LASER and light amplification

Lasers are everywhere. In science, medicine, industry… application field are large, very large. LASER means Light Amplification by Stimulated Emission of Radiation. Used by the military, the arms manufacturer Lockheed Martin has, for example, developed a laser anti-missile turret system for the protection of ships. It is also used in medicine, for example in refractive surgery to correct myopia. Industry uses it for machining and for sensors. But do we really know how these systems work?

How LASER Works ?

To understand the functioning process of LASER, it’s important to visualise the composition of matter. Every element is made up of atoms linked together in various ways. An atom can be represented as an element composed of nucleons (protons and neutrons) and layers of electrons. The electrons in the different layers have different energy levels.

Diagram of an atom and its electronic layers

It is also important to understand what a quantum is. It is the smallest indivisible quantity used in physics. For electromagnetic waves, the energy quanta is the photon, which is the smallest indivisible energy level associated with electromagnetic waves. This applies to radio waves, X-rays or UV rays...

To understand, a pictorial demonstration

To understand the rest, there are two other points to assimilate. Small demonstration and manga references: do you know Dragon Ball and Naruto? Funny question, isn't it? But there is a link with the subject. In the manga Dragon Ball, when the hero Goku concentrates, he can increase his energy level and reach another stage. He accumulates his energy and uses the Kaioken. Later on, it is discovered that he is able to go to a higher level called super Saiyan, then Saiyan 2, etc. With each passage Goku gives off more and more power.

Progressive evolution of San Goku's states: Normal, Kaioken, Saiyan 1, Saiyan 2 and Saiyan 3

For the younger ones, we can also use the analogy of Naruto. When he loses control and Kyubi takes control of his body, we see a shakra tail growing. The more he loses control, the more the number of tails increases, and the more the character gains in power and energy.

Naruto's progressive transformation: Normal, 1-tail demon, 2-tail demon and 3-tail demon

Here is the link with lasers: when an electron with an energy level E1 receives a photon (of energy E=hv ), the electron will absorb the energy of the photon. It then passes to a level E2. This is called absorption. In this phase, the electron gains energy and moves to another level.

If we bring it back to Goku or Naruto:

Goku concentrates and absorbs energy, he goes into Kaioken mode

Scheme of Goku absorbing energy

In Naruto's case, he absorbs fox shakra and ends up as a fox demon with only 1 visible tail

Scheme of Naruto's energy absorption

Our two characters will go from the E1 stage to the E2 stage after having absorbed energy (a photon E=Hv). Then they will use this extra energy to launch super attacks (kame hamé ha, razengan, etc.), but after having spent a large amount of energy, they will fall back to their starting level.

For electrons, it is the same phenomenon. They absorb photons and pass from an E1 level to an E3 level. They will then release energy to go to an E2 level which is between E1 and E3. This energy release is a non-radiative emission, which means that they release energy that is not visible. Finally, to move from the E2 level to the E1 level, they release energy in the form of photons of a specific frequency. This emission is the radiative emission. This is our famous LASER emission.

Level variation of energy in an electron of stimulated emission

Immersion in a LASER

A LASER is made of three main elements

- - A pump

- - An active medium

- - A cavity

Photon of energy E= h*ν

Energy electron E2

Re-emitted photons of energy E=h*ν

The pump is the starting point of the LASER emission. It will generate photons and send them to the system. These photons will be absorbed by the electrons of the active medium which, when sufficiently charged, will emit two identical photons, after having absorbed one.

Scheme of an electron during stimulated emission.

The photons emitted by these electrons will then be absorbed by nearby electrons. These electrons will change their energy level, and then discharge by spontaneous emission. This generates two additional photons for each electron that de-excites. This phenomenon will propagate from electron to electron and create a snowball effect. This is why we speak of amplified light. The more the phenomenon propagates, the more electrons pass from the E1 level to the E3 level: this is called the pumping phase. In the active medium, when there are more electrons at the E3 level than at the E1 level in the cavity, the population inversion phase is reached

Scheme on population inversion

It's a bit like a zombie invasion. The first zombie bites a person. This infected person will bite another, who will bite another, and so on. This is the pumping phase. In the end, when there are more zombies than humans, there is a "population inversion".

The active medium is a medium made up of a material that amplifies light. To create the reaction explained above, media composed of crystals are most often used. For example, sapphire, chromium, silicate... Not just any material can be used: only certain materials can create this light amplification.

Exemple of material that can constitute an active medium

Cavity, where the light amplification takes place

The LASER cavity is the part that contains the active medium and allows the flux to be directed. At one end of the cavity there is a mirror, opposite this at the other end of the cavity there is a semi-reflecting mirror (the output mirror). During the pumping phase, the photons emitted by the different electrons will "bounce" back and forth in the cavity against the mirrors. If a photon hits one of the two mirrors, it is reflected back to the other side of the cavity, and must therefore pass through the active medium again. As a result, it can still participate in the stimulated emission. This is how the snowball effect is created, and thus the amplification of the light.

Scheme of optical cavity of a LASER

The distance between the two mirrors on either side of the cavity must be perfectly controlled to ensure that the laser beam leaving the cavity has the finest possible wavelength. We are looking for a monochromatic wave. We will then try to have a distance between the mirrors equal to x*λ (with λ the wavelength we want to favour).

Future prospects for LASER:

LASER is a technology that has been widely democratised. Although this tool is fairly well mastered, there are many limitations to its use. The power emitted by lasers can be increased by reducing the beam emission time. This is the principle of ultrashort lasers. These give off significant energy thanks to their short emission time. In the future, we could think of increasing their power, but also the fineness of the wavelength of the beam emitted. In view of all the fields of application, this would make it possible to increase production in the field of laser machining and its precision with increasingly shorter emissions. Better control of laser beams will also make it possible to make great strides in the quality of images obtained in laser holography. All the potential of laser technology has not yet been explored, but it is highly likely that this technology will bring further great progress in many areas.

Sources :

1.http://lamh.gmc.ulaval.ca/opus/physique534/optique/laser08.shtml

2.http://www.perrin33.com/biochanalys/photons/absfluo-uvvisible-3.php

3.https://fr.wikipedia.org/wiki/Saphir

4.https://www.lockheedmartin.com/en-us/capabilities/directed-energy/laser-weapon-systems.html

5.https://www.medicalexpo.fr/prod/limmer-laser/product-69117-672245.html