Waveguide Submillimeter Mixers

Abstract

Astronomical spectroscopy in the submillimeter spectral region has been dominated by heterodyne radiometers, despite the difficulties of developing appropriate coupling structures, nonlinear mixing elements, and local oscillator sources. This is a result of the narrow linewidth of the emission produced by relatively cold portions of the interstellar medium which can be advantageously studied in this frequency range. Most molecular and atomic transitions reported to date have FWHM linewidths < 10 km/s (λ/Δλ = 3 x 104) although regions of active star formation have emission covering velocity widths as large as 100 km s -1. There is important information about the structure and dynamics of interstellar molecular regions at velocity widths down to the sound speed of 0.5 km/s which corresponds to λ/Δλ = 6 x 105. Thus, the high spectral resolution afforded by heterodyne systems is essential for extracting the full information available in spectral lines. In the short wavelength submillimeter (or far infrared) region, broadband incoherent detectors and frequency selective filters are dominant at the present time, although their resolution (approximately 30 km/s; λ/Δλ = 104) is not adequate for resolving most spectral lines Currently, the dividing line between these approaches is about 300 micrometers, but efforts are underway to increase the sensitivity of heterodyne systems at short wavelengths so that astronomers can take advantage of their superior resolution.