In 1954 the invention of the ammonia molecular beam maser oscillator, through Charles Hard TOWNES and co-workers and Nikolai Gennadievich BASOV and Alexander Mikhailovich PROKHOROV, opened a new area of applied physics, the field of quantum electronics. Ten years later the importance of the invention of the new device which consists essentially of a microwave resonator and excited ammonia molecules which pass through the resonator was acknowledged by the award of the 1964 NOBEL Prize in physics to the above workers. \nSoon after the invention of the maser principle, an ammonia beam maser utilizing separated emission fields was brought into operation, a device which consists of two separated microwave resonators through which the ammonia beam maser passes in succession. It is now well known that this two-cavity variant has great practical advantages over the original one-cavity ammonia beam maser. \nHowever, not all of the macroscopic effects observed using the two-cavity ammonia beam maser could be explained in a straightforward manner. One of the more obscure effects presents itself as follows: If the frequency of the auto-oscillation in the first resonator is detuned from one side of the possible frequency range in which generation occurs to the other then the intensity of the electromagnetic radiation in the second cavity can go through two maxima, with a gap in the middle. \nThis effect, the "double-hump detuning phenomenon", which was first observed by the Russian research group of BASOV in 1962, has since then been a challenge to several theoretical and experimental workers. BASOV and co-workers themselves made two different attempts in order to explain this phenomenon. At the Third International Conference on Quantum Electronics, held in Paris in 1963, this team reported that the effect might occur because the transition probability of the molecules passing through the second resonator depends on the degree of excitation of the first cavity. However, in 1967 BASOV and co-workers published a paper where it is suggested that the double-hump detuning phenomenon might well arise because of the interference which occurs between the emissions from molecules which move with different velocities and radiate fields with different phases. \nOn the other hand, research activities in fields other than microwave spectroscopy have made it possible to devise masers which are similar to the ammonia beam maser in that excited particles pass through a tuned resonant component, but which emit at quite different frequencies, e.g. in the audio-frequency ranee or in the mm wave range. However, up to now none of these different masers has been furnished with a successive resonator. \nIt is the purpose of this thesis to report for the first time the successful operation of a tandem maser which is not a two-cavity molecular beam maser. The author devised and investigated a maser where excited protons in water pass successively through two tuned solenoids. \nThe system is based on a conventional proton magnetometer maser oscillating in the magnetic field of the earth at a frequency near to 2 kHz. \nThe general result of the investigation is that despite \nthe enormous frequency difference (audio-frequency compared with a microwave frequency) the two coil nuclear maser behaves in much the same way as a two cavity ammonia beam maser. For the two-coil nuclear maser the double-hump detuning phenomenon can be explained \nin terms of the motion of the macroscopic nuclear magnetization, and an analysis by means of BLOCH'S equations is possible. It is shown that the double-hump detuning phenomenon is produced by a fundamental radiation process occurring in the first emission coil and 1 therefore, that it is not a second-order effect such as could occur because of the non-uniform velocity distribution of the excited particles. \nIt is also shown that for the two-resonator ammonia beam maser the detuning phenomenon can be explained in terms of quantum mechanical probabilities, when a uniform velocity distribution of the ammonia molecules is assumed. Those results agree qualitatively with the results which have been obtained using BLOCH's equations and which have been verified experimentally by means of the two-coil nuclear maser. \nA second distinctive feature of the conventional two-cavity ammonia beam maser is that if the first resonator is sufficiently detuned, generation in the second resonator can take place simultaneously at two different frequencies. The analogous effect has been observed using the two-coil nuclear maser. \nThe conventional one-coil nuclear maser which is currently used as a magnetometer has the disadvantage that it does not offer a suitable criterion for tuning the oscillation frequency accurately to the LARMOR frequency. For the two-cavity ammonia beam maser it has previously been proposed to modulate mechanically the inter-cavity distance and to use zero phase modulation as a means of tuning the maser frequency accurately to the centre frequency of the molecular transition. It is shown that by modulating mechanically the intercoil distance of the two-coil nuclear maser. a suitable tuning criterion can be obtained in an analogous manner. A second tuning criterion is offered by the double-hump detuning phenomenon. \nIn Chapter I the maser principle is discussed using a classical model, and the different maser types which are of interest in the context of this thesis are enumerated. Chapter II contains a review of previous work on two-cavity ammonia beam masers. \nChapter III reviews the conventional theory of nuclear masers. In Chapters IV to IX the two-coil nuclear maser is investigated. Chapter X contains a general discussion of the two-resonator maser problem in terms of quantum-mechanical probabilities. Some general remarks concerning practical applications of two-resonator masers \nare made. The conclusions are summarized in a final section.
