HISTORY OF FIBER OPTICS
This week’s “History of Fiber Optics” comes from George Gilder…Einstein realized that on many occasions a photon would strike an electron already in an excited state. In this situation the electron cannot absorb the new photon. Therefore Einstein proposed, the electron will emit a separate, but identical photon moving in exactly the same direction as the photon that struck it.
He stopped short of inventing the laser, but multiplying this Einstein effect many times produces what is termed population inversion, with zillions of electrons pushed at once to a higher state of energy. Bombarded by photons from other electrons similarly excited in a chain reaction, they end up as a polarized coherent stream of light and this laser light is the key foundation of fiber optics and photonics. That was stimulated emission, as Einstein conceived it and also stimulated emission as it works in nature in the phenomenon known as aurora borealis or Northern Lights, which in effect are a vast oxygen laser, excited by unusually intense discharges of charged particles from the sun. Stimulated, coherent light defies the forces of entropy and equilibrium that tend to pull atoms to their lowest available energy state.
At Bell Laboratories in the middle 1950s, it was Charles H. Townes’ luminous dream. Look…nature does it too, in the northern latitudes where glowing vapors are common. Aurora borealis for example, dawns like an oxygen gas laser created by charged particles from storms in the sun, pulled to the earth’s poles by its magnetic field and held briefly in luminous splendor. As the excited electrons tumble back down the quantum energy steps they emit photons creating dancing and shooting towers of greenish and reddish lights that suddenly appear above our planet’s polar horizon. For a lingering moment, the photons have assumed a meta-stable state, or a population inversion. This population inversion of photons is created when electrons are suddenly lifted to higher quantum energies and then tumble back down to lower energy levels causing a chain reaction of photon streams long enough for the world below to see the coherent glow.
For more than three decades, Einstein’s 1917 idea, stood as a challenge to physicists, mathematicians and engineers. One possible route to making it work lay with charged gases in a controlled version of the aurora. With artful manipulation, an enclosed cavity could theoretically produce the necessary population inversion, via excited electrons stimulated to create intensified streams of photons bouncing back and forth between mirrors. And then, by opening a small hole, like a light filled magic flute—a pure tone would stream forth. This stream of coherent, exponentially propagated light or photons of precisely identical wavelengths is called a laser.
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