maser


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maser

(mā`zər), 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 radiation, microwaves being radio waves of short wavelength, or high frequency. The maser is an oscillatoroscillator, electronic
, electronic circuit that produces an output signal of a specific frequency. An oscillator generally consists of an amplifier having part of its output returned to the input by means of a feedback loop; the necessary and sufficient condition for
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 in which the basic frequency control arises from an atomic resonance rather than a resonant electronic circuit. The waves produced by the maser are coherent, that is, all of the same frequency, direction, and phase relationship, while the waves produced by most sources of 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.
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 are emitted in all directions over a wide range of frequencies and have all possible phase relationships. Maser radiowaves are much closer to an ideal single-frequency source than those of ordinary radio transmitters. As a result, the maser output can be transmitted over fairly large distances with relatively little loss. The principle of the maser was conceived of in the early 1950s, based on the developments of 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.
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, and the first maser was operated in 1954 by C. H. Townes, J. P. Gordon, and H. J. Zeiger. In 1960 the first optical maser was developed by T. H. Maiman (the optical maser is now called a laserlaser
[acronym for light amplification by stimulated emission of radiation], device for the creation, amplification, and transmission of a narrow, intense beam of coherent light. The laser is sometimes referred to as an optical maser.
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). Beginning in 1965 a number of masers have been found in space; the first such natural laser discovered lies in the Great Nebula of Orion and is driven by the hydroxyl (OH) molecule. Masers have been developed to operate at many different wavelengths, so that the original designation "microwave" is no longer strictly accurate. In the maser, electromagnetic radiation is produced by stimulated emission; an atom or molecule in an excited state (i.e., a state of increased energy) emits 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.
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 of a specific frequency when struck by a second photon of the same frequency. The emitted photon and the bombarding photon emerge in phase and in the same direction. For such emissions to take place in sufficient numbers to produce a steady source of radiation, many atoms or molecules must first be "pumped" to the higher energy state. The first maser used molecules of ammonia gas, which oscillate at a characteristic natural frequency between two energy states. Paramagnetic ions in crystals have also been used as the source of coherent radiation for a maser. A maser may be used as an amplifier or as an oscillator, the latter application requiring a higher power level. One of the most useful types of maser is based on transitions in atomic hydrogen occurring at a frequency of 1,421 megahertz. The hydrogen maser provides a very sharp, constant oscillating signal, and thus serves as a time standard for an atomic clockatomic clock,
electric or electronic timekeeping device that is controlled by atomic or molecular oscillations. A timekeeping device must contain or be connected to some apparatus that oscillates at a uniform rate to control the rate of movement of its hands or the rate of
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.

Bibliography

See M. Bertolloti, Masers and Lasers (1983).

Maser

A device for coherent amplification or generation of electromagnetic waves by use of excitation energy in resonant atomic or molecular systems. “Maser” is an acronym for microwave amplification by stimulated emission of radiation. The device uses an unstable ensemble of atoms or molecules that may be stimulated by an electromagnetic wave to radiate energy at the same frequency and phase as the stimulating wave, thus providing coherent amplification. Amplifiers and oscillators operating on the same principle as the maser exist in many regions of the electromagnetic spectrum. Those operating in the optical region were once called optical masers, but they are now universally called lasers (the “l” stands for “light”). Amplification by maser action is also observed arising naturally from interstellar gases. See Coherence, Laser

Maser amplifiers can have exceptionally low internally generated noise, approaching the limiting effective input power of one-half quantum of energy per unit bandwidth. Their inherently low noise makes maser oscillators that use a narrow atomic or molecular resonance extremely monochromatic, providing a basis for frequency standards. The hydrogen maser, which uses a hyperfine resonance of a gas of hydrogen atoms as the amplification source, is the prime example of this use. Also, because of their low noise and consequent high sensitivity, maser amplifiers are particularly useful for reception and detection of very weak signals in radio astronomy, microwave radiometry, and the like. A maser amplifier was used in the experiments that detected the cosmic microwave radiation left over from the big bang that created the universe. See Frequency measurement, Uncertainty principle

