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 emits photons in a narrow, well-defined 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 (i.e. it is highly collimated). The beam will eventually spread due to the effect of diffraction but much less so 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.
A laser 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 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.
The basic physics of lasers centres around the idea of producing a population inversion in a laser medium. The medium may then amplify light by the process of stimulated emission, which if the light is fed back through the medium by means of a cavity resonator, will continue to be amplified into a high-intensity beam. 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 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, who was also credited with lucrative patent rights in the 1970s, following a protracted 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.
Even low power lasers can be hazardous to a person's eyesight. 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. Certain wavelengths of laser light can cause cataracts or even boiling of the vitreous humor, the fluid in the eyeball. Infrared and ultraviolet lasers are particularly dangerous, since the body's "blink reflex", which can protect an eye from excessively bright light, works only if the light is visible. Lasers are classified by wavelength and maximum output power into the following safety classes:
The laser powers mentioned above are rough indications; the classification is also dependent on the wavelength and on whether the laser is pulsed or continuous. The use of eye protection when operating lasers of class III and IV is strongly recommended. However, it is common practice in scientific research that knowledgeable operators do not use eye protection while working with class-IV lasers. This is often the only alternative with laser systems that use more than one wavelength, for example when a Ti:sapphire laser at 800 nm is pumped by a frequency-doubled Nd:YLF laser at 527 nm. It is nearly impossible to construct eye protection filters that completely block these two wavelengths and at the same time allow the operator to see what he/she is doing.
- class I: inherently safe; no possibility of eye damage. This can be either because of a low output power (in which cases eye damage is impossible even after hours of exposure), or due to an enclosure that cannot be opened in normal operation without the laser being switched off automatically, such as in CD players.
- class II: the blinking reflex of the human eye will prevent eye damage. Most laser pointers are in this category, with output powers of around 1 milliwatt.
- class IIIb: can cause damage if the beam enters the eye directly. This applies to laser powers up to a few milliwatts.
- class IIIa: similar to IIIb, but with large beam diameters, such that the pupil will only allow a 'class-II'-amount of light to enter the eye. Lasers in this class are mostly dangerous in combination with optical instruments which change the beam diameter.
- class IV: highly dangerous; even non-direct scattering of light from the beam can lead to eye or skin damage. This applies to laser powers of more than a few milliwatts.
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.
- Commonly used laser types for dermatological procedures including removal of tattoos, birthmarks, and hair:
- Argon (488 or 514.5 nm)
- 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, used in laser pointers, laser printers, and CD/DVD players;
- Dye lasers
- Quantum cascade lasers
- Carbon dioxide lasers - used in industry for cutting and welding
- Excimer lasers, producing ultraviolet light, used in semiconductor manufacturing and in LASIK eye surgery;
- 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;
- Erbium-doped YAG, 1645 nm
- Thulium-doped YAG, 2015 nm
- Holmium-doped YAG, 2090 nm, a high-power laser operating in the infrared, it is explosively 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.
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