Development of a terahertz mixer for measurement of stratospheric OH

Passive microwave sensing of the atmosphere is recognised to be an important technique for determining the chemistry, dynamics and evolution of the atmosphere. A common feature of passive microwave instruments is that they require well defined field-of views, good signal-to-noise performance and high spectral resolution. These are all features provided by waveguide mixer radiometers and it is this type of technology which is invariably preferred at lower frequencies. At terahertz frequencies, however, where simple scaling indicates that circuit structures need to be less than 100 μm in size, waveguide mixers have been generally considered to be impractical to build. In this paper we demonstrate that not only is it realistic to build a waveguide mixer similar to those at lower frequencies, but that it promises similar advantages. Our mixer consists of single mode waveguide and a corrugated feedhorn structure to couple the incident signal into the waveguide. A dot-matrix GaAs Schottky diode is used as the nonlinear circuit element. In order to avoid fabrication and fragility problems associated with scaling miniature wire whiskers, we have developed a new planar whisker which incorporates RF and IF filtering and is fabricated entirely lithographically. The mixer is straightforward to assemble, and has good potential for further improvement