Eight-millimeter traveling-wave maser and maser-radiometer system

A low-noise Ka-band traveling-wave maser (TWM) has been developed that tunes over a very wide frequency range (35.4 to 40 Gc), has net gains of 20 db, an instantaneous bandwidth of 75 Mc, and noise temperature near 20°K. The maser uses chromium-doped rutile (Cr³⁺:TiO2) as the active material. By employing push-pull pumping, a relatively low pump frequency (in the 4-mm band) is used. Dielectric slowing is obtained from a rutile-loaded waveguide structure which simultaneously generates the broad-band circular polarization for the ferrite isolators. The isolators used nickel and nickel of zinc ferrite and had reverse-to-forward loss ratios of 70 to 1 at liquid-helium temperature. The maser and associated components were packaged into an operational unit which is described. Design considerations and measured electrical characteristics are presented. The maser has been used to demonstrate a substantial sensitivity improvement in an 8-mm radiometer system.