Development of a coherent imager with a large field of view for synthetic aperture interferometry

Quality control and inspection of large precision-engineered parts in reflection and transmission modes require the use of non-contact techniques with high spatial resolution over large fields of view (FOV). Conventional approaches for measuring areal profiles of reflective surfaces over large areas include optical techniques such as phase shifting interferometry, coherent scanning interferometry and focus variation using high numerical aperture (NA) objectives and stitching the small measured FOVs. In these methods, as the objective NA increases, the spatial resolution increases and the FOV decreases. Other approaches adapted to transmission mode and suitable to measure phase and amplitude include scanning confocal microscopy, large lens systems and lens-less digital holography (DH). In the latter, the spatial resolution and FOV are determined by the photodetector array dimensions and the pixel pitch, respectively. Synthetic aperture interferometry (SAI) offers an alternative solution by enabling the measurement large FOVs with high spatial resolution by coherently combining several low-resolution reconstructions, each with a large FOV. This requires the acquisition of multiple digital holograms, captured with multiple illumination and/or observation directions.

The Optical Engineering Group at Loughborough University has been investigating a SAI system through the project “Synthetic aperture interferometry: High-resolution optical measurement over an exceptionally large field of view” (EP/M020940/1). The purpose of the project was to develop a SAI system based on an array of phase-locked coherent imagers and illumination sources to synthesize a large numerical aperture ~0.5 over a FOV ~ 150 mm × 150 mm at a working distance of ~150 mm. Each coherent imager works as an independent holographic camera connected to a Raspberry Pi computer for image acquisition and data processing. It is a scalable approach, in which signals from all coherent imagers can be processed in parallel to reconstruct the measured surface with high-resolution over a large FOV.

This Thesis explores the design, construction, and characterization of a coherent imager (CI) with a large FOV, as a building block for future modular SAI setups of different nature (multiple sensors, multiple illumination directions, reflection or transmission mode, etc). The proposed CI is a compact lens-less holographic camera with a FOV of ~ ±0.57 rad (the largest reported to date). The CI includes: 1) a low-cost colour sensor with 1.12 mm pixel pitch that behaves as a high-resolution monochrome sensor at near infrared wavelengths; 2) an entrance pupil to increase the sensor effective FOV while avoiding aliasing; and 3) a high NA reference source close to the sensor. The CI requires a bespoke model, data analysis and optimization approach to demodulate the holograms and reconstruct the complex field of the object with nearly diffraction limited performance. Once fully characterized, the CI is used in conjunction with multiple illuminations to demonstrate SAI in transmission mode, to obtain high-resolution reconstructions in a large FOV, beyond the limitations given by the sensor’s pixel pitch.