Probably everyone has already heard of lasers as a strange and forceful form of light, used in CD players or in tattoo-removal, which have the potential to destroy entire planets, as seen in George Lucas' Star Wars movie, or at least incoming nuclear missiles, as seen in Reagan's Star-Wars programme. Most people have actually seen laser light in presentations, parties or rock concerts, and probably everyone thinks of lasers as being something essentially groovy.

However, few people realise that the laser is an interesting piece of science and technology which has its roots way back in 1917, and that a laser is more than just bundled light with some amazing properties. Even fewer people understand how a laser works, even though it is, to a certain extent, quite simple.

Summary

LASER is, in fact, an acronym for Light Amplification by Stimulated Emission of Radiation¹ coined by Gordon Gould in the 1950s. Laser light is formed by light being generated in a very particular way. A light-generating medium is kept between two mirrors, where it generates 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 back inwards, with the rest (1%) let through. That 1% is the laser beam. A more exact explanation can be found in How a Laser Works.

The usage of the word 'laser' has evolved to refer to the machine generating the beam, rather than the beam itself. 'Laser light' is now seen to refer to the original meaning of 'laser' - the actual laser beam. There are different types of lasers, ranging from ultra-light and small micro-lasers to enormous high-precision, high-powered lasers. They can be used in numerous applications such as CD players or microchip lithography.

The History of the Laser

The concept of stimulated emission was brought to us by none less than Albert Einstein in 1917. The way a laser should work was, at least in theory, clear by then. However, it remained a technical problem until 1954, when it was eventually solved by CH Townes for microwave radiation² and later in 1964 by TH Maiman for the visible part of the spectrum.

The Nobel prize of 1964 for the work on 'lasers' went to Townes, NG Basov and AM 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, supposedly the first person to suggest that a laser could work (at least it was him who coined the word 'laser'). In 1977, he eventually won the decisive patent suit³ and his company, Patlex, received 5% of the cost of each laser produced in the 20 intervening years. Whether Gould did genuinely invent the laser is still being debated, but he does hold the patents on optically and electrically pumped lasers (80% of all used lasers) and on fiber-optic communication.

In the 1970s a raging 'lasermania' swept through many research labs around the world, leading to the invention of a variety of new types. But the common opinion of the time was that the laser was a cool invention looking for a job.

What is so Cool about Laser Light?

Because of the way this light is generated, there are some properties that people find very interesting. Perhaps its most important property is coherence. Coherence is generally a measure of perfection of anything, in this case the light. It describes how parallel it is, if the light is in phase, how monochromatic it is. Another cool thing about lasers is that they are really intense. These points are expanded upon below.

• Parallel - The light generated in the laser medium is highly ordered, with all photons having exactly the same direction. For this reason, laser light has an enormous range (depending on the laser type (see below) - up to hundreds of kilometres without losing too much in intensity. It can also be focused very sharply. This has two consequences: first, the light intensity in the focus can be increased by 10 orders of magnitude, and second, the optical resolution is dramatically increased in optical devices such as microscopes.

• Phase - Normal light can be manipulated to become quite parallel, but it will never be 'in phase' - ie have all the waves with their maxima and minima in the same place. 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' using some tricks, but it will only be in phase for a distance of about 10cm. Laser light 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 line-width (that is, the variation of colours) can be thousands of times smaller than the best non-coherent monochromatic light source (like Neon-Lamps). For that reason, laser light can be used in high precision spectroscopy. It is a common misconception that one of the definitions of laser light is its monochromacy. 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. The mirror arrangement can be designed 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 laser pulse is quite short (picoseconds or 10^-12 seconds), but humongously intense (the most powerful lasers are pulsed polychromatic solid state lasers which have an output of up to 1TW (one terawatt) - which is more than enough energy to start nuclear reactions).

Laser Types and Laser Light Generating Methods

The working principles of a laser are 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 light's properties (ie 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 durable lasers, as found in the typical red laser pointers or in a CD player. The medium used to generate the light is a semiconducting material (like Indium- or Gallium-doped Arsenic). The excitation is generated by electricity. Diode lasers have an inferior coherence and intensity (up to 0.5W), but diode laser arrays can produce up to 1000W and can be 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 (eg green laser pointers are frequency doubled NdYAG lasers), easy to maintain, and can be quite small (green laser pointers for example) 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 missile or bomb guidance systems

• Gas lasers were among the first lasers to be developed (Townes' Ammonium maser 1954, 1960 He-Ne (Helium-Neon) laser by Ali Javal). The light emission comes either from electronic transitions eg Xe-laser (Xenon - ultraviolet), N₂-laser (Nitrogen - ulraviolet), Kr-Ion-Laser (Krypton - blue, green or red), He-Cd-Laser (Cadmium - 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 and are commonly found in industrial, technological and medical applications. The big disadvantage of gas lasers is that they are not tuneable and very expensive to operate due to their very low efficiency (smaller than 0.01%).

• Dye lasers are of very high quality, offering 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 tuneable (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 spectrum, but nowadays they 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 2-5MW (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 equipment like linear accelerators and 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 wavelengths (X-Rays).

Some numbers

Uses of Lasers

Lasers can be used:

• To read CDs in CD players (diode lasers, infrared)
• As pointers (diode: red; frequency doubled NdYAG: green)
• For rock show light effects (He-Ne lasers, many colours)
• In medicine for surgery⁶ (CO₂-laser, infrared)
• To remove tattoos (CO₂-laser, infrared)
• As a light source for endoscopes (many types and colours)
• As a light source for confocal microscopy (any laser)
• As a switch in alarm systems (diode lasers, infrared)
• In computer laser printers (diode lasers, infrared)
• In the navigation of planes and missiles (NdYAG, infrared)
• In telephonic communication (diode lasers, infrared)
• For welding, cutting and drilling materials (CO₂-laser)
• To measure distances (many laser types)
• To measure time (many laser types)
• For high-resolution lithography in chip manufacturing (Xe-lasers, UV)
• In advanced science (any type)
• As a reference for adaptive optics in astronomy (high power lasers)
• In holography (mostly gas lasers)

Lasers and Safety

Like all radiation, lasers can be dangerous above a certain intensity. Most lasers (except for the rare high power 'Star Wars' lasers used to ignite nuclear reactions) are quite harmless to the skin, but focused lasers can cause severe burns. At most risk however are the eyes. Even low power laser can irreversibly damage the retina, since the beam is further focused 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 information on safety can be found at the following Internet site: laserinstitute.org (Laser Institute of America)