Lasers are indispensable to the computer and medical industries, surgeons, billing counters and Superman. Lasers are so staggeringly coherent and concentrated that a beam directed at a mirror on the moon by NASA departed and arrived without any dispersion. With absolutely no room for error, the Apollo team could therefore calculate the distance between Earth and the moon with the utmost precision. However, how does a laser achieve such unbelievable consistency?
The word LASER is actually an acronym for Laser Amplification by Stimulated Emission of Radiation. The major component of the apparatus is a laser medium, which can be a tube sealing a gas inside, like the mixture of helium and neon or a solid crystal such as Nd: YAG (Neodymium-doped Yttrium Aluminum Garnet). Fixed above the medium is a source of energy, either electrical or photonic (light).
When atoms experience a surge in energy, their electrons will use it to climb to higher energy states. However, atoms find these surges agitating, as they compulsively seek stability, which they will achieve by forcing the electrons down to lower energy states. However, this relegation comes at the cost of losing energy; as the excited electrons make the leap, the atoms emit energy in the form of light.
The atoms of the medium are excited by either a powerful electric current or a UV lamp. As all the atoms get excited, they achieve what is called a population inversion: the configuration of electrons, rather than residing in their stable or habitable lower energy levels, is inverted such that they now occupy unstable higher energy levels.
However, as explained, the electrons will immediately jump down and emit light, but this light isn’t allowed to escape yet. What the apparatus has accomplished is merely spontaneous radiation, not stimulated radiation.
The medium is confined within two mirrors: one completely reflective and one only partially reflective. The spontaneous radiation will vigorously reflect between these mirrors, which will re-excite the medium’s atoms and thereby produce more radiation. What ensues is a tumultuous chain reaction: a single photon produces, for example, two photons, which then produces four, which then produces eight and so on until billions and billions of photons have been spawned. This torrent of photons being produced is called stimulated radiation.
Stimulation has two outstanding advantages: first, a ridiculous amount of amplification is achieved, and second, every photon exhibits the same wavelength, phase and direction. To explain how goes far beyond the scope of this article.
Now, because the light exhibits only a single wavelength and phase, it is monochromatic or of a single color, if it is visible, of course, or comprising a single spectrum of invisible electromagnetic radiation, including UV, infrared, X-ray or any one of the remaining others. The absolute monochromaticity and tremendous coherence are a laser’s most critical characteristics.
Remember that photons can be compressed and packed this way because, unlike electrons, they aren’t charged. If electrons behaved this way, if they didn’t obey Pauli’s exclusion principle, matter could never have achieved volume.
The stimulated radiation will escape through the mirror that is only partially reflective. It is then further converged and concentrated with powerful lenses and finally directed on compact disks, tissues, barcodes and Superman’s enemies.