Signal processing for a multispectral fringe detector for imaging

Abstract

In synthetic aperture imaging, an interferometer measures the Fourier transform of the image rather than the image itself. For a two-element interferometer with base line L, the fringe amplitude and phase is the Fourier component at spatial frequency L/λ. For an optical interferometer with a wide optical bandpass, the measured amplitude and phase is the average amplitude and phase over the bandpass. For pupil plane interferometers, the use of a wide bandpass will limit the field of view, while in an image plane interferometer, the use of a wide bandpass will lower the signal-to-noise ratio of the measurement of amplitude and phase. To preserve both signal-to-noise ratio and field of view, the fringe detector in a long base line interferometer (L/D ≫ 1, where L is the base line, D is the collecting aperture) must measure the fringe amplitude and phase at a number of wave-lengths simultaneously. For the Mark III interferometer, we are using a pupil plane configuration with a low dispersion spectrometer (λ/Δλ = 50–250) and a photon counting imaging detector. A PZT controlled mirror is used to vary the optical path length. A microcoded signal processor is used to demodulate the signal to obtain the quadrature components of the complex fringe visibility at each spectral channel.