laser

[acronym for light amplification by stimulated emission of radiation], device for the creation, amplification, and transmission of a narrow, intense beam of coherent lightlight,
visible electromagnetic radiation. Of the entire electromagnetic spectrum, the human eye is sensitive to only a tiny part, the part that is called light. The wavelengths of visible light range from about 350 or 400 nm to about 750 or 800 nm.
..... Click the link for more information. . The laser is sometimes referred to as an optical masermaser
, device for creation, amplification, and transmission of an intense, highly focused beam of high-frequency radio waves. The name maser is an acronym for microwave amplification by stimulated emission of r
..... Click the link for more information. .

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 radiationelectromagnetic radiation,
energy radiated in the form of a wave as a result of the motion of electric charges. A moving charge gives rise to a magnetic field, and if the motion is changing (accelerated), then the magnetic field varies and in turn produces an electric field.
..... Click the link for more information. (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 theoryquantum theory,
modern physical theory concerned with the emission and absorption of energy by matter and with the motion of material particles; the quantum theory and the theory of relativity together form the theoretical basis of modern physics.
..... Click the link for more information. , the electromagnetic energy is transmitted in discrete amounts (i.e., in units or packets) called quanta. A quantum of electromagnetic energy is called a photonphoton
, the particle composing light and other forms of electromagnetic radiation, sometimes called light quantum. The photon has no charge and no mass. About the beginning of the 20th cent.
..... Click the link for more information. . 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 rubyruby,
precious stone, the transparent red variety of corundum, found chiefly in Myanmar, Thailand, and Sri Lanka and classified among the most valuable of gems. The Myanmarese stones are blood red, the most valued tint being the "pigeon's blood.
..... Click the link for more information. 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 diodediode
, two-terminal electronic device that permits current flow predominantly in only one direction. Most diodes are semiconductor devices; diode electron tubes are now used only for a few specialized applications.
..... Click the link for more information. 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 as well as welding and soldering, and for inspecting optical equipment. In medicine, they have been used in surgical operations.

CDs and DVDs read and written to using lasers, and lasers also are employed in laser printers and bar-code scanners. They are used in communications, both in fiber optics and in some space and open-air communications; in a manner similar to radio transmission, the transmitted light beam is modulated with a signal and is received and demodulated some distance away. The field of holographyholography
, method of reproducing a three-dimensional image of an object by means of light wave patterns recorded on a photographic plate or film. Holography is sometimes called lensless photography because no lenses are used to form the image.
..... Click the link for more information. 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 been used in a number of areas of scientific research, and 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. Lasers also have been developed experimentally as weaponry.

Bibliography

See S. Leinwoll, Understanding Lasers and Masers (1965); F. T. Arecchi and E. O. Schulz-Dubois, Laser Handbook (1973); J. Walker Light and Its Uses (1980).

Laser

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 Maser

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 Optical pumping

Structure of a parallel-plate laser

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, where lasing 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.

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 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 Frequency measurement, 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.

Laser

a genus of plants of the family Umbelliferae. They are tall perennial herbs with twice- and thrice-ternate leaves. The petals are white, with tips that curl inward. The fruits are ovoid or elongated, with thickened ribs. In Europe and Southwest Asia there are three species; one species, Laser trilobum, is found in the USSR—in the European USSR and inthe Caucasus—growing in light forests, along forest edges, and in thickets. The young shoots are used in cooked formas food. The fruits contain an essential oil.

Laser

a source of electromagnetic radiation in the visible, infrared, and ultraviolet regions that is based on the stimulated radiation of atoms and molecules. The word “laser” comes from the acronym for the expression in English “light amplification by stimulated emission of radiation.” The term “optical quantum generator” is also used in the Soviet literature. The development of lasers (1960) and, somewhat earlier, of masers (1955) served as the basis of development of a new trend in physics and technology called quantum electronics. In 1964 the Nobel Prize in physics was awarded to the Soviet physicists N. G. Basov and A. M. Prokhorov and the American physicist C. Townes for work in quantum electronics.

The laser as a light source. The laser has a number of unique properties that are associated with the coherence and high directivity of its radiation and are absent in the radiation of “nonlaser” light sources. The power radiated by a hot body is determined by its temperature T. The greatest possible value of the radiation flux attainable for an ideal blackbody is W = 5.7 × 10⁻¹² × T⁴ watts per sq cm (W/cm²). The radiated power increases rapidly with increasing T and reaches extremely high levels for high T. For example, each square centimeter of the sun’s surface (T = 5800° K) radiates W = 6.4 × 10³ W of power. However, the radiation of a heat source propagates in all directions from the source—that is, it fills a solid angle of 2π radians. The formation of a directed beam from such a source, which is accomplished by means of a system of apertures or optical systems consisting of lenses and mirrors, is always accompanied by a loss of energy. In no optical system can a radiated power greater than that in the light source itself be produced on the surface of an illuminated object.