In 1954 the invention of the ammonia molecular beam maser oscillator, through Charles Hard TOWNES and co-workers and Nikolai Gennadievich BASOV and Alexander Mikhailovich PROKHOROV, opened a new area of applied physics, the field of quantum electronics. Ten years later the importance of the invention of the new device which consists essentially of a microwave resonator and excited ammonia molecules which pass through the resonator was acknowledged by the award of the 1964 NOBEL Prize in physics to the above workers. \nSoon after the invention of the maser principle, an ammonia beam maser utilizing separated emission fields was brought into operation, a device which consists of two separated microwave resonators through which the ammonia beam maser passes in succession. It is now well known that this two-cavity variant has great practical advantages over the original one-cavity ammonia beam maser. \nHowever, not all of the macroscopic effects observed using the two-cavity ammonia beam maser could be explained in a straightforward manner. One of the more obscure effects presents itself as follows: If the frequency of the auto-oscillation in the first resonator is detuned from one side of the possible frequency range in which generation occurs to the other then the intensity of the electromagnetic radiation in the second cavity can go through two maxima, with a gap in the middle. \nThis effect, the "double-hump detuning phenomenon", which was first observed by the Russian research group of BASOV in 1962, has since then been a challenge to several theoretical and experimental workers. BASOV and co-workers themselves made two different attempts in order to explain this phenomenon. At the Third International Conference on Quantum Electronics, held in Paris in 1963, this team reported that the effect might occur because the transition probability of the molecules passing through the second resonator depends on the degree of excitation of the first cavity. However, in 1967 BASOV and co-workers published a paper where it is suggested that the double-hump detuning phenomenon might well arise because of the interference which occurs between the emissions from molecules which move with different velocities and radiate fields with different phases. \nOn the other hand, research activities in fields other than microwave spectroscopy have made it possible to devise masers which are similar to the ammonia beam maser in that excited particles pass through a tuned resonant component, but which emit at quite different frequencies, e.g. in the audio-frequency ranee or in the mm wave range. However, up to now none of these different masers has been furnished with a successive resonator. \nIt is the purpose of this thesis to report for the first time the successful operation of a tandem maser which is not a two-cavity molecular beam maser. The author devised and investigated a maser where excited protons in water pass successively through two tuned solenoids. \nThe system is based on a conventional proton magnetometer maser oscillating in the magnetic field of the earth at a frequency near to 2 kHz. \nThe general result of the investigation is that despite \nthe enormous frequency difference (audio-frequency compared with a microwave frequency) the two coil nuclear maser behaves in much the same way as a two cavity ammonia beam maser. For the two-coil nuclear maser the double-hump detuning phenomenon can be explained \nin terms of the motion of the macroscopic nuclear magnetization, and an analysis by means of BLOCH'S equations is possible. It is shown that the double-hump detuning phenomenon is produced by a fundamental radiation process occurring in the first emission coil and 1 therefore, that it is not a second-order effect such as could occur because of the non-uniform velocity distribution of the excited particles. \nIt is also shown that for the two-resonator ammonia beam maser the detuning phenomenon can be explained in terms of quantum mechanical probabilities, when a uniform velocity distribution of the ammonia molecules is assumed. Those results agree qualitatively with the results which have been obtained using BLOCH's equations and which have been verified experimentally by means of the two-coil nuclear maser. \nA second distinctive feature of the conventional two-cavity ammonia beam maser is that if the first resonator is sufficiently detuned, generation in the second resonator can take place simultaneously at two different frequencies. The analogous effect has been observed using the two-coil nuclear maser. \nThe conventional one-coil nuclear maser which is currently used as a magnetometer has the disadvantage that it does not offer a suitable criterion for tuning the oscillation frequency accurately to the LARMOR frequency. For the two-cavity ammonia beam maser it has previously been proposed to modulate mechanically the inter-cavity distance and to use zero phase modulation as a means of tuning the maser frequency accurately to the centre frequency of the molecular transition. It is shown that by modulating mechanically the intercoil distance of the two-coil nuclear maser. a suitable tuning criterion can be obtained in an analogous manner. A second tuning criterion is offered by the double-hump detuning phenomenon. \nIn Chapter I the maser principle is discussed using a classical model, and the different maser types which are of interest in the context of this thesis are enumerated. Chapter II contains a review of previous work on two-cavity ammonia beam masers. \nChapter III reviews the conventional theory of nuclear masers. In Chapters IV to IX the two-coil nuclear maser is investigated. Chapter X contains a general discussion of the two-resonator maser problem in terms of quantum-mechanical probabilities. Some general remarks concerning practical applications of two-resonator masers \nare made. The conclusions are summarized in a final section.
