Calculation of heterodyne detection with focused radiation

The principal problems associated with heterodyne detection when use is made of focused radiation are examined. Focusing the radiation represents one way of attenuating the demands made on the mechanical structure of an installation. Several possible arrangements of this type are explained and the signal-to-noise ratios associated with them are computed. The basis for this computation is constituted by the three-dimensional formulation of the focus region (Lommel-function). The manner in which the signal-to-noise ratio depends on the distance between the detector and the focal plane is investigated, and the results and the limitations of the computational methods are discussed.