LASER: Light Amplification by Stimulated Emission of Radiation
In 1957, Gordon Gould began his notes on the feasibility of a LASER by spelling out the acronym: light amplification by stimulated emission of radiation. The acronym, like the concept itself, was inspired by the maser, the laser's microwave counterpart, invented earlier by Charles Townes. Gould's catchy acronym gives the gist of the idea, but is not a full definition.
Light sources from candles to LEDs emit light spontaneously, when excited atoms or molecules release energy on their own, so the photons are not identical and travel in various directions. Stimulated emission occurs when a photon stimulates an excited atom to emit an identical photon, traveling in the same direction and coherent with the first. The effect amplifies the light of the first photon, and the additional photon also can stimulate emission, producing a beam of identical photons.
However, the acronym glosses over an essential point -- a laser also requires a resonant cavity to generate a beam. A pair of mirrors facing each other bounce light back and forth, so it oscillates through a laser medium, amplifying the light that emerges through the output mirror as the laser beam. Oscillation makes the laser beam directional, coherent and monochromatic, conditions considered essential for a laser.
Townes and Arthur Schawlow saw the laser as a variant of the maser, and in 1958 they described it as an "optical maser" in a pioneering paper. The terms "laser" and "optical maser" competed for public favor, as the two sides competed for patent rights and credit for the invention. Schawlow was quick to point out that Gould's choice was a poor one because the device needed an oscillator in order to generate a beam. He said that meant the "laser was a loser," because it should be spelled out as "Light Oscillation by Stimulated Emission of Radiation."
Schawlow had a point, but laser won the popularity race. In time, Townes won a Nobel Prize, and Gould got rich from his patents. But Schawlow got the most laughs.