Time-Stretched Supercontinuum Swept-Sources for High-Speed Optical Coherence Tomography

Since its introduction, optical coherence tomography (OCT) has become a widely used imaging technique for non-invasive, three-dimensional visualisation of biological tissues. Although resolution has steadily improved, imaging speed remains a key limitation-particularly for reducing motion artefacts and enabling real-time volumetric acquisition. Swept-source OCT (SS-OCT), which uses rapidly wavelength-tunable lasers, has enabled imaging at MHz-scale A-scan rates. However, most conventional swept-source designs rely on mechanically tuned elements, which fundamentally restrict further increases in speed. To overcome this, recent research has turned to akinetic swept-source technologies, which avoid moving parts entirely.

This thesis explores an akinetic swept-source approach based on time-stretched supercontinuum generation. In this technique, ultrashort, high-peak-power pulses are passed through a nonlinear medium to generate a broadband supercontinuum, which is then dispersed through a highly chromatic medium. This process creates a passive wavelength sweep through group delay dispersion. First proposed by Moon et. al. in 2006, the approach was initially limited by high noise levels inherent to conventional supercontinuum sources. However, with the development of all-normal dispersion (ANDi) fibres, it became possible to generate broadband light with much lower noise. This work builds on that foundation, investigating whether an ANDi-based time-stretched supercontinuum source at 1060 nm could be suitable for high-speed SS-OCT.

Two swept-source systems were developed as part of this work. The first, operating at 80 MHz, used a fibre-based dispersive stretcher, with the nonlinear broadening optimised through numerical simulations. OCT imaging was demonstrated at an A-scan rate of 40 MHz, and full-volume imaging at 400 Hz was achieved using a fast electro-optic scanner. Although promising, this high-speed setup proved less suited to imaging weakly scattering samples, such as the retina, due to shot and electronic noise. A second source operating at 10 MHz was therefore developed. This version used a custom-fabricated chirped fibre Bragg grating to provide the required dispersion while avoiding the losses associated with long fibre stretchers. A pulse picker was also implemented to reduce the laser's repetition rate, and the supercontinuum generation process was further refined. Using this lower-rate source, low-noise in vivo imaging was successfully carried out on dermal and retinal tissue. Fine structures such as sweat ducts, the retinal nerve fibre layer, and the choroid were clearly visualised. These results demonstrate the feasibility of ANDi-based time-stretched supercontinuum sources for high-speed OCT and highlight their potential for a wide range of biomedical imaging applications.