Lasers are widely used in technology, science and entertainment. Almost everyone has already used one (at least in the CD player), but only a few people know how it works although it is - to a certain extent - quite simple. The history of the development, the uses and types of laser can be found in the Laser entry. This entry will focus on the working mechanism of a laser. For a reasonable understanding of the involved mechanisms, a little digression about light emmitting processes is included (the first two sections) and can be skipped (a brief summary is included right after these chapters).

Absorbtion (or Excitation)

Things can absorb light, and this is the reason why things look like the way they look like. Taking a closer look on how the absorbtion takes place, will later make the concept of emission understandable. To make it less complicated, it is best just to accept that certain colours are absorbed better than other colours. When a flash of light, or a photon, with the best fitting colour hits a very small piece of matter like a molecule (or an atom) it may be absorbed. Since the Energy cannot just be eliminated, the molecule will start behaving differently. Scientists say that the molecule is in an excited state (it can redirect the energy into motion: it can start wiggling about, rotate, send the electrons into more complicated orbits or whatever). The energy of the absorbed photon is now in the excited molecule (or atom) and the photon is lost. That is absorbtion. An example to illustrate this: White light can be analysed using a prism, what one sees is that the white light is composed of all colours of the rainbow. When shining white light through a red-dyed glass it changes its colour. When analysing this light one will see that the green and blue part of the spectrum are missing. Why? These components have been absorbed by the dye molecules in the glass.

Relaxation

After the molecule (or atom) has been excited it is in a so called high-energy state. This energy can dissipate mechanically, causing the material to heat up (this process is called radiationless relaxation), or after a while the molecule can 'decide' to emit a photon (which usually has a lower energy as the original photon used to excite the molecule) and relax. There are two light emission mechanisms:

Spontaneous Emission

After a certain while the photon is emitted in any direction. This process is the one observed for Neon-lamps. It is not influenced by anything except for the molecule's (or atom's) intrinsic properties and statistics. That is: How long in average it takes for this process to occur and how strong it is compared to the radiationless relaxation described above.

Stimulated Emission

Another photon coming from somewhere else can trigger an excited molecule to relax and emit the excitation-photon. In order for that to happen both photons (the triggering one and the one to be released) must have the same energy (i.e. colour). In contrast to the spontaneous emission, the emission is now governed by the incoming trigger-photons, and the emitted photon will have exactly the same direction, energy and phase (because the releasing and the triggering occur in a synchronized way) as the incoming trigger-photon. This kind of emission will be used to generate laser-light.

Summary

There are four processes involved in the light emmission process. The absorbtion of photons by a material (which will be henceforth called medium), which is then in an excited energy-state; the radiationless relaxation, the spontaneous emission and the stimulated emission (latter two are responsible for light generation in a laser). Materials can be analysed or designed to have a good absorbtion yield, a small radiationless relaxation rate, and a high fluorescence rate (which is the spontaneous and the stimulated emissions together). When that is the case almost all input energy can be converted into light, such a material is said to have a good quantum yield (the percentage of the energy that can be converted into light¹).

How exactly is laser-light generated?

The medium where the laser-light is going to be generated is typically one with a high quantum yield. This medium can be excited² to emit light by many means (applying electric currents, allowing chemical reactions, flashing light into it, etc.). Right after the medium is excited for the first time, spontaneous emission will take place: Photons are generated and they will fly away in any direction. This procedure is continuously repeadted many millions of times in one second, so that one could think of a medium that is constantly in an excited-state and at the same time emitting photons.

This medium is placed between two mirrors. Eventually the trajectory followed by one of the many photons that are comming out of the medium will be aligned, so that it will lead light exactly to one of the mirrors, back to the medium and to the other mirror. A photon on that trajectory will follow it a few million times before it is scattered away. Such a photon when passing the medium will be trigger the stimulated emission. The two photons, i.e. the trigger photon and the emitted photon will have the same direction, energy and phase, the light is for that reason called coherent. At this moment there will be two photons on exactly the same path. On the next moment (when the two photons return) they will force two more photons onto the same path, next time there will be 8 photons on the path, then 16, 32, 64 and so forth. Eventually one or two photons can be lost³, so next time there will not be 128 but 126 photons on the path, then 250, 498, 990 and so forth. The number of photons will grow until a certain limit is reached. The limit is reached when: The excitation rate is equal to the emission rate minus the loss of photons. That light kept between the mirrors is laser-light.

The next thing to do to getting the light out of the mirror-medium-mirror arrangement, which in analogy to acoustic resonators is also called an optical resonator⁴ (sometimes the resonator is also called 'cavity', which is way shorter, and therefore widely used). In order to do that one of the mirros is designed to reflect only 99% of all the light, so each time 1% of the laser light is let out. That implies that the treshold for laser activity must be lowered accordingly (the loss of photons term).

A further consideration concerns the distance between the two mirrors. It must be an integer multiple of the wavelength of the laserlight (otherwise the light will annulate due to destructive interference). For that reason most lasers can be tuned, that is, the colour coming out of the laser can vary. The tuning range can be very low, if the emission band is narrow (e.g. 1nm - no visible colour difference⁵), and it can be wide when the emission band is broad (e.g. 200nm - a very noticeable change). The width of emission bands depends on the used medium, and is except for gas-lasers usually quite broad.

A further thought considering the amplification: In order for light to be amplified, there must be more photons coming from the relaxation process (stimulated emission) than being used to excite the molecule (by absorbtion). For this to happen the number of excited molecules must be greater than the number of relaxed molecules (otherwise the photon will be absorbed and not used to stimulate emission). This particular situation is called population inversion. Which is easily achieved when the light emitting relaxation process does not lead directly to the lowest (or ground) state, but to one above that, which is best rapidly depopulated (e.g. by radiationless relaxation) and therefore almost always empty (this ensures the constant population inversion).

Recap

Roughly how a laser works: Light is generated somewhere between two mirrors (which is an arrangement called an optical resonator) and kept exactly between them, it bounces back and forth and generates ever more light by stimulated emission. For that to happen there must be more excited molecules in the medium than relaxed ones (otherwise the photon will be absorbed and not used to stimulate emission), a situation called population inversion. Light genetarted by stimulated emission has the same energy, direction and phase as the light that was used to stimulate the emission, it is therefore called coherent light, this is laser-light. If the generation of light is stronger than the loss (by mirrors or air), more and more light will accumulate between the mirrors. One of the two mirrors is designed only to reflect 99% of the light, so everytime 1% of the resonating laser-light will be let out. More details, like the history, the uses and types of lasers can be found in the Laser entry.