A laser (Light Amplification by Stimulated Emission of Radiation) is a device which uses a quantum mechanical effect, stimulated emission, to generate a coherent beam of light from a lasing medium of controlled purity, size, and shape. The output of a laser may be a continuous, constant-amplitude output (known as CW or continuous wave), or pulsed, by using the techniques of Q-switching, modelocking, or gain-switching. In pulsed operation, much higher peak powers can be achieved. A laser medium can also function as an optical amplifier when seeded with light from another source. The amplified signal can be very similar to the input signal in terms of wavelength, phase, and polarisation; this is particularly important in optical communications. The verb "to lase" means "to produce coherent light" or possibly "to cut or otherwise treat with coherent light", and is a back-formation of the term laser.
Common light sources, such as the incandescent light bulb, emit photons in almost all directions, usually over a wide spectrum of wavelengths. Most light sources are also incoherent; i.e., there is no fixed phase relationship between the photons emitted by the light source. By contrast, a laser generally emits photons in a narrow, well-defined, polarised, coherent beam of near-monochromatic light, consisting of a single wavelength or hue.
Some types of laser, such as dye lasers and vibronic solid-state lasers can produce light over a broad range of wavelengths; this property makes them suitable for the generation of extremely short pulses of light, on the order of a femtosecond (10-15 seconds). A great deal of quantum mechanics and thermodynamics theory can be applied to laser action (see laser science), though in fact many laser types were discovered by trial and error.
Physics and history
The first working laser was made by Theodore H. Maiman in 1960 at Hughes Research Laboratories in Malibu, California, beating several research teams including those of Townes at Columbia University, and Schawlow at Bell laboratories. Maiman used a solid-state flashlamp-pumped ruby crystal to produce red laser light at 694-nanometres wavelength. In the same year the Iranian physicist Ali Javan invented the gas laser. He later received the Albert Einstein Award.
The basic physics of lasers centres around the idea of producing a population inversion in a laser medium by "pumping" the medium; i.e., by supplying energy in the form of light or electricity, for example. The medium may then amplify light by the process of stimulated emission. If the light is circulating through the medium by means of a cavity resonator, and the gain (amplification) in the medium is stronger than the resonator losses, the power of the circulating light can rise exponentially. Eventually it will get so strong that the gain is saturated (reduced). In continuous operation, the intracavity laser power finds an equilibrium value which is saturating the gain exactly to the level of the cavity losses. If the pump power is chosen too small (below the "laser threshold"), the gain is not sufficient to overcome the resonator losses, and the laser will emit only very small light powers.
Population inversion is also the concept behind the maser, which is similar in principle to a laser but works with microwaves. The first maser was built by Charles H. Townes and graduate students J. P. Gordon, and H. J. Zeiger in 1953. Townes later worked with Arthur L. Schawlow to describe the theory of the laser, or optical maser as it was then known. The word laser was coined in 1957 by Gordon Gould. Gordon also coined the words iraser, intending "aser" as the suffix and the spectra of light emitted at as the prefix (examples: X-ray laser = xaser, UltraViolet laser = uvaser) but these terms never became popular. Gordon was also credited with lucrative patent rights for a gas-discharge laser in 1987, following a protracted 30 year legal battle.
The first maser, developed by Townes, was incapable of continuous output. Nikolai Basov and Alexander Prokhorov of the USSR worked independently on the quantum oscillator and solved the problem of continuous output systems by using more than two energy levels. These systems could release stimulated emission without falling to the ground state, thus maintaining a population inversion. In 1964, Charles Townes, Nikolai Basov and Alexandr Prokhorov shared a Nobel Prize in Physics "for fundamental work in the field of quantum electronics, which has led to the construction of oscillators and amplifiers based on the maser-laser principle."
Laser light can be highly intense—able to cut steel and other metals. While the beam emitted by a laser often has a very small divergence (highly collimated), a perfectly collimated beam cannot be created, due to the effect of diffraction. Nonetheless, a laser beam will spread much less than a beam of light generated by other means. A beam generated by a small laboratory laser such as a helium-neon (HeNe) laser spreads to approximately 1 mile (1.6 kilometres) in diameter if shone from the Earth's surface to the Moon. Some lasers, especially semiconductor lasers due to their small size, produce very divergent beams. However, such a divergent beam can be transformed into a collimated beam by means of a lens. In contrast, the light from non-laser light sources cannot be collimated by optics as well or much. Using a waveguide such as an optical fibre though, diffraction laws governing divergence no longer apply. Other interesting effects happen in nonlinear optics.
An unforeseen discovery counter to expected and long-held laser properties, lasing without maintaining the medium excited into a population inversion, was discovered in sodium gas in 1992 and again in 1995 each in sodium and rubidium gas by various international teams. Normally, electrons in the ground state absorb the pumping and emitted radiation, thwarting the laser gain by heating up the medium. So media with electron levels and transitions amenable to the driving current are desired, and generally those which involve three or four energy levels rather than two make better lasers because the electrons are kept above the ground state, excited, and optically-transparent so as not to heat up, but such media are prone to noisy beams. By using an external maser to induce "optical transparency" in the media by introducing and destructively interfering the ground electron transitions between two paths, the likelihood for the ground electrons to absorb any energy has been cancelled. Now that less energy is needed to drive the lasing process, lasers are expected to run more efficiently than the .01 to .3 for typical media and wavelengths.  
