Dispersion in optical configurations and sources for Optical Coherence Tomography

Abstract

Optical coherence tomography is a biomedical imaging technique employed to visualize tissue structure, ocular vasculature and blood flow. An OCT system is a non-invasive biomedical imaging system that provides bi-dimensional and three-dimensional images of biological tissue with micrometer scale resolution and millimeter scale depth range. In clinical application, OCT is employed in vivo imaging of the human eye. Swept source optical coherence tomography (SS-OCT) is the latest and fastest method of scanning. However, there are several disadvantages of the SS-OCT such as: the decrease of the roll-off sensitivity as the depth of scanning increases and the presence of mirror terms in the OCT images. The main objective of the work presented throughout this thesis was to evaluate the effects of dispersion on the performance of OCT. This study extends the research from the optical configurations at the core of OCT systems to the optical sources, where we show that dispersion can be usefully employed to obtain sweeping. We prove that an akinetic swept source (AKSS) can be devised for the important band of OCT, 800 nm, where there is still no MHz swept source available. In a Michelson interferometer a Fourier domain optical delay line was used for dispersion compensation that subsequently was employed to control the dispersion in an OCT system. A first goal of this thesis was to increase dispersion in order to evaluate the possibility of removing mirror terms from the OCT images. Therefore, three methods of dispersion measurement were evaluated. The first method measures the full width half maximum of the autocorrelation function. The second method uses a super continuous laser and an acoustic-optic tunable filter to measure the path dispersion in the position of the autocorrelation peak. A third method consists in a fitting method applied to channeled spectra collected from the interferometer when using a mirror. A second goal of this thesis was to prove the usefulness of employing dispersion in building a SS. In the process of akinetic swept source optimization, several types of dispersive fibres were tested and the most optimum conditions for driving a semiconductor optical amplifier were established. A dual mode locking scheme was used to tune the akinetic swept source at MHz rates. The axial range of the swept source was evaluated by scanning through the channeled spectrum of a Michelson interferometer.