TABLE OF CONTENTS The first working laser was built in 1960 by Maiman, using a ruby crystal as the amplifying or active medium. Ruby belongs to the family of gems consisting of sapphire or alumina (Al 2O 3) with various types of impurities. For example, pink ruby contains about 0.05 % Cr atoms. Similarly, Al 2O 3 doped with Ti, Fe, or Mn results in variously colored sapphire. Most of these materials can be grown as single crystals.
Ruby crystals are available in rods several inches long, convenient for forming an optical cavity (Fig. 18.12). The crystal is cut and polished so that the ends are flat and parallel, with the end planes perpendicular to the axis of the rod. These ends are coated with a highly reflective material, such as Al or Ag, producing a resonant cavity in which light intensity can build up through multiple reflections. One of the end mirrors is constructed to be partially transparent so that a fraction of the light will "leak out" of the resonant system. This transmitted light is the output of the laser. Of course, in designing such a laser one must choose the amount of transmission to be a small perturbation on the resonant system. The gain in photons per pass between the end plates must be larger than the transmission at the ends, as well as any other losses due to light scattering and absorption. The arrangement of parallel plates providing multiple internal reflections is...
