Source：2^nd UK/Europe-China Workshop on Millimetre Waves and Terahertz Technologies

Derek H Martin
School of Electronic Engineering & Computer Science, Queen Mary University of London, Mile End Road, London E1 4NS, UK
d.h.martin@qmul.ac.uk

    Directed free-space signal-beams, rather than guided-wave modes, are used in the signal-conditioning circuits of mm-wave/terahertz measurement systems to provide high-efficiency and/or large spatial or temporal bandwidths.

    The widths, and the separations, of the optical components in such a quasiopticalsystem must appreciably exceed the signal-beam wavelengths. The size, and the weight, of a measurement system must nevertheless usually be minimized so the designer of such a system must be able to determine with some precision the least size/scale that will allow compliance with the system’s performance specifications.

    This means finding the size-scale for each signal-conditioning optical component that will sufficiently limit the beam-aberration there. Signalconditioning components are of three main types:

    beam-launching and receiving feed-horns,
    collimating and condensing (focusing) reflectors, and
    planar dichroics or polarisers,

    and I draw attention in this short presentation to the aberrating properties of each of these types (matters that have received less attention in the literature than they deserve) and I indicate the implications for the size/scale of a system containing them. In order to give some context for these remarks I refer in particular to AMSU-B mm-wave radiometers that are in use on several space-borne platforms for determining the global atmospheric water-vapour distribution.