In addition, the radiation of a heat source is nonmono-chromatic and fills a broad range of wavelengths (Figure 1). For example, the radiation spectrum of the sun encompasses the ultraviolet, visible, and infrared bands. Monochromators, which make possible the separation of a comparatively narrow region from a continuous spectrum, or low-pressure gas-discharge light sources, which produce discrete narrow atomic or molecular spectral lines, are used to increase the monochromaticity of the radiation. However, the intensity of radiation in the spectral lines may not exceed the radiation intensity of an ideal blackbody whose temperature is equal to the excitation temperature of the atoms and molecules (Figure 1). Thus, in both cases the radiation is made monochromatic at the expense of extremely large energy losses. The narrower the spectral line, the lower will be the radiated energy.

A different situation exists in the radio-frequency region. Sources of radio waves are capable of forming high-power directed and monochromatic radiation. The difference between

Figure 1. (1) Radiation spectrum of an ideal blackbody at temperature T = 10⁴ °K: (λ) wavelength, (v) frequency of oscillations, (l) radiated power. (2) Spectral lines of a low-pressure gas-discharge source at an excitation temperature k = 10⁴ °K for atoms or molecules.

radio-wave sources and nonlaser light sources is fundamental in character. Antennas—radio-wave radiators powered by a common generator of electrical oscillations—can be stimulated coherently. Atoms and molecules are the elementary radiators of light waves. The radiation of any light source represents the aggregate effect of the radiation of a vast population of atoms and molecules, all of which radiate entirely independently of each other (incoherently). The incoherence of the radiation of atoms is related to the independence and randomness of the elementary events of atomic excitation and to their random distribution in space. Collisions are the main reason for the excitation of atoms in hot bodies and in a gas discharge. The moments of collisions are randomly distributed over time, leading to random distribution of the phases of the waves radiated by individual atoms—that is, to incoherence of their radiation.

The problem of developing a source of coherent light was solved only with the advent of the laser, which makes use of a fundamentally new method of de-exciting excited atoms that makes possible the production of coherent light beams with very small divergence, despite the incoherent character of the excitation of individual atoms. If the radiation intensity of the laser is compared with that of an ideal blackbody in the same spectral and angular ranges, extraordinarily high temperatures are produced that exceed by a factor of billions or more the temperatures actually attainable with thermal light sources. In addition, the small divergence of the radiation makes possible concentration of the light energy in negligibly small volumes by means of ordinary optical systems, thus creating extremely high energy densities. The coherence and directivity of the radiation open up fundamentally new opportunities for the use of beams of light where nonlaser light sources cannot be used.

Principle of operation. An excited atom may jump spontaneously to a lower energy level, radiating a quantum of light in the process. The light waves radiated by hot bodies are formed precisely as a result of such spontaneous transitions of atoms and molecules. The spontaneous radiation of the various atoms is incoherent. However, radiation events of another type exist, in addition to spontaneous emission. Upon propagation in a medium of a light wave with a frequency ν corresponding to the difference of some two energy levels ∊₁ and ∊₂ of the atoms or molecules of the medium (hν = ∊₂ − ∊₁ where h is Planck’s constant), other radiation processes are added to the spontaneous emission of particles. The atoms at the lower energy level ∊₁ move to level ∊₂ as a result of the absorption of photons with energy hv (Figure 2,a). The number of such transitions is proportional to ρ(v)Ni, where ρ(v) is the spectral radiation density in ergs per sq cm and N₁ is the concentration of the atoms located at level ∊₁ (the population of the level). The atoms at the higher energy level ∊₂ are stimulated to move to level ∊₁ under the action of the quanta hν (Figure 2,b). The number of such transitions is proportional to ρ(v)N₂, where N₂ is the concentration of atoms at level ∊₂. As a result of the ∊₁ → ∊₂ transitions, the wave loses energy and is attenuated. However, as a result of ∊₂ → ∊₁ transitions, the light wave is amplified. The resulting change in the energy of the light wave is defined by the difference (N₂ − N₁) Under conditions of thermodynamic equilibrium the population of the lower level N₁ is always greater than that of the upper level N₂. Therefore, the wave loses more energy than it acquires, that is, absorption of the light takes place. However, in some special cases, conditions may be created under which a population inversion of levels ∊₁ and ∊₂ takes place, in which N₂ > N₁. In the process the stimulated ∊₂ → ∊₁ transitions predominate and deliver more energy to the light wave than is lost as a result of the ∊₁ → ∊₂ transitions. In this case the light wave is not attenuated but rather amplified.

Figure 2. (a) Quantum transitions corresponding to the absorption of a wave, (b) transitions corresponding to induced radiation

The waves radiated by the atoms as a result of the forced ∊₂ → ∊₁ transitions are identical to the primary wave with respect to the frequency v, the direction of propagation, and polarization and phase and consequently are coherent with respect to each other, regardless of the way in which the excitation of the atoms at level ∊₂ took place. It is the coherence of the stimulated radiation that leads to the amplification of the light wave in the medium with a population inversion rather than merely to additional radiation of new waves. A medium with a population inversion of some pair of levels ∊₁ and ∊₂ that is capable of amplifying radiation of frequency ν = (∊₂ − ∊₁)/h is usually called an active medium.

