What are the principles of a laser


Every laser essentially consists of three components: the Laser medium, the pump and the Resonator. The structure, tasks and functions of these three components are described below.

The laser medium

The laser medium is used to amplify the laser light through the stimulated emission. How this works in principle was described in the previous section.

Various substances can serve as the laser medium. There are solid, liquid and gaseous variants. The search for suitable materials took many years and research laboratories around the world are still looking for new laser media today. In any case, a laser medium must have the property that a so-called Population inversion can be produced in it. What is meant by this population inversion?

One can easily consider that the light amplification in a laser only works if the number \ (N _ {\ rm {stimulates}} \)   The number of photons generated by stimulated emission per unit of time is greater than the number \ (N _ {\ rm {absorbed}} \) of photons absorbed by the laser medium per unit of time. So that light amplification is possible, \ [N _ {\ rm {stimulated}}> N _ {\ rm {absorbed}} \] EINSTEIN was able to show that the probability for the occurrence of the stimulated emission and the absorption is equally high. If light amplification is to take place, more atoms or molecules must be in the energetically higher state than in the energetically lower state.

Normally (in thermal equilibrium) the occupation number of the energetically lower level is always higher than that of the energetically higher level (see adjacent picture). In order to achieve light amplification, this state must be reversed, one needs the so-called Population inversion. However, this only works with certain media (e.g. ruby ​​crystals or a gas mixture of helium and neon) by using special atomic properties. These properties are explained in more detail in the "Laser Media" section.

The pump

The task of all pumps is always the same: to bring energy into the system and thereby generate a population inversion. This process will pump called because energy is "pumped" into the system.

The excitation can result from light irradiation (optical pumping), but also electrical processes, heat, collisions with other particles and chemical reactions can transfer the required energy into the system and generate a population inversion. For example, the ruby ​​laser is pumped by a powerful flash lamp, while the He-Ne laser is pumped by electron impacts.

The resonator

If photons have been released by the process of spontaneous emission in the laser medium and if these have subsequently released further photons with the same energy and direction of movement through stimulated emission, then two problems arise:

• The photons move in a purely random direction.

• There are still very few photons, so the laser light is still very weak.

The so-called resonator solves both problems in an ingeniously simple way. In addition, its name suggests another task that will be described in more detail in a moment. And finally, the exit of the laser beam from the laser is also regulated by the resonator.

The resonator consists of an arrangement of two mirrors. These are set up exactly parallel to each other. The laser medium is located between the two mirrors. If the medium consists of a solid, the mirrors are often glued or vapor-deposited directly onto two opposite surfaces.

1. The gain and direction of the laser beam