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Stand-off imaging using active illumination is challenging as the intensity of the backscattered light drops with the square of the distance to the target. Homodyne or heterodyne detection, by interfering with the low intensity signal with a much stronger reference, is capable of amplifying the signal above the noise floor of a detector. However, the low spatial coherence of the backscattered light from an optically rough surface makes imaging the spatially resolved interference problematic. We deliberately over-resolve the image so that each speckle in the back-scattered light falls across many pixels and then by using off-axis holography we can reconstruct phase-images even when the collected light signal is below the noise floor of the detector. We achieve a sensitivity of ∼1 photon per pixel per frame at a frame rate of up to 50 kHz. We demonstrate how the phase imaging capability of the system can be applied to stand-off vibrometry measurements, and extend their range by being able to reconstruct the phase below the noise floor of the camera, achieving a signal-to-noise ratio of 22.4±0.5 dB at a range of 6 m.
Wolley et al. (2024) studied this question.