Abstract
There is a need to remotely measure the full phase and amplitude information of small-scale acousto-seismic vibrations in order to detect the presence of buried objects (e.g., tunnels, etc.), or for other purposes. This remote sensing information may need to be collected with a large area coverage rate and at a safe standoff distance. To accomplish this, we have implemented a shearographic imaging system that incorporates phase stepping in a novel way, automatically separating random speckle noise from surface motion, without requiring an intermediate unwrapping step. This method, which we call surface-phase-resolved shearography, is especially effective for very low-amplitude motions that generate less than one light-wavelength of phase change. In laboratory studies, we have demonstrated sensitivity of two nanometers RMS with 532-nm-wavelength light.
Highlights
In conventional nonphase-resolved (NPR) shearography, the variations in the image intensity and contrast add noise that must be mitigated by algorithms to maximize the speckle contrast
We have developed an advanced phase-resolved (PR) shearography method that separates the random phases from the desired signal phase
For coherent light reflected from an optically rough surface, the optical field at any given locus on the focal plane represents a summation of many complex scalars with a statistical distribution of phases. This summation is entailed by the finite-sized optical spread function (OSF)
Summary
In conventional nonphase-resolved (NPR) shearography, the variations in the image intensity and contrast (due to surface variations or beam inhomogeneity) add noise that must be mitigated by algorithms to maximize the speckle contrast. Current state-of-the-art phase-stepped (PS) shearography separates optical phase, composed of random speckle and signals of interest, from the variations in intensity and contrast, producing clearer fringes. Even with phase stepping, shearographic fringes are dominated by random speckle-to-speckle phase variations This random specklephase noise consumes dynamic range and adds significant processing burden to derive clear continuous fringes. We have developed an advanced phase-resolved (PR) shearography method that separates the random phases from the desired signal phase. Is it possible to recover both amplitude and phase of the ground motion, PR shearography improves the sensitivity by up to an order of magnitude.
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