Coherence or phase balance of a laser beam

In the natural sciences, coherence or coherent radiation refers to electromagnetic waves that have a fixed phase relationship in terms of their spatial and temporal propagation. In terms of how this relates to everyday life, this theoretical and somewhat cumbersome definition simply refers to the light beam generated by a laser source.

Term localisation

To explain coherence more specifically, we need to first take a look at the laser beam itself. The “light amplification by stimulated emission of radiation” is a collective term from physics. The term laser represents both the physical effect, i.e. the light beam itself, and the corresponding laser device (laser machine, laser source).

Laser beams

  • Are electromagnetic waves
  • Often have a very narrow frequency range (narrow-band emitter)
  • Consist of a sharply focused light beam
  • Have a long coherence length, depending on the type of laser

In everyday life, lasers are found and used in a wide variety of applications. It could be as simple as a laser pointer, which is often used in presentations and in power tools, or it could be for reading optical storage media such as Blu-rays or CDs: The laser beam has become indispensable in modern life.

The phase balance

The term coherence, derived from Latin, roughly means “connected". The term refers to specific properties of electromagnetic waves in physics. These waves have a fixed phase relationship between two wave trains. If this phase relationship remains constant, it is possible to generate a stable interference pattern. 
With coherent light, a further distinction is made between temporal and spatial characteristics. Both characteristics can be well illustrated by using a small thought pattern. 

Temporal coherence

If you were to stand next to an electromagnetic wave that consisted of several wave trains and let it pass by, the phase relationships of two wave trains would not change. They remain unchanged in the propagation direction of the wave.

Spatial coherence

If there were a frame of reference for light and you were able to place yourself in it (and connect the frame of reference with the electromagnetic wave) and then you looked perpendicular to the wave, you would discover that the phase shifts between two waves do not change.

The difference to incoherent light

An ordinary light source, e.g. a ceiling light, emits light that is made up of many individual wave trains. For all natural light sources, the emitted wave trains are not coherent. The reason for this lies in the actual source of the light being emitted: the atoms. If a single wave train is emitted in a light emission process, this takes around 0.0000000001 seconds. From this, the theoretical length of this wave train can be calculated: 3 meters. Now if we go back to the atomic level and look at an atom that emits a wave train. If we then stand next to the path that the light travels and look at the first wave train passing us. At some point (this period of time is not defined) the atom emits the next wave train. This wave train also has “mountains” and “valleys”, which are in a well-established but completely arbitrary phase relationship with the first wave train. This also applies to all other emitted wave trains. It is for this reason that there is no fixed phase relationship between any of the individual wave trains emitted by an atom - it changes from wave train to wave train. In addition to this: ordinary light sources emit light with different wavelengths. For wave trains with different wavelengths, the phase difference changes naturally. And: Ordinary light does not radiate in a parallel, but in a multitude of different directions.

The temporal and spatial in-phase light of a laser

Laser light is electromagnetic waves that are coherent both temporally and spatially. Here, a fixed phase relationship can be seen in both the propagation and perpendicular direction. In laser light, the individual wave trains are very long, and at the same time, the adjacent wave trains oscillate in a common mode.

Properties of the laser light

Laser machines emit extremely focused light beams. These beams run together in a straight line and show virtually no scattering. In contrast, there are conventional light sources that emit light waves scattered in all directions. With a laser beam, all light waves are of the same colour. This condition is called monochromatism. During the movement of the light waves in a laser beam they oscillate in perfect synchronisation.

Classification of lasers in different classes

Laser beams can be extremely dangerous for humans depending on the light emitted. Therefore, laser machines have been divided into different machine classes, whereby the classification is carried out by the respective manufacturer according to DIN EN 60825-1.
Class 1 refers to lasers whose radiation is completely harmless. From class 2 onwards, serious damage to the eyes and retina can occur if the laser beam is aimed directly at the eye for a period of time that exceeds 0.25 seconds. Class 3B lasers are also extremely dangerous to the eye and can even damage the skin. Finally, class 4 refers to machines whose lasers will damage the eye extremely quickly, are also dangerous to the skin, and even the lasers scattered radiation is dangerous and capable of causing fires or explosions.

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