Laser welding welding with higher energy density， narrow， small， welding heat affected zone distortion and melting metal quantity is little， beam directional good， can carry on the fine processing etc， and is the most promising NiTi alloy welding
A device that uses the principle of amplification of electromagnetic waves by stimulated emission of radiation and operates in the infrared， visible， or ultraviolet The term laser is an acronym for light amplification by stimulated emission of radiation， or a light However， just as an electronic amplifier can be made into an oscillator by feeding appropriately phased output back into the input， so the laser light amplifier can be made into a laser oscillator， which is really a light Laser oscillators are so much more common than laser amplifiers that the unmodified word “laser” has come to mean the oscillator， while the modifier “amplifier” is generally used when the oscillator is not See also Amplifier; Maser; O The process of stimulated emission can be described as follows: When atoms， ions， or molecules absorb energy， they can emit light spontaneously (as with an incandescent lamp) or they can be stimulated to emit by a light This stimulated emission is the opposite of (stimulated) absorption， where unexcited matter is stimulated into an excited state by a light If a collection of atoms is prepared (pumped) so that more are initially excited than unexcited (population inversion)， then an incident light wave will stimulate more emission than absorption， and there is net amplification of the incident light This is the way the laser amplifier A laser amplifier can be made into a laser oscillator by arranging suitable mirrors on either end of the These are called the Thus the essential parts of a laser oscillator are an amplifying medium， a source of pump power， and a Radiation that is directed straight along the axis bounces back and forth between the mirrors and can remain in the resonator long enough to build up a strong (Waves oriented in other directions soon pass off the edge of the mirrors and are lost before they are much ) Radiation may be coupled out by making one mirror partially transparent so that part of the amplified light can emerge through it (see illustration) The output wave， like most of the waves being amplified between the mirrors， travels along the axis and is thus very nearly a plane See also Optical Continuous-wave gas lasers Perhaps the best-known gas laser is the neutral-atom helium-neon (HeNe) laser， which is an electric-discharge-excited laser involving the noble gases helium and The lasing atom is The wavelength of the transition most used is 8 nanometers; however， many helium-neon lasers operate at longer and shorter wavelengths including 3390， 1152， 612， 594， and 543 Output powers are mostly around 1 A useful gas laser for the near-ultraviolet region is the helium-cadmium (HeCd) laser， wherelasing takes place from singly ionized Wavelengths are 325 and 442 nm， with powers up to 150 mW The argon ion laser provides continuous-wave (CW) powers up to about 50 W， with principal wavelengths of 5 and 488 nm， and a number of weaker transitions at nearby The argon laser is often used to pump other lasers， most importantly tunable dye lasers and titanium:sapphire For applications requiring continuous-wave power in the red， the krypton ion laser can provide continuous-wave lasing at 1 and 4 nm (as well as 521， 568， and other wavelengths)， with powers somewhat less than those of the argon ion The carbon dioxide (CO2) molecular laser has become the laser of choice for many industrial applications， such as cutting and Short-pulsed gas lasers Some lasers can be made to operate only in a pulsed Examples of self-terminating gas lasers are the nitrogen laser (337 nm) and excimer lasers (200–400 nm) The nitrogen laser pulse duration is limited because the lower level becomes populated because of stimulated transitions from the upper lasing level， thus introducing absorption at the lasing Peak powers as large as 1 MW are possible with pulse durations of 1–10 Excimer lasers are self-terminating because lasing transitions tear apart the excimer molecules and time is required for fresh molecules to replace Solid-state lasers The term solid-state laser should logically cover all lasers other than gaseous or Nevertheless， current terminology treats semiconductor (diode) lasers separately from solid-state lasers because the physical mechanisms are somewhat With that reservation， virtually all solid-state lasers are optically Historically， the first laser was a single crystal of synthetic ruby， which is aluminum oxide (Al2O3 or sapphire)， doped with about 05% (by weight) chromium oxide (Cr2O3) Three important rare-earth laser systems in current use are neodymium:YAG， that is， yttrium aluminum garnet (Y3Al5O12) doped with neodymium; neodymium:glass; and erbium: Other rare earths and other host materials also find Semiconductor (diode) lasers The semiconductor laser is the most important of all lasers， both by economic standards and by the degree of its Its main features include rugged structure， small size， high efficiency， direct pumping by low-power electric current， ability to modulate its output by direct modulation of the pumping current at rates exceeding 20 GHz， compatibility of its output beam dimensions with those of optical fibers， feasibility of integrating it monolithically with other semiconductor optoelectronic devices to form integrated circuits， and a manufacturing technology that lends itself to mass See also Integrated Most semiconductor lasers are based on III–V The laser can be a simple sandwich of p- and n-type material such as gallium arsenide (GaAs) The active region is at the junction of the p and n Electrons and holes are injected into the active region from the p and n regions Light is amplified by stimulating electron-hole The mirrors comprise the cleaved end facets of the chip (either uncoated or with enhanced reflective coatings) See also Electron-hole recombination; Semiconductor; Semiconductor Monochromaticity When lasers were first developed， they were widely noted for their extreme They provided far more optical power per spectral range (as well as per angular range) than was previously It has since proven useful to relate laser frequencies to the international time standard (defined by an energy-level difference in the cesium atom)， and this was done so precisely， through the use of optical heterodyne techniques， that the standard of length was redefined in such a way that the speed of light is In addition， extremely stable and monochromatic lasers have been developed， which can be used， for example， for optical communication between remote and moving frames， such as the Moon and the E See also Frequency measurement; Heterodyne principle; Laser spectroscopy; LTunable lasers Having achieved lasers whose frequencies can be monochromatic， stable， and absolute (traceable to the time standard)， the next goal is Most lasers allow modest tuning over the gain bandwidth of their amplifying However， the laser most widely used for wide tunability has been the (liquid) dye This laser must be optically pumped， either by a flash lamp or by another laser， such as the argon ion Considerable engineering has gone into the development of systems to rapidly flow the dye and to provide wavelength About 20 different dyes are required to cover the region from 270 to 1000 Free-election lasers The purpose of the free-electron laser is to convert the kinetic energy in an electron beam to electromagnetic Since it is relatively simple to generate electron beams with peak powers of 1010 W， the free-electron laser has the potential for providing high optical power， and since there are no prescribed energy levels， as in the conventional laser， the free-electron laser can operate over a broad spectral
