Introduction and Scientific Background
Albert Einstein spent World War I in Berlin, where he developed a theory that described electromagnetic radiation in equilibrium with atoms that could emit and absorb radiation. The innovation in Einstein's work, which was published in 1916 and 1917, was that he used the newly developed quantum theory to obtain his results. The most important result was not only that the atoms in the assembly could absorb and emit radiation spontaneously but also that atoms in certain excited states could be induced to emit radiation. (1) Einstein called this discovery the stimulated emission of radiation. Einstein's discovery provided the basis for the development of lasers, though the phenomenon would not be observed in the laboratory for many years.
The development of radar during World War II required intensive research in microwave radiation. The need for highly sensitive radar receivers led to isolating and observing for the first time Einstein's stimulated emission of radiation. In 1954, Charles H. Townes, J.P. Gordon, and H.J. Zeiger were the first to amplify a microwave signal by using stimulated emission. (2) They called their device the maser, which stood for Microwave Amplification by the Stimulated Emission of Radiation. This work led many to speculate about applying the same principles to radiation in other regions of the electromagnetic spectrum. This effort turned out to be successful, and 6 years later, a positive result was achieved with visible light.
The first laser (Light Amplification by the Stimulated Emission of Radiation) was developed by Theodore H. Maiman in 1960 at the Hughes Aircraft Corporation research laboratory. (3) 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. (4) 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.
Solid-state and gas lasers of the type described here are limited in terms of the energy that the laser beam contains because the population inversion is eventually destroyed by melting or other change in the state of the medium in which it is created. The best continuous energy both in pulsed and continuous wave beams is of the order of kilowatts. Lasers of this kind are useful for many purposes, including meteorology, bar-code scanning, and target designation. One of the most fascinating applications of a solid-state laser with energy in the kilowatt range is the lunar laser ranging experiment conducted for the past 30 years at the University of Texas McDonald Observatory. …