The spontaneous radiation of one of the excited atoms of an active medium (that is, of an atom at level ∊₂) may, before leaving the volume V, induce transitions of other excited atoms and, consequently, be amplified (Figure 3). It is essential that the amplification depends on the path traveled by the wave in the medium (on the direction). If the active medium is placed in a very simple optical resonator—that is, between two parallel semitransparent mirrors placed at a certain distance from each other, as in a Fabry-Perot interferometer (Figure 4)—conditions will be most favorable for a wave traveling along the axis of the interferometer. As it is amplified, it will reach the mirror, be reflected from it, and travel in the opposite direction, continuing to be amplified; then it will be reflected from the second mirror, and so on. During each “passage” the intensity of the wave increases by a factor of e^kL, where k is the amplification factor in cm⁻¹ and L is the length of the wave’s path in the active medium. If the amplification over the distance L is greater than the losses experienced by the wave on reflection, with each passage the wave will be amplified still further until the energy density p(v) in the wave reaches some maximum value. The increase in p(v) stops when the energy expended on excitation of the atoms cannot compensate for the energy released as a result of the stimulated transitions, which is proportional to p(v). As a result, a standing wave is formed between the mirrors, and a flux of coherent radiation emerges through the semitransparent mirrors.

Figure 3. Amplification of a light wave by atoms of the active medium

A Fabry-Perot interferometer filled with an active medium with a sufficiently high amplification factor is the simplest laser. Optical resonators of other types—with plane or spherical mirrors, with combinations of plane and spherical mirrors, and so on—also are used in lasers. Only some specific types of oscillations of the electromagnetic field, which are called the resonator’s natural oscillations, or modes, can be stimulated in optical resonators that provide feedback in a laser. The modes are characterized by frequency and shape, that is, by the spatial distribution of the oscillations. In a resonator with plane mirrors (Figure 4), mainly types of oscillations that correspond to plane waves propagating along the axis of the resonator are excited. Such a resonator makes possible the production of highly directional radiation. The solid angle ΔΩ in which the radiation flux is concentrated can be made ~(λ/D))², where D is the diameter of the mirrors. For λ ≈ 1 micron (μ) and D = 1 cm, the quantity (γ/D)² is approximately equal to 10⁻⁸ (for heat sources ΔΩ ~ 2π).

Figure 4. Active medium in a Fabry-Perot interferometer

An optical resonator imposes limitations on the spectral composition of the radiation. For a resonator of given length L, waves of frequencies ν = (c/2L)n, where c is the speed of light and η is an integer, are excited in the resonator. As a result, the radiation spectrum of a laser is usually a set of narrow spectral lines with intervals between them that are identical and equal to c/2L. The number of lines (components) for a given length L depends on the properties of the active medium—that is, on the spontaneous radiation spectrum in the quantum transition used—and may reach several tens or hundreds (Figure 5). Under certain conditions it is possible to single out one spectral component (that is, to produce single-mode generation). The spectral width δv_l of each of the components is determined by the energy losses in the resonator and, above all, by the transmission and absorption of the light by the mirrors. Since the quantity δν_ι can be made many times smaller than the width of the spectral lines of the spontaneous radiation of the atoms, the radiation of a laser in single-mode operation is characterized by high monochromaticity.

Figure 5. Modes of an optical resonator

Modern lasers are distinguished according to the method of producing a population inversion in the medium, or, in the jargon, by the method of pumping (such as optical pumping, the electron-collision method, and chemical pumping; see below); according to the working medium (solid dielectrics, semiconductors, gases, and liquids); according to the resonator design; and according to the mode of operation (pulsed or continuous). The differences are determined by the requirements of the lasers’ uses, which often make totally different demands on their characteristics.

Methods of producing a population inversion. Selective excitation of atoms, which ensures the preferential population of one or several energy levels, is necessary for the production of an active medium. One of the simplest and most effective methods is optical pumping, which was used in the first ruby laser. The ruby is a crystal of aluminum oxide, Al₂O₃, with an additive (approximately 0.05 percent) of Cr³⁺ ions, which replace atoms of aluminum. The energy levels of the Cr³⁺ ion in ruby are shown in Figure 6. Absorption of the light corresponding to the blue and green regions of the spectrum transfers Cr³⁺ ions from the ground level ∊₁ to excited levels that form two broad bands (1) and (2). Then the nonradiative transition of these ions to levels ∊₂ and ∊′₂ is accomplished in a comparatively brief time (~ 10⁻⁸ sec). In the process the excess energy is transferred to lattice vibrations. The lifetime of Cr³⁺ ions at the ∊₂ and ∊′₂ levels is 10⁻³ sec. Only after this period do the ions return once again to the ground level ∊₁. Radiation in the red portion of the spectrum corresponds to ∊₂ → ∊₁ and ∊′₂ → ∊₁ transitions. If a ruby crystal is illuminated by the light of a source having sufficiently great intensity in blue and green parts of the spectrum (pumping bands), accumulation of Cr³⁺ ions takes place in the ∊₂ and ∊′₂ levels, and a population inversion of these levels with respect to the ground level ∊₁ occurs. This made possible the development of a laser that runs on ∊₂ → ∊₁ and ∊′₂ → ∊₁ transitions and generates light of wavelength λ ≈ 0.7 μ.

