Laser Basics

Population Inversion and Stimulated Emission

In most materials at room temperature, the majority of the electrons in atoms are in an unexcited energy level or a ‘ground’ states. By putting sufficient energy into the material it is possible to have more electrons in higher energy states than in the lower energy states. When this is the case it is called a population inversion.

The electrons become excited by absorbing energy, but cannot retain this energy for long and then release it in the form of photons. These photons small packets of energy that are very mobile. They can go on and may collide with other excited electrons causing them to release an identical photon. This is stimulated emission. The light energy is amplified by the photons that collide with other excited electrons, which causes them to simultaneously release yet more photons. Because of this they will move in step or phase with one another which a is a further property of laser light known as coherence.

This amplification process continues only as long as there is a population inversion in the laser material. In other words, energy needs to be ‘pumped’ into the system to keep the process going. If the laser tube has mirrors placed at either end, the light can be made to travel backwards and forwards along the length of the tube and continue this process of stimulated emission. The front mirror of the laser tube is partially coated which allows a thin, parallel beam of laser radiation to be emitted from the laser tube. As long as the mirror reflects back enough photons to keep up the amplification process, the lasing action will continue.