The Airborne Laser from Theory to Reality : An Insider ’ s Account

Hughes Aircraft Corporation research laboratory. To develop the laser, a material had to be found in which an assembly of atoms, most of which were in a higher energy state than the ground state (or lowest energy state) of the atom, could be created. Such a condition is called a population inversion. Where it exists, a light pulse can be amplified by stimulating the emission of radiation by the atoms in the higher energy state. Thus, a strong light pulse can be obtained using a small stimulus—hence the term amplification. Maiman found that a population inversion could be produced within certain atoms in an appropriately designed ruby rod by irradiating it with a strong pulse of light. The atoms in the higher energy states could be stimulated to emit radiation by a very weak light signal of the proper frequency, which would create a cascade that would stimulate the emission of light by all other atoms in the higher energy state, producing a strong pulse of red light. About the same time, Ali Javan and his collaborators at Bell Laboratories discovered a way to create a population inversion in a mixture of helium, neon, and other gases. The tube in which these gases were placed was irradiated continuously with light of the appropriate wavelength. A population inversion was created in the gas mixture, which created a tightly focused beam at a sharply defined wavelength. This tight focus bore out Einstein’s 1916–1917 prediction. His calculations revealed that the light quanta or photons created by the stimulated emission are in exactly the same quantum state as the photon that initiated the stimulated emission. This means that all the photons in the process are moving in exactly the same direction, thus creating the tightly focused beam. Introduction and Scientific Background