Tomographic sensing of displacement fields

A wavelength scanning interferometry system is used to measure all the orthogonal components of the displacement field inside semitransparent scattering materials. A near infrared tunable laser illuminates a sample from multiple directions. The image of the sample is recombined with a reference beam on a photodetector array. As the laser frequency is linearly tuned during a scan, a sequence of speckle interferograms is recorded. In order to reconstruct the sample structure, Fourier transformation is performed on a pixel by pixel basis along the temporal axis of the 3-D data cube obtained. Multiple displacement sensitivities are achieved by introducing different optical delays between the reference and the illumination beams, which separate the reconstruction signals in the frequency domain. Phase changes due to mechanical loading of the sample can finally be measured and combined to obtain all orthogonal components of the displacement field in a convenient coordinate system. Controlled rigid body rotations of an epoxy phantom have been used to validate the methodology.