Design and evaluation of a full-sensitivity tilt scanning interferometry system for displacement field tomography and profilometry
2013-11-22T13:24:05Z (GMT) by
This thesis reports the investigation and further development of a tilt scanning interferometry system for surface profilometry and sub-surface tomographic applications. A new 3D full sensitivity interferometry system extends the work carried out on a previous prototype that was capable of measuring displacement along one lateral plus the axial component. Depth-resolved imaging is achieved by the acquisition of a sequence of 2D interferograms whilst the illumination beam undergoes a constant rate of tilt and full sensitivity displacement is achieved by performing scans from multiple illumination directions. The comparison of phase volumes from two successive series of scans enables 3D displacement fields to be determined. The working principle that describes the technique is presented, covering the reconstruction of a depth-resolved sample from the detected intensity distribution. The system performance is studied, including measurement repeatability and factors that affect the depth resolution and depth range. Depth resolution is fundamentally limited by the range of the illumination tilting angle and the new system design enables a larger range. However, the resolution is degraded by a frequency chirp that appears in the temporal interference signal when a large tilting range is scanned. It is shown through a numerical simulation that the chirp depends on the curvature of the illumination wavefront and also on the position of the pivot axis of the illumination beam. Data processing methods are proposed to overcome these limitations and their effects are illustrated with experimental measurements of opaque surfaces and a weakly scattering phantom with internal features. Displacement measurements involving a controlled rigid body rotation and tilt of a weakly scattering phantom were completed to validate the expected deformations. Both in-plane and out-of-plane components were measured.