Figure 6. Structure of energy levels of a ruby crystal: (∊₁), (∊₁). (∊₂′) levels of the Cr³⁺ ion

To create a population inversion of levels ∊₁ and ∊’₂ with respect to ∊₁, more than half of the Cr³⁺ ions must be transferred to the ∊₂ and ∊′₂ levels in a period not longer than 10⁻³ sec. This imposes great demands on the power of the pumping source. Xenon flash lamps are used as such sources. The duration of the pumping pulse is usually about 10⁻³ sec. In this interval several joules of energy are absorbed in each cubic centimeter of the crystal.

The optical pumping method has several advantages. First, it can be used to stimulate mediums with a high concentration of particles (solids and liquids). Second, it is extremely selective. For example, in a ruby the part of the radiation spectrum of the pumping tubes that is responsible for excitation of the Cr³⁺ ions is mainly absorbed. All other radiation falls into the region of transparency and is absorbed to a relatively slight degree. Therefore, the ratio of the total energy applied per unit volume of the working substance to the useful energy expended to create a population inversion of the levels is determined mainly by the specific features of the system of levels used. All other energy losses are reduced to a minimum. In a ruby only the part of the energy that goes to excite the natural oscillations of the crystal lattice as a result of nonradiative transitions is lost (the wavy arrows in Figure 6). The reduction of parasitic energy losses is essential for reducing the thermal loads on the substance. The specific energy of the generation pulse in solid-state lasers is as high as several joules from each cubic centimeter of matter. Approximately the same amount of energy remains in the working substance. For a monatomic gas at atmospheric pressure, 1 J of energy corresponds to a temperature of 10,000°K. As a result of the great heat capacity of a solid, the release of approximately 1 J/cm³ of energy produces heating by tens of degrees.

Low efficiency is a shortcoming of the optical pumping method. The ratio of the pulse power of the laser to the electric power fed to the exciting lamp is no greater than a few percent at best because of the incomplete use of the spectrum of the exciting lamps (of the order of 15 percent) and as a result of losses to the conversion of electric power into light energy in the lamps themselves.

The method of creating an active medium directly in an electrical discharge in various gases has become widespread. The possibilities of producing high-energy generation pulses by this method are limited primarily by the low density of the working medium; it is easier to produce a population inversion in comparatively rarefied gases. However, this method does make possible the use as the active medium of very diverse atomic and molecular gases and mixtures of them, as well as various types of electric discharges in gases. As a result, it has been possible to develop lasers that operate in the infrared, visible, and ultraviolet regions of the spectrum. In addition, stimulation in an electric discharge makes possible the realization of a continuous mode of laser operation with high efficiency of conversion of electric power into the power of laser radiation.

In the most powerful continuous-wave gas-discharge laser, which uses a mixture of the molecular gases CO₂ and N₂ (with a number of other components as additives), the mechanism of effecting population inversion is as follows. The electrons of the gas-discharge plasma, accelerated by an electric field, excite oscillations of N₂ molecules upon collision. Then, as a result of collisions between excited N₂ molecules and molecules of CO₂ the population of one of the oscillatory levels of CO₂ takes place. This facilitates the formation of the population inversion. All stages of the process are very efficient, and the overall efficiency reaches 20–30 percent.

Subsequently it proved possible to develop a gas dynamic laser using a mixture of CO² and N₂ in which the gaseous mixture is heated to a temperature k ~ 2,000°K and forms a supersonic flow that expands upon emerging from the nozzle and thus is rapidly cooled. A population inversion of the working levels of the CO₂ arises as a result of the fast cooling. The efficiency of conversion of thermal energy into the radiation of a gas dynamic laser is low (~1 percent). Nevertheless, gas dynamic lasers are extremely promising, since the task of developing large high-power lasers is facilitated in this case and the question of the efficiency of the laser is much less acute when thermal energy sources are used than in the case of electric-discharge lasers. Upon combusion of 1 g of fuel (such as kerosine), energy of the order of 10,000 J is released; the electric power stored in the capacitors feeding the flash lamps is of the order of 0.1 J/cm³ of volume of the capacitors.