In 1985 at the University of Rochester's Laboratory for Laser Energetics a breakthrough in creating ultrashort-pulse, very high-intensity (terawatts) laser pulses became available using a technique called chirped pulse amplification , or CPA, discovered by Gérard Mourou . Later, in 1994, it was discovered by Mourou and his team at University of Michigan that the balance between the self-focusing refraction (see Kerr effect) and self-attenuating diffraction by ionization and rarefaction of a laser beam of terawatt intensities in the atmosphere creates "filaments" which act as waveguides for the beam thus preventing divergence. If a light filament drops below the intensity needed for this dynamic balance, called modulation instability, it can merge with another filament and continue propagating without broadening as with all earlier means of sending light. The filaments, having made a plasma, though turn the narrowband laser pulse into a broadband pulse having a wholly new set of applications.  
Uses of lasers
At the time of their invention in 1960, lasers were called "a solution looking for a problem". Since then, they have become virtually ubiquitous, finding utility in thousands of highly varied applications in every section of modern society from vision correction to guidance for transportation and spacecraft to thermonuclear fusion. They have been widely regarded as one of the most influential technological achievements of the 20th century.
The exceptional utility which lasers have found in scientific, industrial and commercial applications stems from their coherency, high monochromaticity, capability for reaching extremely high powers, or a confluence of these factors. For instance, a laser beam's coherence potentially allows it to be focused down to its diffraction limit, which at visible wavelengths corresponds to only a few hundred nanometers. This property is what allows a laser to record gigabytes of information in the microscopic pits of a DVD. It is also what allows a laser of modest power to be focused to very high intensities and used for cutting, burning or even vaporizing materials. For example, a frequency doubled neodymium yttrium aluminum garnet (Nd:YAG) laser emitting 532 nanometer (green) light at 10 watts output power is theoretically capable of achieving an intensity of megawatts per square centimeter. In reality however, perfect focusing of a beam to its diffraction limit is very difficult. See: Laser applications for more information.
The representation of lasers in popular culture, especially science-fiction or other action movies, as well as their criticism are generally very misleading. For instance, contrary to what appears in movies such as Star Wars, a laser beam is never visible in the vacuum of space and usually does not glow in air either; the ray only glows if some obstacle, such as dust, lies in its path, in much the same way that a sunbeam glows in a dusty atmosphere. Very high intensity beams can be visible in air due to Rayleigh scattering or Raman scattering.
Furthermore, science-fiction film special effects often depict weapon laser beams propagating at only a few feet per second—i.e., slowly enough to see their progress, in a manner reminiscent of conventional tracer ammunition—whereas in reality a laser beam travels at the speed of light, and would be instantly visible along its entire length.
Some action movies depict security systems using red lasers (and being foiled by the hero, typically using mirrors); the hero may see the path of the beam by sprinkling some white dust in the air. It is actually easier to build infrared laser diodes than visible light laser diodes; therefore such systems have no reason to work in visible light.
Even low-power lasers with only a few milliwatts of output power can be hazardous to a person's eyesight. At wavelengths which the cornea and the lens can focus well, the coherence and low divergence of laser light means that it can be focused by the eye into an extremely small spot on the retina, resulting in localised burning and permanent damage in seconds or even faster. Lasers are classified into safety classes numbered I, inherently safe, to IV, even scattered light can cause eye and/or skin damage. Laser products available for consumers, such as CD players and laser pointers are usually in class I or II. See also laser safety.
Common laser types
For a more complete list of laser types see list of laser types.
HeNe (543 nm and 633 nm)
- Argon(-Ion) (458 nm, 488 nm or 514.5 nm)
Carbon dioxide lasers (9.6 µm and 10.6 µm) used in industry for cutting and welding, up to 100 kW possible
Carbon monoxide lasers, must be cooled, but extremely powerful, up to 500 kW possible
Excimer gas lasers, producing ultraviolet light, used in semiconductor manufacturing and in LASIK eye surgery; F2 (157 nm), ArF (193 nm), KrCl (222 nm), KrF (248 nm), XeCl (308 nm), XeF (351 nm)
- Commonly used laser types for dermatological procedures including removal of tattoos, birthmarks, and hair: ruby (694 nm), alexandrite (755 nm), pulsed diode array (810 nm), Nd:YAG (1064 nm), Ho:YAG (2090 nm), Er:YAG (2940 nm)
Semiconductor laser diodes
- small: used in laser pointers, laser printers, and CD/DVD players
- bigger: bigger industrial diode lasers are available used in the industry for cutting and welding, up to 10 kW possible
Neodymium-doped YAG lasers (Nd:YAG), a high-power laser operating in the infrared, used for cutting, welding and marking of metals and other materials
Ytterbium-doped lasers with crystals such as Yb:YAG, Yb:KGW, Yb:KYW, Yb:SYS, Yb:BOYS, Yb:CaF2, or Yb-doped glasses (e.g. fibers); typically operating around 1020-1050 nm; potentially very high efficiency and high powers due to a small quantum defect; highest laser power in ultrashort pulses achieved with Yb:YAG
Erbium-doped YAG, 1645 nm
Thulium-doped YAG, 2015 nm
Holmium-doped YAG, 2096 nm, a efficient laser operating in the infrared, it is strongly absorbed by water-bearing tissues in sections less than a millimeter thick. It is usually operated in a pulsed mode, and passed through optical fiber surgical devices to resurface joints, remove rot from teeth, vaporize cancers, and pulverize kidney and gall stones.
Titanium-doped sapphire (Ti:sapphire) lasers, a highly tunable infrared laser, used for spectroscopy
Erbium-doped fiber lasers, a type of laser formed from a specially made optical fiber, which is used as an amplifier for optical communications.
- External-cavity semiconductor lasers, e.g. for generating high power outputs with good beam quality, wavelength-tunable narrow-linewidth radiation, or ultrashort laser pulses
- Quantum cascade lasers
Last updated: 05-06-2005 14:18:43