41.8.2 The Ruby Laser

The first laser to be developed was a ruby laser. FIGURE 41.34a shows the energy-level structure of the chromium atoms that gives ruby its optical properties. Normally, the number of atoms in the ground-state level $E_{1}$ far exceeds the number of excited-state atoms with energy $E_{2}$ . That is, $N_{2} ≪ N_{1}$ . Under these circumstances 690 nm light is absorbed rather than amplified. But suppose that we could rapidly excite more than half the chromium atoms to level $E_{2}$ . Then we would have a population inversion $(N_{2} > N_{1})$ between levels $E_{1}$ and $E_{2}$ .

The figure labeled ("a") illustrates the different energy levels for the optical pumping and emission of a ruby laser.

The figure labeled (b) illustrates a flash lamp-pumped ruby laser.

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This can be accomplished by optically pumping the ruby with a very intense pulse of white light from a flashlamp. A flashlamp is like a camera flash, only vastly more intense. In the basic arrangement of FIGURE 41.34b, a helical flashlamp is coiled around a ruby rod that has mirrors bonded to its end faces. The lamp is fired by discharging a high-voltage capacitor through it, creating a very intense light pulse lasting just a few microseconds. This intense light excites nearly all the chromium atoms from the ground state to the upper energy levels. From there, they quickly $(\approx 10^{- 8} s)$ decay nonradiatively to level 2. With $N_{2} > N_{1}$ , a population inversion has been created.

Once a photon initiates the laser pulse, the light intensity builds quickly into a brief but incredibly intense burst of light. A typical output pulse lasts 10 ns and has an energy of 1 J. This gives a peak power of

P = \frac{Δ E}{Δ t} = \frac{1 J}{10^{- 8} s} = 10^{8} W = 100 MW

One hundred megawatts of light power! That is more than the electric power used by a small city. The difference, of course, is that a city consumes that power continuously but the laser pulse lasts a mere 10 ns. The laser cannot fire again until the capacitor is recharged and the laser rod cooled. A typical firing rate is a few pulses per second, so the laser is “on” only a few billionths of a second out of each second.

Ruby lasers have been replaced by other pulsed lasers that, for various practical reasons, are easier to operate. However, they all operate with the same basic idea of rapid optical pumping to upper states, rapid nonradiative decay to level 2 where the population inversion is formed, then rapid buildup of an intense optical pulse.