I. Introduction
Sensitive terahertz-frequency heterodyne receivers are needed to detect and resolve spectral lines that originate from the interstellar medium and from the atmospheres of planets, including the Earth's. Beyond 1.25 THz, the most sensitive heterodyne receivers are based on superconducting hot-electron bolometer (HEB) mixers. In several practical implementations both waveguide [1]–[3] and quasioptical architectures [4] are successfully used, typically using an interferometer diplexer when local oscillator (LO) power is scarce or a simple dielectric beam splitter when there is sufficient LO power margin. Looking toward operation at higher frequencies, we must contend with the combination of shortage of local oscillator power from frequency-agile electronic sources and the difficulty of efficiently coupling it to the mixer. Further, the desire for imaging capability with multi-pixel systems makes further demands on the LO diplexer as it should not affect mixer performance that would otherwise be obtained with a single-pixel receiver.