Stimulated emission is a process similar to absorption, but operates in the opposite direction. In absorption, an incoming photon is absorbed by an atom, leaving the atom in an excited state and annihilating the photon in the process. In stimulated emission, an incoming photon stimulates an excited atom to give up its stored energy in the form of a photon that is identical in wavelength, direction, polarization, and phase to the stimulus photon. If the excited atom is unable to produce a photon that matches the incoming photon, then stimulated emission cannot take place.
For laser action to occur, a majority of the atoms in the active medium must be excited into an energetic state, creating a population inversion of energized atoms ready to emit light. This is generally accomplished by pumping the atoms optically or electrically. As a photon passes through the collection of excited atoms, it can stimulate the generation of many trillions of photons, or more, creating an avalanche of light. The active medium can thus be regarded as an amplifier that takes in a small signal (one photon, say) and delivers a large signal (many photons, all identical to the first) at the output. This amplification, or gain, is provided by stimulated emission; hence the term laser, which is actually an acronym for light amplification by stimulated eission of radiation.
To illustrate laser operation, consider the well-known helium neon (HeNe) laser, once a staple of super-market scanners. The active medium is a mixture of helium and neon gases, enclosed in a glass tube a few inches long, with an electrode and a mirror at each end. The atoms in the gas mixture are excited, or pumped, by by an electrical discharge that runs through the gas, in much the same way that a neon sign is lit. The conditions in the HeNe laser have been optimized so that the maximum number of neon atoms are in the correct state to emit light at the familiar red wavelength, 633 nm.
Science EncyclopediaScience & Philosophy: Laser - Background And History to Linear equationLaser - Background And History, How It Works, Stimulated Emission, Oscillation, Solid State Lasers, Gas Lasers - Applications