A LASER IS BORN

Lasers by Hal Hellman, is part of the HackerNoon Books Series. You can jump to any chapter in this book here. A LASER IS BORN

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.

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

The duration of this flash of red light is quite brief, lasting only some 300 millionths of a second, but it is very intense. In the early lasers, such a flash reached a peak power of some 10,000 watts.

When Maiman’s device was successfully built and operating, a public relations expert was called in to help introduce this revolutionary device to the world. He took one look at the laser and decided that it was too small and insignificant looking and would not photograph well. Looking around the lab, he spotted a larger laser and decided that that one was better.

Dr. Maiman informed him in his best scientific manner that laser action had not been achieved with that one. But the world of promotion won out, and Dr. Maiman allowed the larger device to be photographed on the assumption—or was it hope?—that he would be able to get it to operate in the future. (He did.)

The device shown in Figure 16 is the true first laser. The all-important crystal rod is seen at the center. These crystals, incidentally, must be quite free of extraneous material; hence they are artificially “grown”, as shown in Figure 17. The single large crystal is formed as it is pulled slowly from the “melt”, after which it is ground to size and polished.

Figure 16 Dr. Maiman’s first laser. Output was 10,000 watts.

Figure 17 An exotic crystal of the garnet family is “grown” from a melt at a temperature of 3400°F.