The quantum theory describes discrete particles such as atoms or molecules as existing in one or more members of a discrete set of energy levels, corresponding to the various possible internal motions of the particle (vibrations, rotations, and so forth). Thermal equilibrium of an ensemble of such particles requires that the number of particles n1 in a lower energy level 1 be related to the number of particles n2 in a higher energy level 2 by the Boltzmann distribution, given by the equation below,

where E1 and E2 are the respective energies of the two levels, k is Boltzmann's constant, and T is the absolute (Kelvin) temperature. See Boltzmann statistics, Quantum mechanics

Particles may be stimulated by an electromagnetic wave to make transitions from a lower energy level to a higher one, thereby absorbing energy from the wave and decreasing its amplitude, or from a higher energy level to a lower one, thereby giving energy to the wave and increasing its amplitude. These two processes are inverses of each other, and their effects on the stimulating wave add together. The upward and downward transition rates are the same, so that, for example, if the number of particles in the upper and lower energy states is the same, the stimulated emission and absorption processes just cancel. For any substance in thermal equilibrium at a positive (ordinary) temperature, the Boltzmann distribution requires that n1 be greater than n2, resulting in net absorption of the wave. If n2 is greater than n1, however, there are more particles that emit than those that absorb, so that the particles amplify the wave. In such a case, the ensemble of particles is said to have a negative temperature T, to be consistent with the Boltzmann condition. If there are not too many counterbalancing losses from other sources, this condition allows net amplification. This is the basic description of how a maser amplifies an electromagnetic wave. An energy source is required to create the negative temperature distribution of particles needed for a maser. This source is called the pump.

Gas masers

In the first known maser of any kind, the amplifying medium was a beam of ammonia (NH3) molecules, and the molecular resonance used was the strongest of the rotation-inversion lines, at a frequency near 23.87 GHz (1.26-cm wavelength). Molecules from a pressurized tank of ammonia issued through an array of small orifices to form a molecular beam in a meter-long vacuum chamber. Spatially varying electric fields in the vacuum chamber created by a cylindrical array of electrodes formed a focusing device, which ejected from the beam the molecules in the lower energy level and directed the molecules in the upper energy level into a metal-walled electromagnetic cavity resonator. When the cavity resonator was tuned to the molecular transition frequency, the number of molecules was sufficiently large to produce net amplification and self-sustained oscillation. This type of maser is particularly useful as a frequency or time standard because of the relative sharpness and invariance of the resonance frequencies of molecules in a dilute gas. See Molecular beams

Solid-state masers

Solid-state masers usually involve the electrons of paramagnetic ions in crystalline media immersed in a magnetic field. At least three energy levels are needed for continuous maser action. The energy levels are determined both by the interaction of the electrons with the internal electric fields of the crystal and by the interaction of the magnetic moments of the electrons with the externally applied magnetic field. The resonant frequencies of these materials can be tuned to a desired condition by changing the strength of the applied magnetic field and the orientation of the crystal in the field. An external oscillator, the pump, excites the transition between levels 1 and 3 [at the frequency &ngr;31 = (E3 - E1)/h], equalizing their populations. Then, depending on other conditions, the population of the intermediate level 2 may be greater or less than that of levels 1 and 3. If greater, maser amplification can occur at the frequency &ngr;21, or if less, at the frequency &ngr;32. Favorable conditions for this type of maser are obtained only at very low temperature, as in a liquid-helium cryostat. A typical material is synthetic ruby, which contains paramagnetic chromium ions (Cr3+), and has four pertinent energy levels. The important feature of solid-state masers is their sensitivity when used as amplifiers. See Paramagnetism

Astronomical masers

Powerful, naturally occurring masers have probably existed since the earliest stages of the universe, though that was not realized until a few years after masers were invented and built on Earth. Their existence was first proven by discovery of rather intense 18-cm-wavelength microwave radiation of the free radical hydroxyl (OH) molecule coming from very localized regions of the Milky Way Galaxy.

