Dispersion-tuned mode-locking and multi-harmonic operation for swept-source optical coherence tomography

Swept sources are central to optical coherence tomography (OCT), which enabled not only high speeds but also longer imaging depths than in the spectrometer-based OCT methods. Among the various types, dispersion-tuned mode-locked (DTML) lasers offer simplified and cost-effective swept source designs. While standard DTML operation has been demonstrated across most wavelength ranges relevant to OCT imaging, its limitations in sweep rate and coherence performance remain a challenge. Moreover, a broader theoretical and experimental understanding of the multi-harmonic DTML operation has been lacking.

This thesis presents a comprehensive theoretical framework and experimental validation of both single- and multi-harmonic DTML regimes. Starting from the analysis of dispersion properties in fibre laser cavities, the principle of wavelength tuning in standard DTML lasers is described. By adopting a spectral filtering approach, notions of the saturation and one round trip operations are introduced, showing trade-offs between the achievable tuning bandwidth and coherence performance. The work then extends to the generalised multi-harmonic DTML operation, introducing the many sweep cycles mathematical framework and establishing the conditions under which wavelength tuning is achieved when sweeping over multiple harmonic bands. Building on the spectral filtering description, the tuning bandwidth limits for the multi-harmonic DTML are derived, showing trade-offs between the achievable tuning bandwidth and coherence performance in this sweeping regime. In the last theoretical chapter, the trends in mathematical modelling of DTML lasers are reviewed, extended with original contributions. Lastly, using the discrete modelling approach, the behaviour of spectral linewidth in a DTML laser as a function of various cavity parameters is investigated. Experimental work includes the development of three DTML swept sources operating at 850nm, 1550nm, 1µm, respectively, with the latter two demonstrating the multi harmonic operation at sweep rates approaching 1MHz. By employing the final 1µm swept source, the first OCT images employing a multi-harmonic DTML swept laser are captured, and the theoretical predictions regarding bandwidth-coherence trade-offs are ii experimentally validated. A set of optimisation steps to improve the multi-harmonic DTML operation to guide the future development is proposed as well. Given the findings in this thesis, the multi-harmonic DTML laser emerges as a promising fully akinetic, simple in design, low cost solution for megahertz SS-OCT applications.