Since the chemical bonds of molecules are exceptionally energy-consuming accumulators of energy, direct use of the energy of chemical bonds to excite the particles—that is, to create an active laser medium as a result of chemical reactions—is promising. The reaction of hydrogen or deuterium with fluorine is an example of chemical pumping. If a small number of F₂ molecules are dissociated in some way in a mixture of H₂ and F₂, the chain reaction F + H₂ → HF + H, H + F₂ → HF + F, and so on occurs. The HF molecules formed as a result of the reaction are in an excited state, and for a number of quantum transitions the conditions for a population inversion are satisfied. If CO₂ is added to the initial mixture, lasers operating on CO₂ transitions (λ = 10.6 μ) may be devised, in addition to lasers operating on HF transitions (λ ~ 3 μ). Here the vibrationally excited HF molecules play the same role as N₂ molecules in gas-discharge CO₂ lasers. In this case a mixture of D₂, F₂, and CO₂ proves to be more efficient. In this mixture the ratio of conversion of chemical energy into the energy of coherent radiation may reach 15 percent. Chemical lasers may run in both the pulsed and continuous modes; various types of chemical lasers, including some that are similar to gas dynamic lasers, have been developed.

The production of an active medium in semiconductors has proved possible by various methods: (1) by injection of current carriers through a p-n junction, (2) by excitation by electron collision, or (3) by optical stimulation.

Solid-state lasers. There are a large number of solid-state lasers, both pulsed and continuous-wave. The ruby laser (see above) and the neodymium-doped glass laser (using glass with an additive of Nd) are the most common pulsed lasers. Neodymium-doped glass lasers operate at a wavelength of λ = 1.06 μ. The production of comparatively large and optically highly homogeneous rods up to 100 cm long and 4–5 cm in diameter has proved possible. One such rod is capable of producing a generation pulse with an energy of 1,000 J in a period of the order of 10⁻³ sec. The ruby and neodymium-doped glass lasers are the most powerful pulsed lasers. The total energy of the generation pulse reaches hundreds of joules for a pulse lasting 10−³ sec. A pulse generation mode with a high repetition frequency (up to several kilohertz) has also been achieved.

Lasers using calcium fluoride, CaF₂, with an additive of dysprosium, Dy, and lasers using yttrium aluminum garnet (YAG), Y₃A1₅O₁₂, with additives of various rare-earth atoms are examples of continuous-wave solid-state lasers. Most such lasers operate at wavelengths λ from 1 to 3 μ. The possibility of realizing the continuous mode in such lasers is usually associated with the fact that the excited level ∊₂, not the ground level ∊₁, is the lower level of the working transition (Figure 7). If level ∊₂ is sufficiently remote from the ground level ∊₁ in terms of energy (and in comparison with kT, where k is the Boltzmann constant and k is the temperature) and is characterized by a sufficiently short lifetime, a population inversion for levels ∊₂ and ∊₃ can be created by means of comparatively low-power optical pumping units. In some such lasers generation is accomplished by sunlight pumping. A typical value of the generation power of solid-state lasers in continuous mode is about 1 W or fractions of a watt, and for YAG lasers it is of the order of tens of watts. If special steps are not taken, the generation spectrum of solid-state lasers is comparatively broad, since multimode generation usually takes place. However, single-mode generation may be produced by introducing discriminators into the optical resonator. As a rule this entails a significant reduction in the generated power.

Figure 7. Typical diagram of the levels of the active medium of a continuous-wave solid-state laser

The difficulty of growing large single crystals or founding large samples of homogeneous and transparent glass led to the development of liquid lasers, in which additives of atoms of the rare earths are introduced into a liquid rather than into crystals. However, liquid lasers have shortcomings and therefore are not used as extensively as solid-state lasers.

Generation of narrow and supernarrow pulses. If a flash lamp with pulse width Δt_p ~ 10⁻³ sec is used to pump a solid-state laser, the generation pulse lasts approximately the same time. The slight delay of the start of generation relative to the flash lamp results from the fact that, in order to develop generation, some threshold value of the population inversion must be exceeded, after which the amplification for one pass through the active volume begins to exceed the total energy losses to reflection of the beam from the resonator mirrors, parasitic absorption, and light scattering. When the pumping power is sufficiently great the threshold of generation is reached in time t « Δt_ρ. When the width of the laser pulse is Δt_l ≈ Δt_ρ, the mode of laser operation is called the free-running mode.

Reduction of the pulse width Δt_l is important for a number of applications, since for a given pulse energy the peak laser power increases with decreasing pulse width. The modulated quality factor (Q-spoiled) method, in which the Q-factor of the resonator is modulated, has been developed for this purpose. The method consists in the following. First, optical pumping is carried out and the start of generation is artificially impeded. This may be done by placing an optical shutter in the resonator. When the shutter is closed, generation is impossible, and energy accumulates in the resonator in the form of a growing number of excited atoms. If the shutter is then quickly opened, the entire stored excitation energy, or most of it, is de-excited in the form of a narrow light pulse. The width of such a laser pulse Δt_l is determined by the speed with which the shutter opens or, if the speed is sufficiently high, by the time necessary for the establishment of an electromagnetic field in the resonator.

Various types of optical shutters are used, among them mechanically rotated mirrors and prisms and electrically controlled Kerr and Pockels cells. Pulses with a duration Δt_l ~ 10−⁷− 10−⁸ sec are usually produced by optical shutters. The total pulse energy in the Q-spoiled mode is less than in the free-running mode. Nonetheless, the power gain resulting from the reduction in Δt_l is several orders of magnitude.