Masers in astronomical objects differ from those generally used on Earth in that they involve no resonators or slow-wave structures to contain the radiation and so increase its interaction with the amplifying medium. Instead, the electromagnetic waves in astronomical masers simply travel a very long distance through astronomical clouds of gas, far enough to amplify the waves enormously even on a single pass through the cloud. It is believed that usually these clouds are large enough in all directions that a wave passing through them in any direction can be strongly amplified, and hence astronomical maser radiation emerges from them in all directions.

Naturally occurring masers have been important tools for obtaining information about astronomical objects. Since they are very intense localized sources of microwave radiation, their positions around stars or other objects can be determined very accurately with microwave antennas separated by long distances and used as interferometers. This provides information about the location of stars themselves as well as that of the masers often closely surrounding them. The masers' velocity of motion can also be determined by Doppler shifts in their wavelengths. The location and motion of masers surrounding black holes at the centers of galaxies have also provided information on the impressively large mass of these black holes. Astronomical masers often vary in power on time scales of days to years, indicating changing conditions in the regions where they are located. Such masers also give information on likely gas densities, temperature, motions, or other conditions in the rarefied gas of which they are a part. See Doppler effect

Maser

 

a term used to designate quantum generators and amplifiers operating in the radio-frequency band. The word “maser” was formed from the initial letters of the English words “microwave amplification by stimulated emission of radiation.”

maser

[′mā·zər]
(physics)
A device for coherent amplification or generation of electromagnetic waves in which an ensemble of atoms or molecules, raised to an unstable energy state, is stimulated by an electromagnetic wave to radiate excess energy at the same frequency and phase as the stimulating wave. Derived from microwave amplification by stimulated emission of radiation. Also known as paramagnetic amplifier.

Maser

A device for coherent amplification or generation of electromagnetic waves by use of excitation energy in resonant atomic or molecular systems. “Maser” is an acronym for microwave amplification by stimulated emission of radiation. The device uses an unstable ensemble of atoms or molecules that may be stimulated by an electromagnetic wave to radiate energy at the same frequency and phase as the stimulating wave, thus providing coherent amplification. Amplifiers and oscillators operating on the same principle as the maser exist in many regions of the electromagnetic spectrum. Those operating in the optical region were once called optical masers, but they are now universally called lasers (the “l” stands for “light”). Amplification by maser action is also observed arising naturally from interstellar gases. See Laser

Maser amplifiers can have exceptionally low internally generated noise, approaching the limiting effective input power of one-half quantum of energy per unit bandwidth. Their inherently low noise makes maser oscillators that use a narrow atomic or molecular resonance extremely monochromatic, providing a basis for frequency standards. The hydrogen maser, which uses a hyperfine resonance of a gas of hydrogen atoms as the amplification source, is the prime example of this use. Also because of their low noise and consequent high sensitivity, maser amplifiers are particularly useful for reception and detection of very weak signals in radio astronomy, microwave radiometry, and the like. A maser amplifier was used in the experiments that detected the cosmic microwave radiation left over from the big bang that created the universe.

The quantum theory describes discrete particles such as atoms or molecules as existing in one or more members of a discrete set of energy levels, corresponding to the various possible internal motions of the particle (vibrations, rotations, and so forth). Thermal equilibrium of an ensemble of such particles requires that the number of particles n1 in a lower energy level 1 be related to the number of particles n2 in a higher energy level 2 by the Boltzmann distribution, given by the equation below,

where E1 and E2 are the respective energies of the two levels, k is Boltzmann's constant, and T is the absolute (Kelvin) temperature.

Particles may be stimulated by an electromagnetic wave to make transitions from a lower energy level to a higher one, thereby absorbing energy from the wave and decreasing its amplitude, or from a higher energy level to a lower one, thereby giving energy to the wave and increasing its amplitude. These two processes are inverses of each other, and their effects on the stimulating wave add together. The upward and downward transition rates are the same, so that, for example, if the number of particles in the upper and lower energy states is the same, the stimulated emission and absorption processes just cancel. For any substance in thermal equilibrium at a positive (ordinary) temperature, the Boltzmann distribution requires that n1 be greater than n2 resulting in net absorption of the wave. If n2 is greater than n1, however, there are more particles that emit than those that absorb, so that the particles amplify the wave. In such a case, the ensemble of particles is said to have a negative temperature T, to be consistent with the Boltzmann condition. If there are not too many counterbalancing losses from other sources, this condition allows net amplification. This is the basic description of how a maser amplifies an electromagnetic wave. An energy source is required to create the negative temperature distribution of particles needed for a maser. This source is called the pump.

