A device that uses the principle of amplification of electromagnetic waves by stimulated emission of radiation and operates in the infrared, visible, or ultraviolet region. The term laser is an acronym for light amplification by stimulated emission of radiation, or a light amplifier. 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 source. 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 intended. See also
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 wave. This stimulated emission is the opposite of (stimulated) absorption, where unexcited matter is stimulated into an excited state by a light wave. 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 beam. This is the way the laser amplifier works.
A laser amplifier can be made into a laser oscillator by arranging suitable mirrors on either end of the amplifier. These are called the resonator. Thus the essential parts of a laser oscillator are an amplifying medium, a source of pump power, and a resonator. 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 oscillation. (Waves oriented in other directions soon pass off the edge of the mirrors and are lost before they are much amplified.) 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 wave. See also Optical pumping.
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 neon. The lasing atom is neon. The wavelength of the transition most used is 632.8 nanometers; however, many helium-neon lasers operate at longer and shorter wavelengths including 3390, 1152, 612, 594, and 543 nm. Output powers are mostly around 1 milliwatt.
A useful gas laser for the near-ultraviolet region is the helium-cadmium (HeCd) laser, wherelasing takes place from singly ionized cadmium. 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 514.5 and 488 nm, and a number of weaker transitions at nearby wavelengths. The argon laser is often used to pump other lasers, most importantly tunable dye lasers and titanium:sapphire lasers. For applications requiring continuous-wave power in the red, the krypton ion laser can provide continuous-wave lasing at 647.1 and 676.4 nm (as well as 521, 568, and other wavelengths), with powers somewhat less than those of the argon ion laser.
The carbon dioxide (CO2) molecular laser has become the laser of choice for many industrial applications, such as cutting and welding.
Short-pulsed gas lasers
Some lasers can be made to operate only in a pulsed mode. 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 wavelength. Peak powers as large as 1 MW are possible with pulse durations of 1–10 nanoseconds. Excimer lasers are self-terminating because lasing transitions tear apart the excimer molecules and time is required for fresh molecules to replace them.
Solid-state lasers
The term solid-state laser should logically cover all lasers other than gaseous or liquid. Nevertheless, current terminology treats semiconductor (diode) lasers separately from solid-state lasers because the physical mechanisms are somewhat different. With that reservation, virtually all solid-state lasers are optically pumped.
Historically, the first laser was a single crystal of synthetic ruby, which is aluminum oxide (Al2O3 or sapphire), doped with about 0.05% (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:glass. Other rare earths and other host materials also find application.
Semiconductor (diode) lasers
The semiconductor laser is the most important of all lasers, both by economic standards and by the degree of its applications. 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 production. See also Integrated optics.
Most semiconductor lasers are based on III–V semiconductors. 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 regions. Electrons and holes are injected into the active region from the p and n regions respectively. Light is amplified by stimulating electron-hole recombination. The mirrors comprise the cleaved end facets of the chip (either uncoated or with enhanced reflective coatings). See also Electron-hole recombination; Semiconductor; Semiconductor diode.
Monochromaticity
When lasers were first developed, they were widely noted for their extreme monochromaticity. They provided far more optical power per spectral range (as well as per angular range) than was previously possible. 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 fixed. 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 Earth. See also Frequency measurement; Heterodyne principle; Laser spectroscopy; Light.
Tunable lasers
Having achieved lasers whose frequencies can be monochromatic, stable, and absolute (traceable to the time standard), the next goal is tunability. Most lasers allow modest tuning over the gain bandwidth of their amplifying medium. However, the laser most widely used for wide tunability has been the (liquid) dye laser. This laser must be optically pumped, either by a flash lamp or by another laser, such as the argon ion laser. Considerable engineering has gone into the development of systems to rapidly flow the dye and to provide wavelength tunability. About 20 different dyes are required to cover the region from 270 to 1000 nm.
Free-election lasers
The purpose of the free-electron laser is to convert the kinetic energy in an electron beam to electromagnetic radiation. 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 range.
A device that produces a very narrow, highly concentrated beam of light. Lasers have a variety of uses in such areas as surgery, welding and metal cut ting, and sound and video recording and reproduc tion. The name is an acronym for light amplification by stimulated emission of radiation.
(Light Amplification by the Stimulated Emission of Radiation) A device that creates a uniform and coherent light that is very different from an ordinary light bulb. Many lasers deliver light in an almost-perfectly parallel beam (collimated) that is very pure, approaching a single wavelength. Laser light can be focused down to a tiny spot as small as a single wavelength.
Laser output can be continuous or pulsed and is used in a myriad of applications. Gas lasers are used to cut steel and perform delicate eye surgery, while solid state lasers create the ultra-high-speed, minuscule pulses traveling in optical fibers traversing the backbones of all major communications networks. Light traveling in an optical fiber is impervious to external interference, a constant problem with electrical pulses in copper wire. See optical fiber.
How Does It Work?
A laser is an optical oscillator, which is made out of a solid, liquid or gas with mirrors at both ends. To make the laser work, the material is excited or "pumped," with light or electricity. The pumping excites the electrons in the atoms, causing them to jump to higher orbits, creating a "population inversion." A few of the electrons drop back to lower energy levels spontaneously, releasing a photon (quantum of light). The photons stimulate other excited electrons to emit more photons with the same energy and thus the same wavelength as the original. The light waves build in strength as they pass through the laser medium, and the mirrors at both ends keep reflecting the light back and forth creating a chain reaction and causing the laser to "lase."
