Femtosecond Laser Inscribed Calibration Phantoms for Optical Coherence Tomography

Abstract

The thesis describes work using a femtosecond laser to fabricate high precision microstructures as multipurpose 3D OCT calibration phantoms that can be used to quantitatively characterise and calibrate across different OCT systems. The research has been focused on three areas in developing the OCT calibration phantom - laser power characterisation, fabrication optimisation and novel multipurpose OCT phantom development and phantom application. For the laser power characterisation process, a detailed in-depth study was performed investigating the correlation between laser power and inscription size. Test phantoms were inscribed with different laser pulse energies. The results showed that with increased laser pulse energy, both the linewidth and the cross-inscription height were increased. The critical power of self-focusing was exceeded when the laser power was around 63% of the total laser average output power. As the non-linear effect caused unexpected control of the inscription size along the axial direction, a laser power range below the self-focusing threshold was selected to inscribe the phantoms which was still enough to extend the design to reach greater depths. The phantom fabrication optimisation with a layer-by-layer inscription method has allowed inscriptions at a greater depth up to 2mm whilst keeping the inscription size uniform and consistent for all depths. For the inscription method, the laser power was reset at beginning of the inscription for each layer which allowed a customised option to be made to get the desired inscription size that can be kept consistent and uniform for all depths. In order to address the challenges of inscribing non-planar samples, a series of multipurpose of OCT phantoms were designed and fabricated. A non-planar phantom was initially proposed that comprised of a grid-like pattern inscribed inside a planoconvex lens as the planar side provided a standard non-distorted image and the curved side provided a distorted image which can be used to detect the scanning errors and the post-processing algorithms of the OCT system for distortion correction. However, the grid-like pattern required a highly degree of alignment under the OCT system. Subsequently, a circle-like pattern was proposed to overcome this angular issue with a landmark layer consisting of a series of radial lines inscribed at the top of the test pattern to guide the location. To investigate the impact of OCT image distortion, a planar phantom was designed and used as a reference to correct the distortion due to the scanning errors caused by the OCT system itself. With a known reference, a phantom-based distortion correction method is possible with the reduction of the scan spacing error by 82% which extends the potential use of the OCT phantoms beyond a qualitive measurement tool.