Gas masers

In the first known maser of any kind, the amplifying medium was a beam of ammonia (NH3) molecules, and the molecular resonance used was the strongest of the rotation-inversion lines, at a frequency near 23.87 GHz (1.26-cm wavelength). Molecules from a pressurized tank of ammonia issued through an array of small orifices to form a molecular beam in a meter-long vacuum chamber. Spatially varying electric fields in the vacuum chamber created by a cylindrical array of electrodes formed a focusing device, which ejected from the beam the molecules in the lower energy level and directed the molecules in the upper energy level into a metal-walled electromagnetic cavity resonator. When the cavity resonator was tuned to the molecular transition frequency, the number of molecules was sufficiently large to produce net amplification and self-sustained oscillation. This type of maser is particularly useful as a frequency or time standard because of the relative sharpness and invariance of the resonance frequencies of molecules in a dilute gas.

Solid-state masers

Solid-state masers usually involve the electrons of paramagnetic ions in crystalline media immersed in a magnetic field. At least three energy levels are needed for continuous maser action. The energy levels are determined both by the interaction of the electrons with the internal electric fields of the crystal and by the interaction of the magnetic moments of the electrons with the externally applied magnetic field. The resonant frequencies of these materials can be tuned to a desired condition by changing the strength of the applied magnetic field and the orientation of the crystal in the field. An external oscillator, the pump, excites the transition between levels 1 and 3 [at the frequency &ngr;31 = (E3 - E1)/h], equalizing their populations. Then, depending on other conditions, the population of the intermediate level 2 may be greater or less than that of levels 1 and 3. If greater, maser amplification can occur at the frequency &ngr;21, or if less, at the frequency &ngr;32. Favorable conditions for this type of maser are obtained only at very low temperature, as in a liquid-helium cryostat. A typical material is synthetic ruby, which contains paramagnetic chromium ions (Cr3+), and has four pertinent energy levels. The important feature of solid-state masers is their sensitivity when used as amplifiers.

Astronomical masers

Powerful, naturally occurring masers have probably existed since the earliest stages of the universe, though that was not realized until a few years after masers were invented and built on Earth. Their existence was first proven by discovery of rather intense 18-cm-wavelength microwave radiation of the free radical hydroxyl (OH) molecule coming from very localized regions of the Milky Way Galaxy.

Masers in astronomical objects differ from those generally used on Earth in that they involve no resonators or slow-wave structures to contain the radiation and so increase its interaction with the amplifying medium. Instead, the electromagnetic waves in astronomical masers simply travel a very long distance through astronomical clouds of gas, far enough to amplify the waves enormously even on a single pass through the cloud. It is believed that usually these clouds are large enough in all directions that a wave passing through them in any direction can be strongly amplified, and hence astronomical maser radiation emerges from them in all directions.

Naturally occurring masers have been important tools for obtaining information about astronomical objects. Since they are very intense localized sources of microwave radiation, their positions around stars or other objects can be determined very accurately with microwave antennas separated by long distances and used as interferometers. This provides information about the location of stars themselves as well as that of the masers often closely surrounding them. The masers' velocity of motion can also be determined by Doppler shifts in their wavelengths. The location and motion of masers surrounding black holes at the centers of galaxies have also provided information on the impressively large mass of these black holes. Astronomical masers often vary in power on time scales of days to years, indicating changing conditions in the regions where they are located. Such masers also give information on likely gas densities, temperature, motions, or other conditions in the rarefied gas of which they are a part.

maser

a device for amplifying microwaves, working on the same principle as a laser

maser

(Microwave Amplification by Stimulated Emission of Radiation) A device that creates a uniform and coherent electromagnetic radiation pattern. It is similar to a laser, except that it emits microwaves or radio waves rather than light. See laser.
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