Development of an Electronic Speckle Pattern Interferometry System for Non-Contact Nanoscale Surface Measurement and Its Application to Acoustic Wave Detection
Abstract
The pursuit of non-invasive, high accuracy biomedical imaging modalities has prompted the exploration of innovative methods such as photoacoustic tomography (PAT). The photoacoustic wave generated from an optical absorber shows promise for measuring physiological processes and diagnostic imaging of medical conditions. However, the application and adoption of PAT for clinical use face challenges. Non-contact interferometric optical techniques have shown potential in addressing these challenges by improving resolution and usability. Nevertheless, there remain gaps in our understanding of the performance of non-contact detection methods. Furthermore, these techniques require further optimization related to faster recording speed, larger field of view (FoV), and broader frequency detection range.
This thesis first presents a digital holographic (DH) instrument that incorporates several modifications to improve performance. The DH setup was assembled on an air-floated optical table and shielded from stray light to effectively isolate it from external disturbances. A large sensor (36.4 mm × 27.6 mm) with matched detection beam size resulted in an enlarged FoV compared to similar systems reported in the literature. Static measurements on a resolution target revealed a resolution of 10 nm axially (out-of-plane). The DH system was also capable of tracking dynamic surface events in both solid and liquid materials.
The thesis also presents a double exposure electronic speckle pattern interferometry (ESPI) system designed with higher imaging speed for acquiring a broad spectrum of surface vibrations. This instrument utilized precise timing to coordinate multiple subsystems to capture signals at microsecond timescales. The ESPI system was tested on phantoms designed to mimic biological materials. Custom ESPI analysis software revealed nanometer-scale surface topography and confirmed the capability of the system to provide broadband detection of frequencies from 1 Hz up to 150 kHz. Additionally, the system was successfully applied to non-contact measurement of photoacoustic waves by capturing the surface motion on phantoms of varying thickness, optical density, geometry, and absorber arrangement.
The findings demonstrated the ability of DH and ESPI to capture out-of-plane nanoscale surface movements over a large field of view and at timescales down to 1 μs. Additionally, successful detection of photoacoustic waves motivates future research on testing ESPI as a possible detection method for non-contact photoacoustic imaging.