Use of Active Material in Three-Level Solid State Masers

In 1956, Bloembergen1 proposed a three-level excitation scheme for obtaining microwave amplification by stimulated emission of radiation in paramagnetic solids, or three-level solid state maser (3LSSM) for short. It is superior to many other microwave amplifiers in that its noise contribution to an amplified signal should be virtually negligible compared to other noise sources in a system. The principal advantage of a solid state maser over a gaseous maser is that the amplified bandwidth should be of the order of megacycles, a magnitude sufficient for many communication applications. In contrast to the two-level solid state masers, its gain factor should be constant in time. * This work was supported in part bv the U. S. Army Signal Corps under Contract DA-36-039 80-73224. 335 336 THE BELL SYSTEM T E C H N I C A L J O U R N A L , MARCH 1959 Experimental work has confirmed the theoretical predictions. It has been shown that microwave energy can be extracted from paramagnetics by stimulated emission.2'3,4 If the circuit employed consists of a microwave resonant cavity, the energy extracted can be used to maintain either oscillations or amplification depending on the choice of the coupling parameter. Three paramagnetic salts have been used as active materials. These are gadolinium ethyl sulfate,2 potassium chromicyanide3 and ruby.4 Gain and bandwidth obtained are in agreement with theoretical expectations. Noise measurements have indicated that maser noise is of the magnitude expected.