Swept source optical coherence tomography system development for bioimaging applications

Abstract

Optical coherence tomography (OCT) is an imaging modality with strengths in high-resolution, label-free, and noninvasive, which strategically fills the gap between ultrasound and microscopy and is superior for some biomedical applications. Swept source OCT (SS-OCT) improves system simplicity and sensitivity compared to previous versions of time domain OCT (TD-OCT) because of a static reference mirror and has faster imaging speed compared to spectral domain OCT (SD-OCT) because of a single photodetector and the availability of high-repetition-rate broadband lasers. Furthermore, functional spectroscopic OCT (SOCT) analyzes materials according to their specific optical properties at different wavelengths. Thus, by utilizing an ultra-wide band OCT and analyzing the sub-band images, it provides us a spectroscopic way to recognize materials. These kinds of newly developed OCT systems are quite promising in clinical bioimaging applications.

One of the most commonly used swept sources in SS-OCT systems is the Fourier domain mode-locked (FDML) laser. It has a narrow instantaneous linewidth that improves imaging range, a broad sweeping range that renders high resolution, fast building up time that enables fast scanning, and a proper output power. However, traditional FDML lasers have limited sweeping range because of a limited amplification window of the gain medium that is usually unable to achieve an ultra-wide wavelength range. It hinders its way in spectroscopic imaging applications. Nevertheless, multi-band illumination can break through the bandwidth limitation of single-band SOCT. Consequently, developing swept sources with various output wavelength ranges is of high interest. The FDML lasers with output at 1.0 µm range are most suitable in water-rich tissues where water dispersion and absorption are minimized. Thus, 1.0 µm FDML laser cavities are firstly proposed with different amplification schemes to compare their performances. Dual-and tri-band SS-OCT systems for SOCT application are further investigated for contrast enhancement and differentiation of materials, such as lipid and porcine artery.

Not only the output wavelength ranges of swept sources are of big concern, the swept speed is another bottleneck yet to be surpassed, especially for endoscopic applications. As the mechanical inertia of FDML lasers will limit the fundamental A-scan rate to hundreds of kilohertz, an ultrafast mode-locked laser together with inertia-free optical time-stretch mechanism is a promising alternative for SS-OCT endoscopic application. It has long imaging range and allows an A-scan rate up to more than ten megahertz. We further demonstrated this kind of amplified optical time-stretch (AOT) OCT with extensional application by endoscopic imaging.

In summary, the goal of this thesis is to develop and extend SS-OCT systems in different aspects for various bioimaging applications. The SS-OCT systems with different advantages are proposed for spectroscopic and endoscopic imaging.