An experimental method is presented for characterizing the full optical behaviour of few-mode power detectors. The technique involves illuminating the detector with a pair of coherent sources. When the phase of one source is rotated relative to the other, the detector output displays a fringe. Measurement of the complex amplitude of this fringe allows an element of a two-point optical response function to be recovered. By repeating the process for different source positions and orientations, the full two-point response function can be mapped out. From this function it is possible to obtain the full spatial form of each of the modes of the field in which the detector is incoherently sensitive to power, along with the responsivity to the power carried in each of these modes. We describe a scanning system assembled for performing such measurements at frequencies in the range 195 - 270 GHz. Results will be presented for measurements made on a detector that was stopped down to simulate and a few mode planar absorber.
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