laser
Device for producing a narrow beam of light, capable of travelling over vast distances without dispersion, and of being focused to give enormous power densities (108 watts per cm2 for high-energy lasers). The laser operates on a principle similar to that of the maser (a high-frequency microwave amplifier or oscillator). The uses of lasers include communications (a laser beam can carry much more information than can radio waves), cutting, drilling, welding, satellite tracking, medical and biological research, and surgery. Sound wave vibrations from the window glass of a room can be picked up by a reflected laser beam. Lasers are also used as entertainment in theatres, concerts, and light shows.
Laser material
Any substance in which the majority of atoms or molecules can be put into an excited energy state can be used as laser material. Many solid, liquid, and gaseous substances have been used, including synthetic ruby crystal (used for the first extraction of laser light in 1960, and giving a high-power pulsed output) and a helium–neon gas mixture, capable of continuous operation, but at a lower power. A silicon-based laser was created in 2004, using the natural atomic vibrations of silicon nanocrystals to generate the light.
Applications
Carbon dioxide gas lasers (CO
2 lasers) can produce a beam of 100 watts or more power in the infrared (wavelength 10.6 μm) and this has led to an important commercial application, the cutting of material for suits and dresses in hundreds of thicknesses at a time. Dye lasers, in which complex organic dyes in solution are the lasing material, can be tuned to produce light of any chosen wavelength over a range of a sizeable fraction of the visible spectrum.
Photon emission An atom can emit a photon of light (an elementary wave train) if it has somehow gained enough energy to do so; if it has gained this energy it is said to be in an excited state and this can occur, for example, by collision with another atom or by irradiation with light of suitable wavelength. The process of providing the atoms with energy is called ‘pumping’. Normally the atom will emit its photon very quickly (in less than 10−6 s) and at random (spontaneous emission), but if a photon of the same wavelength passes while the atom is still in an excited state the atom will emit its photon in phase with the passing photon (stimulated emission). In a laser it is arranged that this process takes place in a manner that causes a rapid build-up of light intensity. |
Helium–neon lasers The helium–neon laser is the commonest and cheapest kind; it consists of a sealed glass tube containing a mixture of helium and neon gases at low pressure and with mirrors sealed onto the tube at either end. An electrical discharge is passed through the tube from two sealed-in electrodes. The energy of the discharge ‘pumps’ the neon atoms, and photons of wavelength 0.6328 × 10−6 m are emitted (red light). The light bounces between the mirrors and is amplified at each pass. One of the mirrors is slightly transparent to allow the pencil beam of red light to emerge. |
Laser coherence Laser light is very coherent and it can be used to demonstrate a variety of interference effects conveniently; it is used for holography and for accurate length measurement by interferometry. On account of its coherence, laser light can be accurately collimated; if a collimated beam 1 m/3.3 ft in diameter is made it will spread by diffraction at an angle much less than 1" of arc. This has made it possible to send laser pulses to the Moon; by detecting the pulses reflected back from mirrors placed on the Moon very accurate measurements of the Moon's distance can be made. Quite different applications are for concentrated heat in surgery and welding, and for many purposes in pure science. Lasers can be made to produce very short pulses of light (10−11 s) with very intense peak power (1014 watts); such pulses are a possible means for initiating the fusion of light elements to produce energy, in the same way that the energy of the Sun is produced. In 1995 the Omega laser was developed at the University of Rochester, New York. It generates 60 trillion watts of ultraviolet light in pulses that last for 0.65 billionths of a second, and is used in researching the civil applications of nuclear fusion. The Lawrence Livermore National Laboratory, California produced a laser with a power of 1.3 petawatts (1,300 trillion watts) in May 1996. |
Atom laser US scientists unveiled the first atom laser in 1997. It emits a beam consisting of atoms cooled close to absolute zero to form a Bose-Einstein condensate – a special type of matter in which the atoms act like lightwaves. The inventors predicted their new laser will lead to advances in computer chips and navigational equipment. |
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