Charles H. Townes of coherent radiation, for which he invented the acronym "maser" (for Microwave Amplification by Stimulated Emission of Radiation). When the same principle was applied to higher frequencies, the term "laser" was used (the word "light" substituting for the word "microwave"). During 1953, Townes, James P. Gordon, and H. J. Zeiger built the first ammonia maser at Columbia University. This device used stimulated emission in a stream of energized ammonia molecules to produce amplification of microwaves at a frequency of about 24.0 gigahertz. From 1959 to 1961, he was on leave of absence from Columbia University to serve as Vice President
Stimulated emission of energy levels 1 and 2, respectively. The intensity (in watts per square meter) of the stimulated emission is governed by the following differential equation: as long as the intensity "I"("z") is small enough so that it does not have a significant effect on the magnitude of the population inversion. Grouping the first two factors together, this equation simplifies as where is the "small-signal gain coefficient" (in units of radians per meter). We can solve the differential equation using separation of variables: Integrating, we find: or where The saturation intensity "I" is defined as the input intensity at which the