The word laser is an acronym for light amplification by the stimulated emission of radiation. The first laser, a ruby laser, was demonstrated in 1960, and several other kinds of lasers appeared within a few months. The driving force behind much of the research was the American physicist Charles Townes. Townes was awarded the Nobel Prize in 1964 for the invention of the maser, an earlier device using microwaves, and his theoretical work leading to the laser.

Today, lasers do everything from being the light source in fiber-optic communications to measuring the distance to the moon and from playing your DVD to performing delicate eye surgery. But what is a laser? Basically it is a device that produces a beam of highly coherent and essentially monochromatic (single-color) light as a result of stimulated emission. Coherent light is light in which all the electromagnetic waves have the same phase, direction, and amplitude. It is the coherence of a laser beam that allows it to be very tightly focused or to be rapidly modulated for communications.

Let’s take a brief look at how a laser works. FIGURE 41.31 represents a system of atoms that have a lower energy level $E_1$ and a higher energy level $E_2$. Suppose there are $N_1$ atoms in level 1 and $N_2$ atoms in level 2. Left to themselves, all the atoms would soon end up in level 1 because of the spontaneous emission $2\to 1$. To prevent this, we can imagine that some type of excitation mechanism, perhaps an electrical discharge, is continuing to produce new excited atoms in level 2.

FIGURE 41.31 Energy levels 1 and 2, with populations $N_1$ and $N_2$.

Let a photon of frequency $f=(E_2-E_1)\text{/}h$ be incident on this group of atoms. Because it has the correct frequency, it could be absorbed by one of the atoms in level 1. Another possibility is that it could cause stimulated emission from one of the level 2 atoms. Ordinarily $N_2\ll N_1$, so absorption events far outnumber stimulated emission events. Even if a few photons were generated by stimulated emission, they would quickly be absorbed by the vastly larger group of atoms in level 1.

But what if we could somehow arrange to place every atom in level 2, making $N_1=0?$ Then the incident photon, upon encountering its first atom, will cause stimulated emission. Where there was initially one photon of frequency $f$, now there are two. These will strike two additional excited-state atoms, again causing stimulated emission. Then there will be four photons. As FIGURE 41.32 shows, there will be a chain reaction of stimulated emission until all $N_2$ atoms emit a photon of frequency $f$.

FIGURE 41.32 Stimulated emission creates a chain reaction of photon production in a population of excited atoms.

In stimulated emission, each emitted photon is identical to the incident photon. The chain reaction of Figure 41.32 will lead not just to $N_2$ photons of frequency $f$, but to $N_2$ identical photons, all traveling together in the same direction with the same phase. If $N_2$ is a large number, as would be the case in any practical device, the one initial photon will have been amplified into a gigantic coherent pulse of light! A collection of excited-state atoms is called an optical amplifier.

As FIGURE 41.33 shows, the stimulated emission is sustained by placing the lasing medium—the sample of atoms that emits the light—in an optical cavity consisting of two facing mirrors. One of the mirrors will be partially transmitting so that some of the light emerges as the laser beam.

FIGURE 41.33 Lasing takes place in an optical cavity.

Although the chain reaction of Figure 41.32 illustrates the idea most clearly, it is not necessary for every atom to be in level 2 for amplification to occur. All that is needed is to have $N_2>N_1$ so that stimulated emission exceeds absorption. Such a situation is called a population inversion. The process of obtaining a population inversion is called pumping, and we will look at two specific examples. Pumping is the technically difficult part of designing and building a laser because normal excitation mechanisms do not create population inversions. In fact, lasers would likely have been discovered accidentally long before 1960 if population inversions were easy to create.