[This is one part of an eight-article series on how lasers work. If you are arriving on this page from a search engine or a link, please click here to see the introduction to the series.]

The Laser/Atom Connection
A laser is a device that controls the way that energized atoms release photons. Laser is an acronym for Light Amplification by Stimulated Emission of Radiation, which describes very succinctly how a laser works!

Although there are many types of lasers, all have certain essential features. In a laser, the lasing medium is “pumped” to get the atoms into an excited state. Typically, very intense flashes of light or electrical discharges pump the lasing medium and create a large collection of excited-state atoms (atoms with higher energy electrons). It is necessary to have a large collection of atoms in the excited state for the laser to work efficiently. In general, the atoms are excited to a level that is two or three levels above the ground state. This increases the degree of population inversion. The population inversion is the number of atoms in the excited state versus the ground state.

Once the lasing medium is pumped, it contains a collection of atoms with some electrons sitting in excited levels. The excited electrons have energies greater than the more relaxed electrons. Just as the electron absorbed some amount of energy to reach this excited level, it can also release this energy. As Figure 3 illustrates, the electron can simply relax, and in turn rid itself of some energy. This emitted energy comes in the form of photons (light energy). The photon emitted has a very specific wavelength (color) that depends on the state the the electon's energy when the photon is released. Two identical atoms with electrons in identical states will release photons with identical wavelengths.

Figure 3

Laser light is very different from normal light. Laser light has the following properties:

1. The light released is monochromatic. It contains one specific wavelength of light (one specific color). The wavelength of light is determined by the amount of energy released when the electron drops to a lower orbit.
2. The light released is coherent. The light is “organized” and each photon moves in step with the others. This means that all of the photons have wave fronts that launch in unison.
3. The light is very directional. A laser light has a very tight beam and is very strong and concentrated. A flashlight, on the other hand, releases light in many directions and the light is very weak and diffuse.

To make these three properties occur takes something called stimulated emission. This does not occur in your ordinary flashlight -- in a flashlight, all of the atoms release their photons randomly. In stimulated emission, photon emission is organized.

The photon that any atom releases (Figure 3) has a certain wavelength that is dependent on the energy difference between the excited state and the ground state. If this photon (possessing a certain energy and phase) should encounter another atom that has an electron in the same excited state, a phenomena called stimulated emission can occur. The first photon can stimulate or induce atomic emission such that the subsequent emitted photon (from the second atom) vibrates with the same frequency and direction as the incoming photon.

The other key to a laser is a pair of mirrors, one at each end of the lasing medium. Photons, with a very specific wavelength and phase, reflect off the mirrors to travel back and forth through the lasing medium. In the process, they stimulate other electrons to make the downward energy jump and can cause the emission of more photons of the same wavelength and phase. A cascade effect occurs, and soon we may have propagated many, many photons of the same wavelength and phase. The mirror at one end of the laser is "half-silvered", meaning it reflects some light and lets some light through. The light that makes it through is the laser light.

You can see all of these components in the following figures, which illustrate how a simple ruby laser works. The laser consists of a flash tube (like you would have on a camera), a ruby rod and 2 mirrors (one half-silvered). The ruby rod is the lasing medium and the flash tube pumps it.

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Figure 5

Go to the next article about How Lasers Work or pick from the table of contents:

• Introduction to How Lasers Work
• The Basics of an Atom
• The Laser/Atom Connection You are here
• Three-Level Laser
• Types of Lasers
• Laser Classifications
• Links
• About the Author

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