The Connection between Lasers and The Atom

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. 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). The key here is critical mass so that the following simplified level population and emission can occur

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 number in ground state. Electrons in excited levels have more energy than electrons in the ground state. As these fall down in energy levels photons are emitted. Now, if your have the "right" kind of atom, then you can get photon emission, via level transitions, that all have very similar wavelengths.

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

• 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.

• The light released is coherent. It is organized -- each photon moves in step with the others. This means that all of the photons have wave fronts that launch in unison.

• 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 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, 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. Hence, a long signal chain of atoms with this property will end up emitting a train of photons all in the same direction with all exactly the same or nearly the same wavelength.

The Key design element (see next page) involves figuring out how to build this long signal chain of atomic emission without failures