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Journal of Applied Physics

Seesaw Maser Operation

Paul Forrester, W. B. Mims

Published Feb 1, 1961• 3 citations
Publisher

Abstract

Cross relaxation between adjacent intervals may be used to secure an advantage in maser operation comparable to that obtained by ``push-pull'' pumping. Equations for the rate of change of populations in a scheme of four levels (A,B,C,D) when AC is pumped and cross relaxation takes place between BC, CD have been formulated, and the steady-state condition corresponding to various assumptions regarding lattice relaxation times is discussed. In an experimental investigation of maser operation based on this scheme and using ruby as the active material, an inversion of 0.9:1 was obtained at a signal frequency of 14.5 kMc when pumping at 24.5 kMc.

DOI
10.1063/1.1735996

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Physical Review•Feb 15, 1961•8 citations
Cross relaxation between adjacent intervals may be used to secure an advantage in maser operation comparable to that obtained by ``push-pull'' pumping. Equations for the rate of change of populations in a scheme of four levels (A,B,C,D) when AC is pumped and cross relaxation takes place between BC, CD have been formulated, and the steady-state condition corresponding to various assumptions regarding lattice relaxation times is discussed. In an experimental investigation of maser operation based on this scheme and using ruby as the active material, an inversion of 0.9:1 was obtained at a signal frequency of 14.5 kMc when pumping at 24.5 kMc.
The use of a quantitative analysis of the dominant source of relaxation in ruby spin systems to make predictions of key maser amplifier parameters is described. The spin-lattice Hamiltonian which describes the interaction of the electron spins with the thermal vibrations of the surrounding lattice is obtained from the literature. Taking into account the vibrational anisotropy of ruby, Fermi's rule is used to calculate the spin transition rates between the maser energy levels. The spin population rate equations are solved for the spin transition relaxation times, and a comparison with previous calculations is made. Predictions of ruby gain, inversion ratio, and noise temperature as a function of physical temperature are made for 8.4-GHz and 32-GHz maser pumping schemes. The theory predicts that ruby oriented at 90 deg will have approximately 50 percent higher gain in dB and slightly lower noise temperature than a 54.7-deg ruby at 32 GHz (assuming pump saturation). A specific calculation relating pump power to inversion ratio is given for a single channel of the 32-GHz reflected wave maser.
The practical applicability of Masers is often hindered by saturation effects of the maser material. Any strong signal of the same frequency as the maser signal frequency will cause saturation, thus paralyzing the maser for a period of time which is often too long for the intended application. These difficulties have been overcome by desaturating a four-level ruby maser immediately after the maser has been saturated. The experiment was carried out at a signal frequency of 2.5 KMc using an X-band pump. Full recovery of the amplification capabilities of the maser was achieved.
Methods using harmonic spin coupling for operating solid-state masers with signal frequencies higher than the pump frequency are discussed. Expressions for the population inversion ratios are presented, and the maximum signal-to-pump-frequency ratios are calculated. Experimental data is presented on a ruby maser which is operated using the symmetrical method. Amplification was obtained at signal frequencies from 10,320 to 10,740 Mc, using pump frequencies ranging from 9580 to 9670 Mc. An experiment in which maser operation is obtained simultaneously at two frequencies is described.
Cross relaxation effects in ruby maser crystals are treated by introduction of a cross relaxation probability in the rate equations. Detailed solutions have been obtained for several specific processes and compared to recent experiments. It is shown that cross relaxation can improve maser performance even in the absence of impurity doping. Pulse experiments at 0.06 and 0.14% chromium ion concentrations in a ruby traveling wave maser are interpreted in terms of a five-spin process in addition to a four-spin process.