Development of advanced label-free optical bioimaging technologies

Abstract

Today label-free bioimaging has been leading to widespread and fast-growing applications, which demands for a more efficient way to keep up such momentum. To this end, the research in this thesis will study the techniques of efficiency improvement for advanced label-free bioimaging, including the time efficiency, cost efficiency and information efficiency. Optical coherence tomography (OCT) is one of the most valuable label-free bioimaging modalities to provide noninvasive cross-sectional assessment of biological tissue. In many occasions, these applications demand for three dimensional (3D) imaging at video-rate in order to perform real-time diagnoses, which can be overcome by MHz-OCT. Here we demonstrate inertia-free all-optical ultrahigh-speed swept-source optical coherence tomography (OCT) based on amplified optical time-stretch (AOT). More importantly, the key significance of AOT-OCT is its broadband amplification stage, which greatly enhances the detection sensitivity compared with the prior attempts to employ optical time-stretch to OCT. We report an AOT-OCT system which is operated at an A-scan rate of multi-megahertz with high sensitivity (>80 dB) and perform time-stretch-based OCT of biological tissue in vivo. Moreover, using a more stable and coherent mode-locked fiber laser, we can achieve better performance without the compromise of averaging for supercontinuum-generation-based AOT-OCT system. It represents a major step forward in utilizing AOT as an alternative for achieving practical time-efficient OCT imaging at multi-MHz speed. For the further development of this ultrahigh-speed OCT, we present a theoretical analysis of the AOT-OCT system. The spectral resolution, coherence length and sensitivity of AOT-OCT system have been discussed in detail. By theoretical model of the noise sources based on Raman amplifier, we also quantify how the input signal, amplifier gain, A-scan rate affect the sensitivity of AOT-OCT imaging. These simulation results are expected to be valuable for optimizing the design of AOT-OCT. We also investigate in cost-effective implementation to realize efficient optical time-stretch process based on dispersive fiber. We explore and demonstrate the feasibility of using the standard telecommunication single-mode fibers as few-mode fibers (FMFs) for optical time-stretch confocal microscopy in the 1m range. It can provide sufficiently high dispersion-to-loss ratios for practical time-stretch imaging at 1 m, without the needs for high-cost specialty 1 m single mode fiber. In addition, Coherent anti-Stokes Raman scattering (CARS) microscopy is another attractive efficient tool for label-free biochemical-specific imaging, which can bypass laborious steps of preparing and staining in routine standard histopathology. Here we further explore ultrabroadband hyperspectral multiplex (HM-CARS) to perform chemoselective histological imaging with efficient information in fingerprint region. In order to unravel the congested CARS spectra, we employ phase-retrieval algorithm based on Kramers–Kronig (KK) transform and principal component analysis (PCA) to display the key cellular structures with components distribution. All these research efforts are aiming at improving the efficiency, from theory to implementation, for label-free bioimaging technology such as OCT and CARS. These schemes demonstrate great potential to realize powerful label-free bioimaging with high efficiency, including ultrafast 3D OCT imaging at video-rate, cost-effective optical time-stretch imaging and HM-CARS imaging with richness of biological fingerprint information.