The use of translucent filters as shutters has opened up new possibilities for reducing the pulse width of lasers. A weak solution of dye usually serves as the filter, and the concentration of the absorbing component is chosen in such a way that when the light intensity is sufficiently great, saturation is achieved and the solution becomes transparent (is clarified). The placement of such a filter in the resonator increases the threshold of generation: when pumping begins, excited particles begin to accumulate in the active volume; the intensity of their spontaneous radiation also increases. As long as this intensity (taking into account amplification in one pass through the active volume) is less than the clarifying intensity, the absorption in the filter impedes the development of generation. As soon as the clarification level is reached, the shutter is opened automatically and the development of generation is no longer impeded in any way. The use of translucent filters has made possible the production of giant light pulses with a duration of as little as 10−⁹ sec and an energy of the order of tens of joules, corresponding to a power of about 10¹⁰ W.

If single-mode generation is accomplished, a single amorphous giant pulse is observed. In all other cases giant pulses have a complex structure. For example, for a neodymium laser they are a sequence of much narrower pulses lasting about 10−¹¹− 10−¹² sec. The origin of such a structure may be explained as follows. The spontaneous radiation of neodymium atoms in glass is characterized by a very broad spectrum, Δν ~ 10¹² Hz (Δλ ~100 Å), that is, it is the sum of a large number of monochromatic oscillations with frequencies in the interval Δν and with random phases. Therefore, the radiation intensity changes randomly over time (Figure 8), and the characteristic time scale of the entire pattern—that is, the duration of typical bursts—is of the order of the quantity 1/2πΔν. Phasing-in of laser modes has been found to be possible by introducing a nonlinear component into the resonator, such as a translucent filter. In the ideal case, when all modes are phased, the laser radiation assumes the form of a regular sequence of pulses of width 1/2πν. The interpulse intervals are determined by the length of the resonator, that is, are equal to the period 2 L/c. This method of producing supernarrow and exceptionally powerful pulses is called the mode self-synchronization method. In practice, the phasing of all laser modes is very difficult. Usually it is possible to phase in only some modes. In this case the pattern of formation of supernarrow pulses is complicated.

Figure 8. Intensity of spontaneous radiation of the active medium of a neodymium laser as a function of time. The horizontal line gives the intensity that causes the filter to become transparent.

The actual process of the formation of supernarrow pulses by means of a translucent filter takes place approximately as follows. At the initial stage of development of generation the radiation is a random process. If the clarifying intensity corresponds to a horizontal straight line (Figure 8), the filter will be deactivated by beams whose intensity is greater than the clarifying intensity. After each such beam passes, the filter once again begins to absorb. Naturally, generation may develop in this manner only if the filter has a sufficiently short time lag. Otherwise, several other successive, weaker peaks will be passed by the filter after each strong peak.

The filter may be chosen in such a way that it will be deactivated only by the strongest bursts. This makes possible the separation of individual supernarrow generation pulses (Figure 9), using some additional apparatus. The energy of each such pulse is usually low, but it can be increased significantly if the initial pulse is amplified by a second laser or several lasers operating in the amplification mode and differing from lasers in the generation mode by the absence of the mirrors or some other reflecting elements that form the resonator. All possible factors causing reflection are eliminated by selection of an appropriate design. The technique of forming supernarrow pulses and of their subsequent amplification makes possible the production of generation pulses with a duration of the order of 10¹¹—10¹² sec and a peak power of the order of 10¹²-10¹³ W.

Figure 9

Further shortening of pulses by at least severalfold may be expected from the neodymium-doped glass laser. However, the measurement of the duration of such short time intervals is difficult. The power is limited by the strength of the laser materials themselves and runs as high as 10¹²-10¹³ W, which greatly exceeds the capacity of the most powerful modern electric power plants. The development of methods of forming narrow and supernarrow pulses has opened up a new class of optical phenomena, such as the self-focusing of light, induced light scattering, parametric conversion of light frequency, and frequency mixing. These phenomena and their applications are the subject of nonlinear optics.

Gas lasers. The high optical uniformity of gases is their main advantage as an active laser medium. Therefore, gas lasers are of greatest interest for scientific and technical applications for which the highest possible directivity and monochromaticity of radiation are of the greatest necessity. A large number of various types of gas lasers using quantum transitions of neutral atoms, molecules, and ions with frequencies ranging from the ultraviolet to the remote infrared parts of the spectrum were developed soon after the first gas laser, which used a mixture of helium and neon (1960). For example, hydrogen lasers operate at λ = 0.17 μ, lasers using Ne³⁺ and Ne²⁺ ions operate at λ = 0.2358 μ and λ = 0.3324 μ, and water-molecule (H₂O) lasers operate at λ = 27.9 μ and λ = 118.6 μ.

