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 in diameter if shone from the Earth's surface to the Moon.
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.
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 safety classes, from class I (inherently safe; no possibility of eye damage even from hours of direct exposure) to class IV (highly dangerous; even non-direct scattering of light from the beam can blind). Users of class III lasers and above must usually wear appropriate eye protection when operating the laser.
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 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.
Types of lasers include:
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.