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.
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.
To treat the laser in a realistic way, we need the density matrix concepts and formalism given in Chap. 7. This material makes it possible to incorporate dipole phase decays due, for example, to collisions and to treat multimode electric fields in a straightforward way. Nevertheless, it is gratifying to find that we can already determine the field intensity for a simplified model of the ammonia beam maser, the first device of the maser-laser genre. The maser was originally conceived in 1951 by Townes, who noted his thoughts on the back of an envelope during a period of contemplation on a bench in Franklin Park, Washington, DC.T The device was realized experimentally by Gordon, Zeiger, and Townes (1954). In this chapter, we treat the maser along the lines given by Lamb (1960). The analysis is not complicated by atomic (molecular) lifetimes, because they are long compared to the Rabi flopping time, which is the optimal transit time for molecules in the maser cavity. Furthermore, by considering single-mode, steady-state operation, we can assume by energy conservation (1) that the energy given up by the atoms due to stimulated emission is gained by the field, and (2) that this energy flow balances the cavity losses.
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 theory of stimulated emission was put forwarded by A. Einstein (1917), which predicted that as light passed through a substance it could stimulate the emission of more light. In 1954, this phenomenonwas used by Charles Townes andArthur Schawlow in the construction of a microwave amplification by stimulated emission of radiation (maser) using ammonia gas and microwave radiation. Later, this operating principle of maser was put forth to the optical frequencies by them from the Bell Telephone laboratories in 1958, which led to the realization of the device, known as light amplification by stimulated emission of radiation (laser; Townes and Schawlow, 1958). A laser is a quantum optical device that produces a nearly monochromatic (single wavelength), and coherent beam of light by exciting atoms to a higher energy level and causing them to radiate their energy in phase.
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.
This paper has two purposes: 1) to clarify the relationship between the quantum theory of radiation, where the electromagnetic field-expansion coefficients satisfy commutation relations, and the semiclassical theory, where the electromagnetic field is considered as a definite function of time rather than as an operator; and 2) to apply some of the results in a study of amplitude and frequency stability in a molecular beam maser. In 1), it is shown that the semiclassical theory, when extended te take into account both the effect of the field on the molecules and the effect of the molecules on the field, reproduces almost quantitatively the same laws of energy exchange and coherence properties as the quantized field theory, even in the limit of one or a few quanta in the field mode. In particular, the semiclassical theory is shown to lead to a prediction of spontaneous emission, with the same decay rate as given by quantum electrodynamics, described by the Einstein A coefficients. In 2), the semiclassical theory is applied to the molecular beam maser. Equilibrium amplitude and frequency of oscillation are obtained for an arbitrary velocity distribution of focused molecules, generalizing the results obtained previously by Gordon, Zeiger, and Townes for a singel-velocity beam, and by Lamb and Helmer for a Maxwellian beam. A somewhat surprising result is obtained; which is that the measurable properties of the maser, such as starting current, effective molecular Q, etc., depend mostly on the slowest 5 to 10 per cent of the molecules. Next we calculate the effect of amplitude and frequency of oscillation, of small systematic perturbations. We obtain a prediction that stability can be improved by adjusting the system so that the molecules emit all their energy h Ω to the field, then reabsorb part of it, before leaving the cavity. In general, the most stable operation is obtained when the molecules are in the process of absorbing energy from the radiation as they leave the cavity, most unstable when they are still emitting energy at that time. Finally, we consider the response of an oscillating maser to randomly time-varying perturbations. Graphs are given showing predicted response to a small superimposed signal of a frequency near the oscillation frequency. The existence of "noise enhancing" and "noise quieting" modes of operation found here is a general property of any oscillating system in which amplitude is limited by nonlinearity.
The equations are generalized governing coherent emission from quantum mechanical amplifiers (microwave, infrared, and optical masers) using either electric or magnetic dipole transitions. Earlier analyses by Gordon, Zeiger and Townes, Feynman, Vernon and Hellworth, and Wang and Singer, are incorporated in this generalization. The principal new result of this analysis is that an amplitude modulation of the output is to be expected from all maser oscillators excepting those in which excited atoms are supplied at a notably greater rate than the depopulation rate due to coherent induced emission. This analysis also suggests that an energy population inversion of electric dipole moments can be performed using coherent light sources in the same manner as with magnetic dipole moments. It appears therefore that modulation of an optical or infrared maser (e.g., for communication purposes) may be easily accomplished by varying the population of excited states through control of the pumping energy. This modulation may be both by absorption and amplification. (Author)