Among the continuous-wave lasers operating in the visible and near infrared regions of the spectrum, the helium-neon laser is most widely used. It is a gas-discharge tube enclosed in an optical resonator and filled with a mixture of helium and neon. It generates radiation with λ = 0.6328 μ, that is, in the red part of the spectrum. Such tubes typically are several dozen centimeters or 1–2 m long and several millimeters in diameter. The generated power is usually tens of milliwatts. The helium-neon laser can also operate on a number of transitions in the near-infrared region—for example, at λ = 1.152 μ and λ = 3.39 μ. Extremely low divergence (diffraction divergence of the light beam) can be achieved comparatively simply in lasers.

The argon laser is the most powerful continuous-wave laser in the visible part of the spectrum. It uses an electric discharge in argon with high current density (up to several thousand A/cm²). It operates on quantum transitions of the argon ion in the blue and green regions of the spectrum: λ = 0.4880 μ and λ = 0.5145 μ. The generated power is tens of watts. The argon laser is much more complex in design than the helium-neon laser (gas cooling and circulation are needed). The carbon dioxide laser (CO₂; λ = 1.06 μ) is the most powerful gas laser. In the continuous mode the carbon dioxide laser attains a power of tens of kilowatts.

A large number of pulsed gas lasers that generally operate in the transitional mode of discharge formation have also been developed. Some of them produce comparatively high peak powers (of the order of 10 kilowatts) in the narrow-pulse mode (with a pulse width of about 10−⁹ sec). The carbon dioxide laser can also operate in the pulsed mode, producing a power of 10¹⁰ W.

Gas lasers are capable of producing much higher monochromaticity of radiation than lasers of any other type. However, a number of difficulties of both a technical and fundamental nature arise in the attempt to increase the monochromaticity and stability of the frequency of laser radiation. The various types of noise that lead to “wobbling” of the laser frequency may be divided into two groups: the technical factors, which affect the natural frequencies of the resonator, and the physical factors, which affect the frequency of the amplifying transition. Vibration of the resonator mirrors and a change in the length of the resonator as a result of thermal expansion belong to the first group; the influence of external electric and magnetic fields and fluctuations of the properties of the active medium and pumping power belong to the second. Corresponding methods of protection are used to reduce the role of most of these factors. For example, special methods for automatic tuning of the resonators using magnetostriction phenomena and the piezoelectric effect are being developed. The basis of these methods is a servome-chanism, which reacts to any change in the resonator parameters and effects appropriate compensation. Fluctuations of the pressure in the active volume are the most important factor limiting the stability of laser frequency. The shape of the spectral line in a given gas depends on the pressure, since collisions of atoms and molecules in the gas lead to widening and displacement of the spectral lines that are proportional to pressure. Fluctuations in pressure lead to fluctuations in the frequency of the amplifying quantum transition. Therefore, the active gas must be kept at the lowest possible pressure. On the other hand, a decrease in pressure leads to a decrease in the amplification factor of the medium. This contradiction can be resolved in part by the method of stabilizing the laser radiation frequency by means of an absorbing cell placed in the resonator. The absorbing cell contains a gas whose spectral absorption line covers the line corresponding to the amplifying transition of the active medium. For example, in the helium-neon laser, methane (CH₄) is such a gas for the line λ = 3.39 μ. Stabilization of the laser radiation frequency with respect to the frequency of the absorption line of methane has proved possible under conditions in which the pressure of the absorbing gas is much less than that of the active gas. The relative stability of the radiation frequency, Δν/ν ~10−¹³-10−¹⁴, has been achieved by using an absorbing cell.

Semiconductor lasers. Semiconductor lasers occupy a special place among lasers in the visible and infrared bands in terms of a number of characteristics. Very high amplification factors (of the order of 10²-10³ cm⁻¹) may be produced in semiconductors; therefore, a semiconductor laser can be made very small—of the order of fractions of a millimeter. The visible and near infrared regions may be covered almost entirely by GaAs, CdS, InAs, InSb, ZnS, and other semiconductor lasers. Semiconductor injection lasers are characterized by very high efficiency of conversion of electric power into coherent radiation (close to 100 percent) and can operate in the continuous mode. An output of the order of 10 W is achieved at the temperature of liquid helium, and about 4–5 W is reached at the temperature of liquid nitrogen. Heterojunction injection lasers, which operate in the continuous mode at room temperature, producing an output of the order of 5 × 10−² W with an efficiency of up to 25 percent, are particularly promising.

Larger volumes of semiconductors may be excited in injection lasers using electron-beam excitation than in the case of injection through p-n junctions. Here the peak output may reach 1 MW, with an average output of the order of 1 W. The efficiency in electron-beam excitation cannot exceed 30 percent.

Shortcomings common to semiconductor lasers are comparatively low directivity of radiation, caused by small size, and the difficulty of producing high monochromaticity. The latter difficulty is associated with the great width of the spectrum of spontaneous radiation in amplifying recombination transitions.

Semiconductor lasers are used to greatest effect when the requirements for coherence and directivity are not very great but small size and high efficiency are essential. Semiconductor lasers surpass all other types of lasers in terms of energy density of radiation and degree of efficiency. The possibility of tuning the radiation frequency and of controlling the light beam—that is, modulation of light intensity with a time constant of about 10−¹¹ sec—is an important property of semiconductor lasers.

