Modern lasers mostly function as intensely focused light

How a Laser Builds Its Beam Inside a Cavity

A laser needs more than a material capable of emitting light. It also needs a way to keep useful light passing through that material long enough to multiply. Most lasers do this with an optical cavity: a space between two mirrors, with the light-producing material—called the gain medium—inside.

Energy supplied from outside, by electricity or another light source, raises the gain medium into an excited state. When light of the right kind passes through, it can trigger excited particles to release energy as additional light. The new light travels in step with the triggering light, so repeated passes can strengthen a coordinated wave rather than simply adding unrelated flashes.

Why the Mirrors Matter

One cavity mirror reflects nearly all the light back through the gain medium. The other reflects most of it but lets a small portion escape. That escaping portion is the beam; the reflected portion continues the buildup. This repeated feedback is why a laser can produce a strong, narrow output even though the beam does not consist of light that was physically compressed into a smaller space.

The cavity also favors light that can make the round trip between its mirrors in step with itself. In effect, the mirrors help select the directions and wavelengths that reinforce one another. The medium supplies the light’s possible colors, while the cavity helps determine which of them become prominent.

A Balance Between Gain and Loss

Light is lost each time it passes through the cavity—some escapes through the output mirror, and some is absorbed or scattered. A laser begins sustained operation only when the energy supplied to the medium allows the light amplification to overcome those losses. Below that point, light may be emitted, but it cannot build into the same self-reinforcing beam.

That balance explains a laser’s distinctive design: the gain medium creates amplification, while the mirrors provide repeated opportunities for it to happen. The visible beam is the small fraction deliberately allowed out of that ongoing loop.