Robust Laser Sources for Ultrafast Imaging Systems

This dissertation, "Robust Laser Sources for Ultrafast Imaging Systems" by Xiaoming, Wei, 韦小明, was obtained from The University of Hong Kong (Pokfulam, Hong Kong) and is being sold pursuant to Creative Commons: Attribution 3.0 Hong Kong License. The content of this dissertation has not been altered in any way. We have altered the formatting in order to facilitate the ease of printing and reading of the dissertation. All rights not granted by the above license are retained by the author. Abstract: Optical imaging has not unleashed its full potential for a better temporal resolution, which is in great demand for the studies of high-speed dynamical phenomena. Traditional imagers incorporated with CCD/CMOS electronic sensors are fundamentally limited by their on-chip storage and readout speed. Time-stretch imaging, on the other hand, has been proved to be a promising imaging modality for high throughput screening and transient dynamics observation. However, it has suffered greatly from the noise issue of those commonly-used laser sources, thus the averaging has been usually performed to suppress the noise. More importantly, the time-stretch imaging has been mostly demonstrated in microscopy for lateral information visualization, while its tomography and spectroscopy modalities for axial and chemical information, respectively, have been rarely studied yet. In this regard, this thesis is to explore robust laser sources to extend the capacity of those ultrafast imaging systems. In brief, it includes: 1) actively-stabilized supercontinuum (SC) laser source, and the comparison with spectral-broadening-free source for ultrafast flow imaging; 2) 10's-MHz swept sources for ultrafast axial imaging; 3) dual-color sources and nonlinear dynamic investigation; 4) broadband fiber parametric optical amplifier for sensitivity and bandwidth enhancements. 1) SC source has been usually employed to conduct the time-stretch imaging. The SC generation, unfortunately, is a noise-initiated process, which delivers strongly-fluctuated time-stretch signal. Here, we introduce a narrow-linewidth CW source to stimulate the spectral broadening in SC generation. We analyze the linewidth influence of CW source on the SC stability and coherence. We then propose a compact additive-pulse mode-locked (APM) fiber laser for the comparison purpose at the bio-favorable window. 2) Regarding the time-stretch imaging has been largely demonstrated in lateral microscopy, we propose a breathing laser as inertia-free swept source (BLISS) for ultrafast axial imaging. In BLISS, time-stretch is employed as an all-optical wavelength-scanner for a 10's-MHz sweep rate. BLISS is designed with a compact configuration with superior intensity and phase performance. Its bandwidth, particularly, can be tuned from 10 nm to 100 nm. It is especially to be applied to time-stretch microscopy, optical coherence tomography (OCT) and phase-sensitive microscopy. 3) Sources mentioned above have been designed for a specific window, while multi-color source is a great demand for those hyperspectral applications. Thus, an all-optically synchronization technique is incorporated to generate dual-color lasers. The capacity of pulse manipulation in between is studied by injecting various optical patterns at a larger wavelength detuning. These nonlinear dynamic behaviors therein inspire us to gain insight into the mystical world of a fiber cavity. Thus we further introduce a nonlinear dynamic platform ― a noise-like mode-locked all-fiber laser, to investigate the stochastic process in broadband fiber lasers. 4) Finally, to strengthen the performances of those described fiber laser sources in ultrafast imaging systems, an all-fiber optical parametric amplifier for life-science (OPALS) at 1.0 m with superior performance outperforming other counterparts is introduced. As a unique feature of OPALS, it can coherently generate a synchronized signal replica, w