TABLE OF CONTENTS All the lasers discussed above have several common characteristics. Originally, in order to qualify as a laser, stimulated emission had to be demonstrated. The convention is now that a laser must demonstrate both stimulated emission and positive optical feedback. These concepts will be addressed in this section.
Most materials exhibit some kind of optical absorption. Absorption is the process by which incident light is converted to electrical potential energy. For example, an electron can be excited from the valence band to the conduction band by absorbing the energy of a photon. Logically, most materials also exhibit some form of light emission. For example, in an ordinary light source such as a light bulb, the emission of radiation occurs through a process called spontaneous emission. In this process, an electron which had been excited into a higher energy state for "some time" falls down to a lower state by emitting a photon. This "time" is called the radiative recombination lifetime.
However, in the presence of photons, an excited electron can be forced or stimulated to fall down to a lower state much faster than in a spontaneous event. The stimulus is provided by a photon with the proper wavelength. This process is called stimulated emission and produces an additional photon of exactly the same direction of propagation, frequency, and polarization (direction of the electric field in a wave) as the stimulating photon.
Spontaneous and stimulated processes are illustrated in Fig. 18.2. An electron...
