the released energy takes the form, in part, of a packet of light, called a photon.

This is in fact the way in which all light is created, i.e., by electrons "relaxing" from excited states to lower energy states, and emitting photons in the process. Each photon has a wavelength specific to the difference in energy between the two electron states. A wavelength can be thought of in terms of a rope being waved up and down on one end, a plucked guitar string, a spring with waves of compression and expansion oscillating through it, or simply a vibration. The important concept is that the specific wavelength of a photon is unique to a specific electron transition, and determines the color of the light. Red light has a longer wavelength than green light, for example.

Light from the sun or a light bulb is composed of many different wavelengths of photons traveling in many different directions. This is why it appears white to the eye, but when passed through a prism, the photons are refracted at different angles, and some of the component colors, or wavelengths, can be seen. In the same way, moisture in the atmosphere can act as a prism on the white light produced by the sun to separate the various wavelengths and create a rainbow. One useful analogy is to think of white light as the noise of striking all the keys on a piano at once. But striking a single key produces one single wavelength of sound or a single tone, just as only one wavelength of light produces a single color.

There are three important differences between white light and laser light. First, laser light is monochromatic, of a single wavelength and therefore a single color. Second, it is very coherent, meaning all the photons are "in phase." The crests and troughs, or vibrations, of their wave patterns are all aligned or "in synch." In this relationship, they reinforce each other, increasing the intensity of the light. Finally, laser light is very directional. All the photons or packets of light are moving in almost exactly the same direction, resulting in a tightly focused beam. To illustrate this, the beam of a flashlight shined onto a wall across the street may be many feet in diameter, but a laser beam won't be much larger than when it left the laser.

So how can we make a laser? One common type of laser is a tube filled with gas: neon, for example. Let's excite the gas atoms by passing electricity through the tube. The excited electrons fall to lower energy states, generating photons. This is known as spontaneous emission. Since all the gas is the same type of atom, neon in this case, all the photons are the same wavelength and therefore the same color. As long we continue to apply the electricity, the electrons keep transitioning between energy levels and emitting photons. We have created a neon light!

Now let's replace one end of the tube with a mirror, and the other end with a "half-silvered" mirror, meaning it will reflect light up to a certain intensity, above which some light will pass through it, as through a window. When we turn on the light, some of the photons reflect off the mirrors, back and forth down the tube, interacting with the excited atoms, and stimulating them to emit photons. Unlike spontaneous emission, this stimulated emission process creates new photons exactly in phase with the stimulating photons. A "chain reaction" ensues, generating a cascade of more and more photons, all of identical wavelength and perfectly in phase. Because only the photons moving along the length of the tube are reflected, the amplified light beam is also very directional. It is monochromatic, coherent, directional light: a laser!

The beam eventually builds up sufficient intensity to burst through the half-silvered end, resulting in a laser beam that can be put to work scanning groceries or performing cosmetic surgery. In the case of our neon laser, the beam will be either red or green, depending on the energy states to which we excite the electrons. Other materials have atoms with electron states of different energies, and therefore generate photons of various wavelengths and colors. Our example was a gas laser, but solids and liquids are also used.

The TechKnow Guy would be remiss if he failed to caution the reader not to play with lasers, unless in possession of the equipment and expertise to do so safely. Lasers are classified according to their ability to damage the human eye. Supermarket checkout scanners are not particularly dangerous (though you should never stare into one). But much more powerful lasers are readily available for purchase, and even brief exposure to such a laser can permanently damage eyes.

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