LASING—A NEW WORD
Now we can begin to put together the various processes and equipment we have been discussing separately. Perhaps the best way to do this is to look again at the word laser and recall its meaning: light amplification by stimulated emission of radiation. Our objective is to create a powerful, narrow, coherent beam of light. Let us see how to do this.
In [Figure 18] we imagine a laser crystal containing many atoms in the ground state (white dots) and a few in the excited state (black dots). Pumping light (wavy arrows in a) raises most of the atoms to the excited state, creating the required population inversion.
Figure 18 Sequence of operations in a solid crystal laser. (a) Pumping light raises many atoms to excited state. (b) Lasing begins when a photon is spontaneously emitted along the axis of the crystal. This stimulates other atoms in its path to emit. (c) The resulting wave is reflected back and forth many times between the ends of the crystal and builds in intensity until finally it flashes out of the partially silvered end.
(a) Ruby crystal Pumping light Atom in ground state Excited atom Partial reflecting mirror Full reflecting mirror (b) Excited atom emits photon parallel to axis (c)
Lasing begins when an excited atom spontaneously emits a photon parallel to the axis of the crystal (b). (Photons emitted in other directions merely pass out of the crystal.) The photon stimulates another atom in its path to contribute a second photon, in step, and in the same direction.
This process continues as the photons are reflected back and forth between the ends of the crystal. (We might think of lone soldiers falling into step with a column of marching men.) The beam builds up until, when amplification is great enough (c), it flashes out through the partially silvered mirror at the right—a narrow, parallel, concentrated, coherent beam of light, ready for....