A LASER IS BORN
Following the maser development, there was much speculation about the possibility of extending the principle to the optical region. Indeed the first lasers—light amplification by stimulated emission of radiation—were called “optical masers”.
The difficulty, of course, was that optical wavelengths are so tiny—about ¹/₁₀,₀₀₀ that of microwaves. The maser principle depended upon a physical resonator, a box a few centimeters (or even millimeters) in length. But at millimeter wavelengths, such resonators are already so small that they are hard to make accurately. Making a box ¹/₁,₀₀₀ that size was out of the question. Another approach was necessary.
In 1958 A. L. Schawlow of Bell Telephone Laboratories and Dr. Townes outlined the theory and proposed a structure for an optical maser. They suggested that resonance could be obtained by making the waves travel back and forth along a relatively long, thin column of amplifying substance that had parallel reflectors at the ends.
After their theory of the optical maser had been published, the race to build the first actual device began in earnest. The winner, in 1960, was Dr. T. H. Maiman, then with Hughes Aircraft Company. (He is now president of Maiman Associates.) The active substance he used was a single crystal of ruby, with the ends ground flat and silvered.
Ruby is an aluminum oxide in which a small fraction of the aluminum atoms in the molecular structure, or lattice, have been replaced with chromium atoms. These atoms absorb green and blue light and hence impart a red color to the ruby. The chromium atoms can be boosted from their ground state into excited states when they absorb the green or blue light. This process, by which population inversion is achieved, has been given the name pumping.[13]
Pumping in a crystal laser is generally achieved by placing the ruby rod within a spiral flash lamp ([Figure 15]) that operates like those used in high-speed (stroboscopic) photography. When the lamp is flashed, a bright beam of red light emerges from the ruby, shining out through one end, which has been only partially silvered.
Figure 15 A ruby laser system.
Ruby Flash lamp Partially silvered end Laser output Power Cooling