On that morning in Franklin Park, the goal of boosting energy gave me an incentive to think more deeply about stimulated emission than I had before. How could one get such a nonequilibrium set up? … Rabi, right at Columbia, had been working with molecular and atomic beams that he manipulated by deflecting atoms in excited states from those of lower energies. The result could be a beam enriched in excited atoms. … I took an envelope from my pocket to try to figure out how many molecules it would take to make an oscillator able to produce and amplify millimeter or submillimeter waves. … Any resonator has losses, so we would need a certain threshold number of molecules in the flow to keep the wave from dying out. Beyond that threshold, a wave would not only sustain itself bouncing back and forth, but it would gain energy with each pass. The power would be limited only by the rate at which molecules carried energy into the cavity.
—Charles H. Townes 
Radio transmitters generate electromagnetic radiation that, to a good approximation, can be regarded as coherent waves. This coherence allows us to modulate the wave's amplitude (AM) or its frequency (FM) to carry sound to far-away radio receivers. Before the invention of the laser, there was no such coherent source for electromagnetic radiation in the optical region of the spectrum.
The laser is often regarded as a direct by-product of the modern ideas of the “quantum revolution.” ...