The original maser of Gordon, Zeiger and Townes, driven by a focussed stream of ammonia molecules in their antisymmetrical state, provides a conveniently explicit example on which to base a discussion of the principles underlying coherent excitation of a vibrator by stimulated emission. It was shown in chapter 18 how a quadrupole electrostatic lens served to separate symmetric from antisymmetric states, and we shall assume that separation is perfect; it is easy to extend the argument to include a proportion of molecules in the symmetric state. In addition we shall ignore any complications arising from the multitude of rotational states leading to the fine structure shown in fig. 18.8, and shall assume that only one line contributes, for example the strong 3,3 line at 23.9 GHz. Since the microwave cavity resonator, if it is to be excited by the molecules, must normally be very closely tuned to their natural frequency this assumption is realistic. The simplest intuitive approach to the maser is by way of Einstein's treatment of radiation in terms of stimulated and spontaneous processes.† Excited molecules passing through the resonator, when it is already in an oscillatory condition, are stimulated by the field; if the resonator frequency is well matched to the molecular levels they may make a transition down to the ground state and on leaving the resonator have 2Δ 0 less energy than when they entered.
A quantum-mechanical treatment is presented of the radiation behavior of molecules in the detuning cavity. The phenomenon that was discovered by Higa (Rev. Sci. Instr., 28, 1957, p. 726) in his experiment with a double-cavity maser is explained; namely, when the detuning frequency of the first cavity reaches a certain critical value, the amonia molecules in the second cavity suddenly break out into oscillation at the central frequency. A critical formula for the detuning that excites such sudden oscillation is obtained. The influence of the cooperation effect is also taken into consideration. Also qualitatively explained is the experiment performed by Strakhovskiy et al. (J. Exp. Theor. Phys., 42, 1962, p. 907) which relates the degree of detuning in first cavity with the radiative power of the molecules in second cavity. (Author)
Introduction The idea of making use of the phenomenon of stimulated emission from an atom or molecule for amplification of the electromagnetic field, and then combining the amplifier with a resonator to make an oscillator, is due to Townes and his co-workers (Gordon, Zeiger and Townes, 1954, 1955) and independently to Basov and Prokhorov (1954, 1955). The former constructed the first MASER, or microwave amplifier by stimulated emission of radiation . In 1958 Schawlow and Townes proposed an application of the same principle to the optical domain (Schawlow and Townes, 1958), in which a two-mirror Fabry–Perot interferometer serves as the optical resonator and an excited group of atoms as the gain medium. The first LASER, or light amplifier by stimulated emission of radiation , was constructed by Maiman (1960). The gain medium was ruby, which was excited by a bright flash of light from a gas discharge tube, whereupon the laser delivered a short optical pulse. The first continuously operating – or CW – laser was developed by Javan and his co-workers (Javan, Bennett and Herriott, 1961); it made use of a He:Ne gas mixture, that was continuously excited by a discharge, as the gain medium. This type of laser is still widely used today. Since that time many different kinds of laser have been developed, ranging in wavelength from the infrared to the ultraviolet. Some produce light at several different frequencies at once, and some are tunable over a wide range.