In simple laser cavities, one mirror has a small transparent area that lets the laser beam out. In semiconductor lasers, both mirrors often transmit a beam, the second one being used for monitoring purposes.
Who Invented It?
In 1957, the laser was conceived by Gordon Gould, a graduate student in physics at Columbia University. When Gould filed for patents in 1959, he found that Columbia professor Charles Townes and Arthur Schawlow of Bell Labs had already filed for them. The year before, AT&T had, in fact, demonstrated a working laser at Bell Labs. In 1977, after years of litigation, a court awarded Gould rights to the first of three patents and later to all of them. He finally reaped millions in royalties.
Coherent Light and Its Emission in Lasers
The coherent light produced by a laser differs from ordinary light in that it is made up of waves all of the same wavelength and all in phase (i.e., in step with each other); ordinary light contains many different wavelengths and phase relations. Both the laser and the maser find theoretical basis for their operation in the quantum theory. Electromagnetic radiation (e.g., light or microwaves) is emitted or absorbed by the atoms or molecules of a substance only at certain characteristic frequencies. According to the quantum theory, the electromagnetic energy is transmitted in discrete amounts (i.e., in units or packets) called quanta. A quantum of electromagnetic energy is called a photon. The energy carried by each photon is proportional to its frequency.
An atom or molecule of a substance usually does not emit energy; it is then said to be in a low-energy or ground state. When an atom or molecule in the ground state absorbs a photon, it is raised to a higher energy state, and is said to be excited. The substance spontaneously returns to a lower energy state by emitting a photon with a frequency proportional to the energy difference between the excited state and the lower state. In the simplest case, the substance will return directly to the ground state, emitting a single photon with the same frequency as the absorbed photon.
In a laser or maser, the atoms or molecules are excited so that more of them are at higher energy levels than are at lower energy levels, a condition known as an inverted population. The process of adding energy to produce an inverted population is called pumping. Once the atoms or molecules are in this excited state, they readily emit radiation. If a photon whose frequency corresponds to the energy difference between the excited state and the ground state strikes an excited atom, the atom is stimulated to emit a second photon of the same frequency, in phase with and in the same direction as the bombarding photon. The bombarding photon and the emitted photon may then each strike other excited atoms, stimulating further emissions of photons, all of the same frequency and all in phase. This produces a sudden burst of coherent radiation as all the atoms discharge in a rapid chain reaction. Often the laser is constructed so that the emitted light is reflected between opposite ends of a resonant cavity; an intense, highly focused light beam passes out through one end, which is only partially reflecting. If the atoms are pumped back to an excited state as soon as they are discharged, a steady beam of coherent light is produced.
Characteristics of Lasers
The physical size of a laser depends on the materials used for light emission, on its power output, and on whether the light is emitted in pulses or as a steady beam. Lasers have been developed that are not much larger than a common flashlight. Various materials have been used as the active media in lasers. The first laser, built in 1960, used a ruby rod with polished ends; the chromium atoms embedded in the ruby's aluminum oxide crystal lattice were pumped to an excited state by a flash tube that, wrapped around the rod, saturated the rod with light of a frequency higher than that of the laser frequency (this method is called optical pumping). This first ruby laser produced intense pulses of red light. In many other optically pumped lasers, the basic element is a transparent, nonconducting crystal such as yttrium aluminum garnet (YAG). Another type of crystal laser uses a semiconductor diode as the element; pumping is done by passing a current through the crystal.
In some lasers, a gas or liquid is used as the emitting medium. In one kind of gas laser the inverted population is achieved through collisional pumping, the gas molecules gaining energy from collisions with other molecules or with electrons released through current discharge. Some gas lasers make use of molecular dissociation to create the inverted population. In a free-electron laser a beam of electrons is “wiggled” by a magnetic field; the oscillatory behavior of the electrons induces them to emit laser radiation. Another device under development is the X-ray laser, which presents special difficulties; most materials, for instance, are poor reflectors of X rays.
Applications of Lasers
The light beam produced by most lasers is pencil-sized, and maintains its size and direction over very large distances; this sharply focused beam of coherent light is suitable for a wide variety of applications. Lasers have been used in industry for cutting and boring metals and other materials, and for inspecting optical equipment. In medicine, they have been used in surgical operations. Lasers have been used in several kinds of scientific research. The field of holography is based on the fact that actual wave-front patterns, captured in a photographic image of an object illuminated with laser light, can be reconstructed to produce a three-dimensional image of the object.
Lasers have opened a new field of scientific research, nonlinear optics, which is concerned with the study of such phenomena as the frequency doubling of coherent light by certain crystals. One important result of laser research is the development of lasers that can be tuned to emit light over a range of frequencies, instead of producing light of only a single frequency. Work is being done to develop lasers for communication; in a manner similar to radio transmission, the transmitted light beam is modulated with a signal and is received and demodulated some distance away. Lasers have also been used in plasma physics and chemistry.
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