What is population inversion in a laser?

When a photon strikes an excited atom, that single photon transforms into two identical photons. Those two photons can then strike other excited photons, resulting in 4 photons, and then 8, and so on. That is basically how a laser works.

But atoms are not naturally excited. Quite the opposite.

When left to their own designs, atoms prefer the stable, low-energy, ground state. Photons that collide with these unexcited atoms are not duplicated, but absorbed. The chain reaction slows and eventually stops completely.

We speak of population inversion when we manage to reverse the natural tendency. We speak of population inversion when more than 50% of atoms are excited and less than half remain in the ground state.

How do we achieve population inversion in a laser?

To obtain population inversion, energy needs to be pumped into the system to excite the atoms. The 2 most popular methods for doing this are electrical pumping and optical pumping.

Threshold pump power

Excited atoms naturally decay back into lower energy states. This decay will happen more or less quickly depending on the specifics of the laser.

To have a buildup of excited molecules (and eventually reach population inversion), it is necessary to excite particles faster than they decay into the ground state. This minimum rate of pumping is called the threshold pump power.

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Electrical pumping

As the name suggests, the pump power used to excite atoms in this case is of electrical origins.

An example of an electrically pumped laser is the CO₂ laser. In CO₂ lasers, a strong electrical current is run through a mix of gases, including carbon dioxide (CO₂), nitrogen, and helium. Each gas contributes to the end goal of having more excited CO₂ molecules than ground-state ones.

Optical pumping

The first-ever laser was an optically pumped laser. Since then, many other optically pumped lasers have been created using flash lamps, lasers, and even the sun as a source of pump power.

One of the advantages of optical pumping is that it can be used even when the lasing material is an electrical insulator. For example, fiber lasers are made out of polymers and glasses which cannot carry electrical current, making electrical pumping impossible.

The basic operating principle of optical pumping is the same as electrical pumping. However, because the pump energy is light (just like the laser itself), optically pumping a 2-level laser is impossible. Only 3-level and 4-level lasers can be optically pumped.

Summary

Population inversion is a necessary condition for all lasers, regardless of their power or wavelength, because all lasers operate on the same basic principle of a chain reaction.

Excited atoms fuel the laser chain reaction.

Ground-state atoms slow it down.

In order to have a functioning laser, there need to be more excited atoms than ground-state ones. We call that population inversion because it’s an inversion of the natural order.