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IEEE Transactions on Microwave Theory and Techniques

Maser Operation at Signal Frequencies Higher than Pump Frequency

Frank R. Arams

Published Jan 1, 1961• 10 citations
Publisher

Abstract

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.

DOI
10.1109/tmtt.1961.1125265

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•Aug 15, 1989•4 citations
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.
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.
The computational results of a comparison of maser materials for a 32 GHz maser amplifier are presented. The search for a better maser material is prompted by the relatively large amount of pump power required to sustain a population inversion in ruby at frequencies on the order of 30 GHz and above. The general requirements of a maser material and the specific problems with ruby are outlined. The spin Hamiltonian is used to calculate energy levels and transition probabilities for ruby and twelve other materials. A table is compiled of several attractive operating points for each of the materials analyzed. All the materials analyzed possess operating points that could be superior to ruby. To complete the evaluation of the materials, measurements of inversion ratio and pump power requirements must be made in the future.
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.
The practical application of masers is often hindered by saturation effects of the maser material. Any strong signal frequency (e.g., the transmitter pulse of a radar system leaking into the maser) will cause saturation. This paralyzes the maser for a period of time, too long for most intended applications. In this report, a method is described which has been used in desaturating a four-level ruby maser. The saturation is caused by equalizing the populations of levels one, two, and three of the maser crystal by the leakage of the transmitter pulse, in addition to the action of the maser pump. The application of a desaturation pulse between levels one and four will then desaturate the maser restoring the excess population density between levels two and one. In a cavity maser, full recovery of the amplification capabilities has been achieved within less than 1 msec after the maser has been saturated. Repetition rates up to 120 cps are tolerable.
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.