Towards the measurement of surfaces on a large scale: synthetic aperture interferometry

Synthetic aperture interferometry has been used in radar for many years, however, it is only recently that technological advancements have allowed this technique to be applied at optical wavelengths. One application for synthetic aperture interferometry is in the field of surface metrology, where optical methods have been used to increase the area and speed of surface measurement when comparted to traditional contacting techniques. With the ability to synthesise large apertures, the area, speed, and resolution of these optical measurements could be further improved.

In this thesis, a proposal for a novel optical instrument is presented. This instrument was based around synthetic aperture interferometry and was designed to measure surfaces over a large area and to an axial and lateral resolution comparable to a commercial coherence scanning interferometer. The size of the measurement area and the estimated speed of measurement for this device would allow for large high value manufactured parts to be measured in real time.

Within this thesis, the performance of the proposed optical system was predicted using linear systems theory. This analysis looked at different illumination arrangements for the system and the effects they had on the point spread function of the instrument. Additionally, this investigation showed an important factor for the novel optical instrument was the close packing of coherent imagers; used to capture light scattered from the surface. This led to the design of a compact coherent imager (compact holographic imager) that was used to measure the phase and amplitude of light scattered from a surface. The design of this imager involved the use of a bespoke reference beam illumination, which required a new method for the calibration of the coherent imagers’ reference beam.

The light measured by a coherent imager that has been scattered from a surface contains information about the surface and can be used to synthesise an aperture. This combining of information was achieved with the knowledge of the illumination conditions and positions of the imagers when the measurement was taken. To correctly combine this data a two-step calibration method for the proposed optical system was developed, allowing for the reconstruction of the measured surface to be performed. The thesis concludes with experimental results demonstrating synthetic aperture interferometry, the various calibration techniques and the compact coherent imager.

This thesis was able to demonstrate the feasibility of a synthetic aperture interferometric system that is capable of measuring surfaces over a large area to a high axial and lateral resolution.