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Transactions of the IRE Professional Group on Electron Devices

Amplification of microwave radiation by substances not in thermal equilibrium

J. Weber

Published Jun 1, 1953• 72 citations
Publisher

Abstract

This paper briefly discusses the possibility of obtaining coherent microwave radiation from crystals and gases. It will be shown that it is possible to obtain coherent microwave radiation by such methods, provided that a certain non equilibrium energy distribution is first produced. Methods are discussed for producing such a distribution. The amount of amplification which can be produced by such methods is very small Tinder ordinary circumstances and does not appear to be able to compete with other methods. The method may have certain special applications.

DOI
10.1109/irepged.1953.6811068

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Amplification of Microwave Radiation by Substances not in Thermal Equilibrium

J. WEBER

Elsevier eBooks•Jan 1, 1970•2 citations
This paper briefly discusses the possibility of obtaining coherent microwave radiation from crystals and gases. It will be shown that it is possible to obtain coherent microwave radiation by such methods, provided that a certain non equilibrium energy distribution is first produced. Methods are discussed for producing such a distribution. The amount of amplification which can be produced by such methods is very small Tinder ordinary circumstances and does not appear to be able to compete with other methods. The method may have certain special applications.
Abstract : A method of amplifying electromagnetic radiation is proposed, based on the use of induced emission phenomena, theoretically developed by A. Einstein in 1917. At the given method of amplification there is no conversion of energy of amplified radiation into other forms of energy. The method is suitable for amplification of ultraviolet, visible, infrared radio range waves. To bring the described amplification method into realization, a medium is produced having a negative absorption factor for radiations. The intensity of the radiation stream, passed through such a medium, rises, which produces the amplification effect. The amplification factor equals e to the (K)L, where K is the absorption coefficient and L is the thickness of the layer.
The subject of masers is introduced by giving an historical account of events which led to the amplification of microwave radiation by an inverted population in a system of weakly interacting ammonia molecules. The introduction also outlines the development of masers up to the present. Sections 2 and 3 are concerned with maser fundamentals. Methods for obtaining inverted populations are discussed in § 2. Section 3 is devoted to the theory of maser gain and noise. This is followed by a discussion of microwave solid state masers. Microwave maser materials and the development of unidirectional travelling wave maser amplifiers are emphasized. Optical masers are treated in § 5. In this section the theory of optical amplifiers and oscillators is outlined. Also included in the theoretical discussion is a rather lengthy consideration of pump power requirements in optically pumped masers. This is followed by a survey of solid state, semiconductor and gaseous optical masers. The review ends with some comments on the extension of masers to new wavelength regions.
By considering a radiating gas as a single quantum-mechanical system, energy levels corresponding to certain correlations between individual molecules are described. Spontaneous emission of radiation in a transition between two such levels leads to the emission of coherent radiation. The discussion is limited first to a gas of dimension small compared with a wavelength. Spontaneous radiation rates and natural line breadths are calculated. For a gas of large extent the effect of photon recoil momentum on coherence is calculated. The effect of a radiation pulse in exciting "super-radiant" states is discussed. The angular correlation between successive photons spontaneously emitted by a gas initially in thermal equilibrium is calculated.
We develop a quantum field-theoretic method to describe the effect of frequency mixing using Josephson junctions or using an electron beam traversing a succession of microwave cavities. One of our purposes is to discuss the basic mechanism of current modulation and the subsequent radiation in the complete quantum mechanical context. We use the Heisenberg picture, so that the field operators depend on time and state vectors are fixed. It is essential to take quantum mechanical expectation values only for the quantities that are actually observed. Thus, in the case of frequency mixing, we do not evaluate the current but rather the radiated power, which is observable. An evaluation of the current would destroy correlations between Fourier components of the current produced in the first cavity or in the first junction. Leaving the current as an operator permits the correlations produced in the first cavity (junction) to affect the radiation in the second cavity (junction). Another purpose of the approach is to find the possible dependence of the output radiation on the quantum state of the incident radiation preexisting in cavities or in Josephson junctions. In the case of frequency mixing using two microwave cavities where the fields are treated as independent, we show that it is possible to distinguish the case where both cavities are in definite phase-certain coherent states from the phase-uncertain number states by the value of the wave function renormalization constant. However, if the relative phase of the two cavities cannot be defined, this distinction disappears. This indicates that the radiation occurs at the beat frequency even when the cavity fields are taken to be in energy eigenstates, so that no phase whatever can be associated with the fields in those cavities and thus a classical velocity modulation picture does not apply.