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: _l_ight
_a_mplification by _s_timulated _e_mission of _r_adiation. 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.
[Illustration: 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....