A unidirectional quantum-mechanical amplifier has been developed in which the amplifying medium consists of a beam of ammonia molecules passing through, in turn, two separated resonant cavities. The cavities are electrically isolated from one another so that the only coupling is provided by the molecular beam. Unlike the various single-cavity amplifiers constructed in other laboratories, in which a common coupling point for input and output signals necessitates the inclusion of isolators or circulators to separate the two signals, the amplifier described herein affords a natural isolation of input and output with independent control of loadings. After a discussion of the physical principles underlying the mechanism of amplification in a two-cavity molecular beam amplifier, a simplified theory of the operation of the two-cavity amplifier is presented, outlining the parameters which influence power gain and noise Figure. The details of an experimental amplifier are described and the results of gain and noise figure measurements are reported.
A two-cavity beam type of maser is studied by a geometrical representation of the Schrödinger equation developed by Feynman, Vernon, and Hellwarth. By this method complicated effects may be readily visualized. As an example, the qualitative explanation will be developed for the experimental observation that a cavity in a maser will oscillate simultaneously at two frequencies (differing by 1000–5000 cps) under certain conditions when the beam is first passed through another cavity. A possible amplifier application is noted.
There are only fistful of systems in physics that can be described by an exact theory from first principles and, at the same time, can be investigated under experimental conditions approaching the idealized theoretical ones. The one-atom-maser or micromaser provides such an example, making a detailed study of the fundamental properties of the atom-field interaction possible. The situation realized in a micromaser is very close to the ideal case of a single two-level atom interacting with a single quantized mode of a cavity field. In the micromaser the atoms have a dual purpose of both pumping the field and also probing it via measurements of the outgoing atoms. Any other means of measuring the field inside the resonator has the detrimental effect of lowering its Q-factor and thereby the photon storage time. The long photon storage time of the resonator allows for the decay of the field to be negligible during the passage of an atom and its interaction with the field. The behavior of atoms in a cavity is governed by the oscillatory exchange of energy between the atoms and the field, which is called Rabi oscillation. For cavity fields in the vacuum and few photon number states (Fock states), Rabi oscillations have been measured in the past. To prevent thermal effects from polluting the pure number states these experiments were performed at low temperatures (below 1 K). However, the observed resolution of the Rabi oscillations measurements was quite disappointing (only about 2% of the predicted theoretical value) and precluded more ambitious experiments like atom-atom correlations or time resolved investigations of the maser field dynamics. The observed contrast of Rabi oscillations can be considered as a figure of merit for the experimental resolution of quantum features of a maser field. Therefore, a high resolution in the measurements of Rabi oscillations is crucial. This thesis describes the methods and improvements applied to enhance the resolution of Rabi oscillations by a factor of 8. The following upgrades and developments proved to be crucial in this work: 1) The base temperature of the cryogenic system was lowered to 0.3 K, i.e. one third of the original value. At the same time, the time during which the system remains at the base temperature was improved by almost of an order of magnitude (to more than 12 h). 2) For the promotion of the atoms to the Rydberg regime new three-step diode laser setup was constructed. The excitation efficiency compared to the old dye laser was increased by two orders. Using top-of-fringe stabilization with new spectroscopic methods, specially designed error detection and feedback schemes as well as state-of-the-art PID regulators the continuous locking of lasers is about 8 h (compared to about 20 min with the old dye laser system). 3) The flexibility of using the three step excitation allows to excite different maser states and for the first time to investigate pure maser transition. 4) Various improvements on the atomic oven and the channeltron detection unit led to the reliable and stable production and detection of the atomic beam. Observed distribution of the atomic beam statistics reaches to within 6% the Poisson limit which is expected in the absence of experimental imperfections. 5) New developed techniques in magnetic field compensation allow to perform such measurements with two orders of magnitude better accuracy compared to the previous experiments. To demonstrate the new capabilities of the apparatus and to apply its improved resolution of quantum fields we performed a previously impossible in-depth analysis of mean photon number and the atomic inversion produced by a micromaser for various detunings between the cavity- and atomic resonance frequency.