The word LASER is simply an acronym for: Light Amplification by Stimulated Emission Radiation. This acronym has become so popular that has been accepted as a word including its verbal form “to lase” and “lasing”, but what does it mean? Let’s take it word by word.

Light refers to electromagnetic radiation whether visible or not. Light comes in quantized packets called photons. The nature of light is such that it exhibits properties of both waves and particles and thus it can be characterized by a wavelength and/or by its photon energy.

Amplification usually refers to a mechanism or device intended to amplify something. In this case we are talking about amplifying light, making it stronger.

Stimulated refers to incite an action. Let’s think about it this way: something that happens on its own without anything else intervening is a spontaneous process; something that happens as a result of an intervening action is a stimulated process.

Emission refers to give off something, in this case the emission of the energy of a photon in the form of light

Radiation is the process by which the emitted photon energy travels through space.

Therefore, a laser is a device that radiates light that has been amplified through stimulated photon energy emission. A laser in general consists of 3 main components: a gain medium, a pumping source, and a resonant cavity. The gain medium can be solid, liquid, gas, or plasma and its purpose is to host the particles that will transition within energy levels to emit light. The pumping mechanism is some type of power input often as electrical power or optical power and its purpose is to create a population inversion in the gain medium. The resonant cavity consists of reflective surfaces and its purpose is to provide positive feedback to the gain medium. As a result of this process, laser light have especial characteristics that make it unique such as: high intensity, spatially coherent, nearly monochromatic, and directional.

Here is a picture of a Helium-Neon laser we built as a lab experiment during a laser engineering course. For this laser the gain medium is the helium-neon gas inside the tube which eventually becomes plasma. The pumping mechanism was electrical power at about 1000V and 2mA. The cavity was made by two reflecting mirrors (100% reflective the back mirror and about 99% reflective the front mirror).


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