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 number in 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 the figure below 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 of the electron's energy when the photon is released. Two identical atoms with electrons in identical states will release photons with identical 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.

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 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 two mirrors (one half-silvered). The ruby rod is the lasing medium and the flash tube pumps it.

1. The laser in its non-lasing state

 

2. The flash tube fires and injects light into the ruby rod. The light excites atoms in the ruby.

 

3. Some of these atoms emit photons.

 

4. Some of these photons run in a direction parallel to the ruby's axis, so they bounce back and forth off the mirrors. As they pass through the crystal, they stimulate emission in other atoms.

 


5. Monochromatic, single-phase, columnated light leaves the ruby through the half-silvered mirror -- laser light!

Three-Level Laser
Here's what happens in a real-life, three-level laser.


In the next section, you'll learn about the different types of lasers.