Probably everyone has already heard of lasers, as being a strange and forceful form of light used in CD-Players or in tattoo-removal and which have the potential to destroy entire planets as seen in George Lucas' 'Star Wars' or at least incoming nuclear missiles as seen in Reagan's 'Star-Wars'. Probably everyone has actually seen laser-light in presentations, parties or rock-concerts. And probably everyone thinks of lasers as being something essentially groovy. Few people realize that the laser is an interesting piece of science and technology which has its roots way back in 1917. Few people realize that laser is more than just bundled light, with some amazing properties. Even less people understand how a laser works, even though it is - to a certain extent - quite simple.

Summary

Laser is an acronym for light amplification by stimulated emission of radiation^1,2 coined by Gordon Gould in the 1950s. A brief and incomplete description of how it works (just to get the picture): Basically laser is light being generated by light in a very particular way. This light generating medium is kept between two mirrors, where it will generate more and more light (that is why it's called light amplification). One of the mirrors is designed so that it will only reflect 99% of the light, the rest (1%) is let through. That 1% is the laserbeam coming out of the laser. A more exact explanation can be found in How a Laser Works.

The usage of the word 'laser' evolved, so that nowadays when referring to a laser a person is probably meaning the machine that generates laser beams. 'Laser light' is nowadays what the original meaning of 'laser' was: The laser light coming out of 'the laser'. There are different types of lasers, ranging from ultra light and small microlasers to enormous high-precision and high-power lasers. Lasers can be used in innumerous applications such as CD-players or microchip lithography.

The history of the 'laser'

The concept of stimulated emission was brought to us by no one less than Albert Einstein in 1917. The way a laser should work was at least in theory clear by then, it remained a technical problem though, which was eventually solved by C.H. Townes in 1954 for microwave radiation (the correct word for this kind of 'laser' is therefore 'maser', because it is not light being amplified but microwaves) and later in 1964 by T.H. Maiman for the visible part of the spectrum (a real 'laser'). The Nobel prize of 1964 for the work on 'lasers' went to Townes, N.G. Basov and A.M. Prokhorov, leaving some celebrities hurt. It should be remarked here, that there has been a huge discussion about 'who really invented the laser'. In the middle of this debate was Gordon Gould, the - supposedly - first person to suggest that a laser could work (at least it was him who coined the word 'laser'). He eventually (in 1977) won the decisive patent-suit³ and his company, Patlex, received 5% of the cost of each laser produced in the 20 years. If it was him or not is still being debated, at least it is him who holds the patents on optically and electrically pumped lasers (80% of all used lasers) and on fiber optics communication. In the 1970s a raging 'lasermania' swept through many research labs around the world, leading to the invention of a variety of new laser-types. But the common posture of the time was that the laser was a cool invention looking for a job.

What is so cool about laserlight?

Because of the way this light is generated, there are some properties that people find very interesting. The perhaps most important property is the coherence. Coherence is generally a measure of perfection of anything, in this case the light. It will describe how parallel it is, if the light (as an electromagnetic wave) is 'in phase' (i.e. maxima and minima are in the same place) and how monochromatic it is. Another cool thing about lasers is that they are really intense

Parallel: The light generated in the laser medium is highly ordered, all photons have exactly the same direction. For this reason laserlight has an enormous range (depending on the laser type - cf. below - up to hundreds of kilometres) without losing too much in intensity. It can also be focussed very sharply. This has two consequences: First, the light intensity in the focus can be increased by 10 orders of magnitude. Second, the optical resolution is dramatically increased in optical devices such as a microscope.

Phase: Normal light can be manipulated to become quite parallel, but it will never be 'in phase'. Interference will not be observable using normal light. Interference in its turn is crucial for holography and interferometry (a highly precise method for measuring distances). Common Neon-light can be generated 'in phase' unsing some tricks, but it will only be in phase for a distance of about 10 cm. Laserlight remains 'in phase' for hundreds of kilometres.

Monochromacy: Because all the generated photons have the same colour, the light is said to be monochromatic. In reality, even laser light is not absolutely monochromatic, but its linewidth (that is, the variation of colours) can be thousands of times smaller than the best non-coherent monochromatic lightsource (like Neon-Lamps). For that reason laserlight can be used in high precision spectroscopy. It is a common misconception that one of the definitions of laserlight is its monochromism. In fact, lasers do not necessarily have to be monochromatic. (There are even white lasers)

Intensity: Since the emission of the photons from the medium is synchronised all the light being generated can be present in one single pulse of light. The mirror arrangement can be desinged in such a way that 100% of the light is kept between the mirrors and then for a fraction of a second all 100% let out. This laserpulse is quite short (ps or 10^-12 seconds), but humongously intense (the most powerful lasers are pulsed polychromatic solid state lasers and they have an output of up to 1 TW - which is more than enough energy to start nuclear reactions).

