Coherent or Heterodyne Detection

Abstract

In the previous chapter we discussed the detection of the incident power in an optical or infrared wave, basically ignoring the actual frequency of the wave except in the value of the quantum energy and the wavelength or frequency cutoff of the photon detector. We now consider heterodyne or coherent detection first demonstrated at optical frequencies by Forrester et al. (1955). By using a single-frequency local oscillator as a reference, we may convert the incoming wave’s amplitude and phase to a low frequency in the rf or microwave region. In the following sections we treat the heterodyne case, first for simple constant-amplitude co-planar waves and then in a more general way for an arbitrary amplitude and phase distribution of the local oscillator over the detector surface. We then derive two important theorems that are applicable to heterodyne detection. These are the antenna theorem, which describes the angular response of the detector, and the mixing theorem, which allows the calculation of the mixing efficiency at any arbitrary surface.