Advanced label-free imaging modalities empowered by novel fiber-based methodology

Abstract

Label-free and non-intrusive imaging modalities have brought about a paradigm shift in the imaging field by eliminating the reliance on exogenous contrast agents and minimizing sample perturbations. However, despite the progress made in label-free imaging, there is still a growing need for higher contrast, resolution, detection sensitivity, and faster imaging speed of these modalities. Considering this, this thesis aims to enhance the performance of label-free microscopies by developing and leveraging novel fiber-based methodology. This methodology, including mode-locked lasers (MLL), modulated pulsed lasers, time-stretch technique, and phase-sensitive amplifier (PSA), offers innovative optical sources, distinctive contrast formation mechanisms, and advanced signal processing modules for imaging, which optimizes the capabilities of label-free modalities by enhancing contrast, resolution, detection sensitivity, and imaging speed. To enhance detection sensitivity for phase signals, a fiber-based PSA is developed as a nonlinear signal processing platform for phase retrieval. Specifically, a fiber-based MLL is constructed and phase-stabilized using a phase-locked loop, and a copier-PSA scheme is established via nonlinear parametric amplification techniques. Numerical simulations are performed to investigate the gain characteristics of the PSA system. Systematic investigations are then carried out to verify its phase retrieval capability and demonstrate its exceptional phase sensitivity at the nanoscale. To augment inherent contrast formation from phase objects, a quantitative phase microscopy (QPM) is developed by leveraging the PSA system. The illumination and signal processing module of the QPM is ingeniously constructed based on the PSA, which forms a nonlinear interferometric scheme with unique phase reading capability. The phase information could be directly retrieved from spectral amplitudes by a self-developed algorithm. To further enhance the imaging speed, the time stretch technique is exploited in conjunction with a line-scanning configuration, boosting the signal acquisition rate to an MHz level. The QPM demonstrates fast and phase-contrast imaging upon various samples. To realize bond-selective imaging with high optical resolution and signal-to-noise ratio (SNR), a tailored pulsed fiber laser source, targeting the first overtone of the C-H bond, is utilized for constructing a photoacoustic remote sensing microscopy (PARS). The laser is based on a modulated Thulium-doped fiber laser, which excites intensive photoacoustic signals from lipids. In addition, the PARS exploits an optical sensing configuration for signal collection, which enables broadband detection and eliminates direct contact with bio-samples. This approach allows for high-quality tissue-scale lipid imaging, simultaneously featuring high SNR, deep penetration depth, and high optical resolution. Finally, to explore label-free vibrational contrast imaging, a novel optical generation at mid-infrared (MIR) is developed to take advantage of its characteristic vibrational spectroscopy. The laser is mode-locked to produce ultrashort pulses by a nonlinear polarization rotation mechanism. Intriguingly, the source showcases a unique spectral transition phenomenon among diverse soliton molecule states. Based on a proposed pump-probe photothermal scheme, the source is expected to enable live cell imaging at MIR wavelength. Overall, high-performance optical imaging systems rely on advanced optical sources, contrast formation means, and signal processing platforms. Fiber-based methodology including MLLs, modulated laser, PSA, and time stretch technique provide powerful and versatile solutions for constructing label-free microscopic systems.