Ways of generating laser-light and types of lasers

The working principles of a laser is described in the entry How a Laser Works. As stated before, the laser light is generated in an arrangement of a light emitting medium placed between two mirrors, which is called an optical resonator (in analogy to acoustic resonators). Much of the laser-light's properties (that is its coherence and intensity) and the design of the laser generating machine (and its weight, efficiency, cost etc..) are determined by the type of medium used to generate the laser.

Diode lasers are the most common, lightweight, cheap and endurable lasers (as found in the typical red laserpointers or in the CD-Player). The medium used to generate the light is a semi-conducting material (like Indium or Gallium doped Arsenic). The excitation is generated by electricity. Diode lasers have a inferior coherence and intensity (up to 0.5W). Diode laser arrays can produce up to 1000W are also used to pump more sophisticated lasers.

Solid state lasers were the first lasers to be invented (Maiman's Ruby laser). The medium consists of solid crystals, like Garnet or Corundum doped with metal ions: Nd (Neodymium)in Y-Al-Garnet⁴ (NdYAG), Ti (titanium) and Fe (Iron) in Corundum (or Ti:Sapphire) Cr in Corundum (Ruby). The metal ions are responsible for the light generation. They are excited by light (like Flashlamps or diode lasers). Such lasers are not too expensive (e.g. green laserpointers are frequency doubled NdYAG lasers), easy to maintain and can be quite small (green laserpointer) but for most applications they are quite big (one would need an extra room and high-voltage outputs). Solid state lasers can be tuned, used in pulsed mode (high energy), and are very stable. NdYAG lasers are also used for military purposes as part of the missile or bomb guideance

Gas lasers were among the first lasers to be developed (Townes' Ammonium maser 1954, 1960 He-Ne⁵ laser by Ali Javal). The light emission comes either from electronic transitions e.g. Xe-laser⁶ (ultraviolet), N₂-laser⁷ (ulraviolet), Kr-Ion-Laser⁸(blue, green or red), He-Cd-Laser⁹ (blue), He-Ne-Laser (red) or vibrational transitions in the CO₂-Laser (infrared). All these transitions are induced electrically (by electrical discharges). Gas lasers are in general easy to maintain and are the only stable lasers in the blue and ultraviolet part of the spectrum (Xe, N₂), they are therefore used in microchip lithography. CO₂-lasers are one of the most powerful laser sources, they are the most common types of lasers found in industrial, technological and medical applications. The big disadvantages of gas lasers is that they are not tunable and very expensive to operate due to their very low efficiency(smaller than 0.01%).

Dye lasers are high quality lasers, they offer high coherence and good intensity. The light emitting medium consists of a dye solution (typically some polyaromatic organic dyes like Rhodamine or Coumarin in organic solvents) and is excited optically (mostly by other lasers). They are difficult to maintain and quite expensive, they have very narrow lines, are not as stable as the other lasers and are tunable (tuning range 20nm per dye). Dye lasers were for a long time the only laser-source for virtually any line in the visible part of the spactrum. Nowadays dye-lasers are only used in scientific research.

Chemical lasers use photochemical reactions for excitation and emission (HF or I₂lasers). This laser class operates in the 1-3µm wavelength range and is really difficult to handle. There is some interest in this area since they can generate 2-5 · 10⁶Watts (a lot) unpulsed.

Other kinds of 'laser' are the Free Electron Laser (FEL) and Synchrotron Radiation (SR) which are not lasers in the classical sense but generate some high power coherent radiation (that cannot be distinguished from the conventional laser-light). These 'lasers' require stuff like linear accelerators and - well - synchrotrons (which can be as big as a small town). Nevertheless these light-sources are quite interesting, since they are the only ones to coherently reach very short wavelenghts (X-Rays).

Uses of Laser

Lasers can be used:

Lasers and Safety

Like all radiation, after a certain intensity also lasers can be dangerous. Most lasers (except for the rare high power starwars-lasers used to ignite nuclear reactions) are quite harmless to the skin. Focussed lasers can cause severe burns. The most endangered organ are the eyes. Even low-power laser can irreversibly damage the retina, since the beam is focussed by the eye's lens. (Laser-pointers and show-lasers are usually approved by the appropriate authorities, and are relatively safe - very low intensity - even for the eye). Most other lasers should be considered dangerous and precautionary measures taken (special laser-protecting glasses). Official info on safety can be found at the following internet-site:
laserinstitute.org (Laser Institute of America)