Masers for systems applications

During the past 5 years, solid-state microwave maser amplifiers have advanced from the conception of the basic idea to their present status of having been successfully used for critical measurements in highly specialized applications such as radio and radar astronomy. Design of maser amplifiers for tracking and communicating with deep-space probes and for satellite communication systems is now in progress in several laboratories. All maser amplifiers operated until the present, however, have used open cryogenic systems. The operating time for these systems with one filling of cryogenic fluids varies from about 4 to 12 hours. The process of refilling the cryogenic dewars is a particularly awkward one in view of the fact that the masers are often used at the focal points of large steerable antennas. In addition to the cryogenic difficulties, the gain and bandwidth performance of early masers, while suitable for important scientific measurements, has not been adequate for more demanding systems applications. It is the purpose of this paper to discuss some of the problems in, and progress being made toward,the evolution of maser technology suitable for a variety of systems. As suggested above, the design of closed refrigeration systems and efficient dewars is one of the more important problems which must be faced in the evolution of a practical maser technology. Examples of other advances which must be made are the following: extension of the useful frequency range for maser operation into the millimeter-wave region; improvement of gain-bandwidth product, particularly for radiometer-type operation; achievement of electronic tuning capabilities; achievement of very high gain stability, for passive detection systems which use large integration times; accomplishment of very high phase stability for use in interferometer-type tracking systems in which the basic information is contained in the phase of the signal. In addition to these electrical performance requirements, maser amplifiers must work over a wide range of mechanical positions since they are often mounted on moveable antennas. The problem of protecting the maser amplifier from the saturating effects of local signals (such as the TR leak-through pulse in a radar) already has- several preliminary solutions.