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How a Laser Works

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Lasers are widely used in the fields of technology, science and entertainment. Almost everyone has used one in daily life (possibly without knowing - there's one in your 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 focuses on the working mechanism of a laser. For a reasonable understanding of the involved mechanisms, a little digression about light-emitting processes is included (the first two sections) and can be skipped - a brief summary is included right after these chapters.

Absorption (or Excitation)

Objects can absorb light, and this is the reason why things look the way they do. Taking a closer look at how light absorption takes place makes the concept of emission more understandable. To make it less complicated, it is best just to accept that certain colours are absorbed better than other colours are.

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, wiggling about, rotating, sending 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. This is absorption.

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 analyzing this light one will see that the green and blue parts 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 '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 than 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, ie 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 (ie 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 is used to generate laser light.

Summary

There are four processes involved in the light emission process:

  • The absorption of photons by a material (which will be henceforth called the medium), which is then in an excited energy-state

  • The radiationless relaxation

  • The spontaneous emission

  • The stimulated emission

The latter two are responsible for light generation in a laser.

Materials can be analysed or designed to have a good absorption yield, a small radiationless relaxation rate, and a high fluorescence rate (which is the spontaneous and the stimulated emissions together). When this is the case, almost all input energy can be converted into light, and such a material is said to have a good quantum yield (the percentage of the energy that can be converted into light). This can be close to 100% for certain dye molecules.

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 excited1 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 repeated many millions of times a 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 coming out of the medium will be reflected by one of the mirrors straight back into 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 trigger the stimulated emission. The two photons, ie the trigger photon and the emitted photon will have the same direction, energy and phase - the light is therefore called coherent.

At this moment there will be two photons on exactly the same path. At the next moment, when the two photons return from a mirror, 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 lost2, 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. The light kept between the mirrors is laser light.

The next stage is to get the light out of the mirror-medium-mirror arrangement, which is called an optical resonator3 in a manner analogous to acoustic resonators. (Sometimes the resonator is also called the 'cavity', which is much shorter, and therefore widely used.) In order to do that, one of the mirrors 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 threshold for laser activity must be lowered accordingly (the 'loss of photons' term).

A further consideration involves the distance between the two mirrors. It must be an integer multiple of the wavelength of the laser light (otherwise the light will annihilate itself due to destructive interference). For that reason, most lasers can be tuned - ie the colour coming out of the laser can vary. The tuning range can be very low if the emission band is narrow (eg 1nm - no visible colour difference4), and it can be wide when the emission band is broad (eg 200nm - a very noticeable change). The width of emission bands depends on the medium used, and is, except for gas lasers, usually quite broad.

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 molecules (by absorption). 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 and 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 most easily and rapidly depopulated (eg 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 generated by stimulated emission has the same energy, direction and phase as the light that was used to stimulate the emission, and is therefore called coherent or 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 every time 1% of the resonating laser-light will be let out.

1In laser jargon this process is often called pumping.2Due to spontaneous emission, absorption or scattering.3The length of typical optical resonators varies from a few millimetres (diode lasers) to one or two metres (dye or crystal lasers).4The visible spectrum ranges from violet (wavelength 400nm) to deep red (wavelength 800nm).

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