The Three-Level Solid State Traveling-Wave Maser

The three-level solid state maser, as proposed by Bloembergen, employs a microwave pump signal to alter the thermal equilibrium of a 1 * This work was supported in part by the U. S. Army Signal Corps under Contract DA-36-039 sc-73224. 305 306 THE BELL SYSTEM TECHNICAL JOURNAL, MARCH 1 9 5 9 paramagnetic salt in such a manner that an otherwise absorptive medium becomes emissive when stimulated by radiation at the signal frequency. Successful application of this principle to produce microwave amplification was reported by Scovil el al., - who used microwave cavities to couple the microwave radiation to the paramagnetic salt. Several laboratories ' have since operated such cavity-type masers. Microwave amplification can also be obtained by stimulating radiation from active material in a propagating structure. Effective coupling of the microwave fields to the paramagnetic salt is obtained by slowing the velocity of propagation of the microwave energy through the structure. The active material produces an equivalent negative resistance in the slow-wave structure, and a propagating wave having an exponentially increasing amplitude is obtained. However, if a slow-wave structure is simply filled with the active material, the device will be reciprocal and have gain in both directions. It would therefore require excellent input and output matches and presumably external isolation to obtain unilateral gain. Unidirectional traveling-wave amplification can be obtained in a slow-wave structure which has definite regions of circular polarization of the magnetic field if a maser material is employed which has circularly polarized signalfrequency transitions.