Uses. Simultaneously with the invention of the first lasers, various trends in their use began to develop. The development of lasers eliminated the qualitative difference between optics and radio electronics. Thus, in principle, all radio-engineering methods may also be realized in the optical band, and the smallness of the wavelengths of laser radiation opens up a number of additional prospects. High-power lasers make possible the study of various phenomena during the interaction of high-intensity light with a medium that previously were totally beyond the reach of experiments. In studies on the molecular scattering of light, laser sources have greatly expanded the possibilities of experimental techniques and, in particular, have made possible the study of the properties of liquid and solid helium (such as second-sound attenuation and the bound states of two rotons in liquid helium) and the conduct of the first studies of the kinetics of the motion of certain biological entities, such as the simplest bacteria. The extremely fast relaxation processes in condensed mediums with a relaxation time of the order of 10−¹³ sec may be studied by means of narrow and supernarrow pulses. The possibility of forming supernarrow light pulses with a width of 10⁻¹¹—10^—12 sec is also of great importance for high-speed photography and a number of other methods of investigating highspeed processes. Unified optical standards of length (wavelength) and time (frequency) may be created using a helium-neon laser, which has a highly stable frequency. To measure the absolute value of the frequency of the helium-neon laser (3.32 μ) the frequency is measured, after conversion, in the units of frequency of the klystron (0.074230 × 10¹² Hz). This makes it possible to obtain a highly accurate value of the speed of light, c = 299,792,456.2 ± 1.1 m/sec.

The exceptionally high effective emission temperature of lasers and the possibility of concentrating energy in a negligibly small volume have opened up unique possibilities for vaporizing and heating matter. The heating of plasma to temperatures sufficient to produce thermonuclear reactions—that is, the production of thermonuclear plasma—is a very important task. Temperatures of 20 × 10⁶ °K have been attained. Under the same experimental conditions, with appropriate selection of the chemical composition of the vaporizing target, it is possible to produce a point source of high-intensity × radiation with an output of the order of 10⁹ W and a pulse duration of several nanoseconds. It is possible that intensive point neutron sources may be developed. Laser-beam heating of plasma has proved to be an effective method of producing multiply charged ions of various elements. The spectra of many multiply charged ions that are of interest to astrophysics have been produced and investigated under laboratory conditions for the first time by this method.

Powerful lasers have also come to be used in manufacturing. The welding, hardening, cutting, and drilling of various materials are possible by means of such lasers, with very high precision (up to a few wavelengths) and without inducing in them the mechanical stresses that are inevitable during ordinary treatment. Materials of any hardness—metals, diamonds, rubies, and so on—can be processed. Lasers are beginning to be used in the cutting of gas pipes.

Great possibilities are opening up for laser technology in biology and medicine.

Methods of laser radar and communications are being intensively developed. Range-finding of the moon by means of ruby lasers and special corner reflectors that have been placed on the moon has made possible an increase in the accuracy of measurement of the earth-moon distance to several centimeters. The total energy consumption in this process is of the order of the energy released upon combustion of a dozen matches. Communication with a satellite has been accomplished by means of a semiconductor laser. Laser methods of geodetic measurements and recording seismic phenomena are being developed. Laser gyroscopes and range finders have been developed and are in use.

A great deal of attention is being paid to the development of tunable lasers. Various types of parametric light generators exist: lasers using induced light scattering and injection lasers operating in a single mode. As a result, virtually the entire range from λ = 1 mm to the visible band has been covered, and a resolution of 10−²−10−³ cm−¹ has been achieved. The extensive use of such lasers in spectroscopy makes possible in many cases the elimination of the need for monochromators, spectrographs, and other equipment. Laser spectroscopy should be of particularly great importance in the study of short-lived products and of chemical reactions and biological transformations.

Encouraging results have been produced in the directed stimulation of chemical reactions. Lasers make possible selective excitation of one of the natural oscillations of a molecule. Molecules have been found to be capable of entering into reactions that cannot be stimulated or are difficult to stimulate by conventional heating. Powerful tunable lasers in the near infrared region of the spectrum are needed to realize all the existing possibilities in this area.

New methods of producing a population inversion (forced-ionization discharge) have made possible an increase in the pressure in the active medium of molecular gas lasers to 10–20 atmospheres. At such pressures the vibrational and rotational levels of the molecules overlap as a result of collisions, thus opening up new possibilities for tuning laser frequencies.

Optical methods of handling data transmission and storage, holographic methods of recording information, and color projection television are being intensively developed with the aid of laser technology.

REFERENCES

Kvantovaia elektronika: Malen’kaia entsiklopediia. Moscow, 1969.

I. I. SOBEL’MAN

laser

[′lā·zər] (optics) An active electron device that converts input power into a very narrow, intense beam of coherent visible or infrared light; the input power excites the atoms of an optical resonator to a higher energy level, and the resonator forces the excited atoms to radiate in phase. Derived from light amplification by stimulated emission of radiation.