Towards high-speed Swept Source Optical Coherence Tomography

Optical coherence tomography (OCT) has evolved into an impactful biomedical imaging modality, with swept-source OCT (SS-OCT) emerging as a key technology for pushing axial scan rates from hundreds of Hz to tens of MHz. However, achieving these high speeds introduces critical challenges: (i) developing light sources capable of emitting stable multi-MHz sweeps, (ii) scaling lateral scanners to match axial speeds, and (iii) enabling real-time processing with digitizers exceeding tens of GSa/s. Resolving these limitations is imperative, as high-speed OCT unlocks transformative applications-from capturing dynamic processes to reducing motion artifacts in clinical imaging and enabling large-volume diagnostics. Advancements in this area promise to expand OCT's role in functional imaging, intraoperative guidance, and high-throughput screening, bridging the gap between laboratory innovation and real-world clinical impact.

At first, it remains hard to produce swept sources that are able to sweep at multi-MHz; for that, a sweep modality that is able to cope with high rates such as time stretch has been investigated in this thesis to overcome the issues above. A commercially available Erbium-doped amplifier is transformed into a mode-locked laser by nonlinear polarization rotation, which seeds a dispersion compensation fibre, stretching the pulse duration to nearly 200 ns. The OCT capabilities of the source are tested, and the main imaging parameters, such as axial resolution, range and drop-off, are characterized. Secondly, a 40 MHz swept source laser based on time-stretch after a supercontinuum is used along with an electro-optic deflector, a potassium tantalum niobate (KTN) crystal, producing a system that was able to capture volumes at 400 Hz. However, the processing to obtain the OCT images is not done in real-time. As such, in the final part of this thesis, alternative processing algorithms are explored to generate real-time in-vivo images of the retina and cornea; i) through numerical procedures, a novel FDML laser at 850 nm is used at 828 kHz with bidirectional sweeping. For the first time, an FMDL laser has been integrated with complex master-slave (CMS) systems; the laser has been fully characterized and enables real-time B-Scans and \textit{en-face} imaging of the human retina. ii) A MEMS-VCSEL swept source centred at 1060 nm working at 1.6 MHz is used in a downconversion scheme where two interferometers are used, and the high frequencies generated in the measuring interferometer are mixed with a reference signal to downconvert the signal to lower frequencies and alleviate the requirements needed for a high sampling rate digitiser. The main similarities and differences between the numerical processing methods and analogue methods are given. Finally, \textit{en-face} OCT images of the retina with the downconversion scheme are presented.