- For alternative meanings see laser (disambiguation).
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. Light from a laser is often very collimated and monochromatic, but this is not true of all laser types.
Overview
Common light sources, such as the electric light bulb, emit photons in 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, coherent beam of light. The light is often near-monochromatic, consisting of a single wavelength or color, is highly coherent and is often polarised. 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).
Laser light can be highly intense — able to cut steel and other metals. 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, but 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 can generally not be collimated.
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 output of a laser may be a continuous, constant-amplitude output (known as c.w. 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.
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.
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.
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, Ultra Violet 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."
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 nanometeres wavelength. In the same year the Iranian physicist Ali Javan invented the gas laser. He later received the Albert Einstein Award.
The verb "to lase" means to give off coherent light or possibly to cut or otherwise treat with coherent light, and is a back-formation of the term laser.
Popular misconceptions
The representation of lasers in popular culture, especially science-fiction or other action movies, is 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 obstacles, such as dust, lie 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.
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), whereas in reality they of course travel at the speed of light.
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 flour 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.
Laser safety
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.
- Gas lasers
- HeNe (543 nm and 633 nm)
- Argon(-Ion) (458 nm, 488 nm or 514.5 nm)
- Carbon dioxide lasers - 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; 157 nm (F_2), 193 nm (ArF), 222 nm (KrCl), 248 nm (KrF), 308 nm (XeCl), 351 nm (XeF).
- 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 laser are available used in the industry for cutting and welding, up to 10 kW possible
- External-cavity semiconductor lasers, e.g. for generating high power outputs with good beam quality, wavelength-tunable narrow-linewidth radiation, or ultrashort laser pulses
- Dye lasers
- Quantum cascade lasers
- 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 optic fiber surgical devices to resurface joints, remove rot from teeth, vaporize cancers, and to 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.