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Soviet Journal of Quantum Electronics

Cross-relaxation quantum amplifier operating at 22.2 GHz

L. Eh. Abramyan, A. A. Akhumyan, R. М. Мartirosyan, V. P. Shakhparyan

Published Sep 30, 1975• 0 citations
Publisher

Abstract

A cross-relaxation ruby quantum amplifier operating at 22.2 GHz was constructed. The 4→3 signal transition and the 2→4 pump transition were used in the θ=76° and 90° orientations. A gain of 18 dB was obtained for a passband of 13 MHz. Possible cross relaxations at the working point were considered. The inversion coefficient was calculated by solving the rate equations in the presence of cross relaxation.

DOI
10.1070/qe1975v004n09abeh011562

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A cross-relaxation ruby quantum amplifier operating at 22.2 GHz was constructed. The 4→3 signal transition and the 2→4 pump transition were used in the θ=76° and 90° orientations. A gain of 18 dB was obtained for a passband of 13 MHz. Possible cross relaxations at the working point were considered. The inversion coefficient was calculated by solving the rate equations in the presence of cross relaxation.
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.
A three-level solid-state maser using ruby has been operated at frequencies near 3000 Mc/s, using a high-efficiency cavity having TEM modes in a rectangular geometry. Two different operating points in the θ = 10° to θ =30 ° region of the ruby spectrum gave maser operation with a pump frequency near 10 500 Mc/s, although the inversion obtained was a. sensitive function of the exact operating point. An operating point at θ = 90°, requiring pumping at 13 500 Mc/s, gave a gain-bandwidth product of better than 50 Mc/s. It was also found possible to pump the 1-4 or ΔM = 3 transition at this same operating point, obtaining 3000 Mc/s amplification with a 23 700 Mc/s pump frequency. Although this mode of operation was not fully explored, it is potentially capable of increasing the gain-bandwidth product by a substantial amount.
Arrays of 10 or 40 series-connected small densely packed Josephson tunnel junctions were successfully operated as parametric amplifiers at 10 GHz. Between 50 and 90% of the junctions in an array were observed to work coherently in the amplification process. Signal gain exceeding 24 dB was achieved with a pump power as low as −50 to −70 dBm. A power width (i.e., the range of the pump power for a gain within 3 dB of the peak gain) of 0.25 dB and a bandwidth of 15 MHz were measured for 20-dB gain. The power width was in accordance with theory but the bandwidth was reduced, probably due to noise saturation effects. The latter effects can be reduced by lowering the input noise temperature or by including an external band limitation. The noise temperature was also increased by saturation; however, with an external bandwidth of 60 MHz, a noise temperature of 30±20 K was measured. The application of a magnetic field has proved extremely useful as a means of tuning the amplifiers to stable high-gain operating points. This tuning was also utilized in evaluating the amplifier parameters, and the theory by Feldman, Parrish, and Chiao has been verified. The upper frequency limit for a lead tunnel junction amplifier ought to be well above 300 GHz.
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.