Advanced temporal imaging systems for ultrafast optical dynamics observation and signal processing

Abstract

Single-shot and real-time characterization of optical waveforms and spectra are desired in various fields of scientific research. In nonlinear optics, such capability helps unveil intriguing dynamics such as optical rogue waves, breather solitons, soliton explosions, modulation instabilities, etc. Fully resolving these dynamic processes in both time and spectral domains will significantly deepen the understanding of the interdisciplinary principles behind. However, there is a large gap between the capability desired and the performance of commercial products currently available. Facing this dilemma, temporal imaging techniques have been developed inspired by the space-time duality, which process the signal-under-test to greatly lower down the requirement for detection equipment. For example, time magnifiers can “slow down” temporal waveforms by more than 100 times such that sub-picosecond temporal resolution can be achieved by off-the-shelf photodetectors and oscilloscopes. Dispersive Fourier transform (DFT) technique and temporal two-focal-length (2f) system convert the spectral information into temporal waveforms. Therefore, spectral characterization can be performed at tens-of-MHz frame rate by real-time measurement of the waveforms. However, the true potential of temporal imaging techniques has not been fully unleashed, which confines their wider application. In this thesis, we aim at not only developing advanced temporal imaging systems with expertise in complex dynamics observation, but also identifying novel applications of existing techniques. In the time domain, a panoramic-reconstruction temporal imaging (PARTI) system is proposed and constructed that is inspired by the spatial panoramic camera. The PARTI system overcomes the inherent limitation in time magnifiers on the record length while maintaining a high temporal resolution. Dissipative-soliton colliding dynamics evolving over 1.5 ns from a microresonator is comprehensively depicted for the first time. In the spectral domain, we utilize new principle to significantly expand the wavelength observation range of temporal 2-f system. Moreover, a more sophisticated spectral analyzing technique is invented to combine the feats of both DFT technique and temporal 2-f system, such that the spectrum of both short pulses and continuous waves can be analyzed simultaneously. Using this technique, the spectral evolution during the mode-locking process of a fiber mode-locked laser is holistically characterized. In addition to developing advanced temporal imaging systems with new principle of operation, novel applications are identified and demonstrated for existing techniques. First of all, the waveform-manipulation capability of temporal Talbot effect is utilized to significantly enhance the performance of temporal cloaking technique. Second, the DFT technique is applied to study a round-trip resolved spectral evolution of a passive mode-locked laser, which provides comprehensive understanding of the vector-pulse mechanism. Third, a 250-MHz temporal 2-f system has been constructed to explore the breather phenomena in a microresonator-based frequency comb. Last but not least, we apply the temporal magnification technique to the bio-imaging area for the first time. An ultrafast optical tomographic imaging modality is invented that reaches an A-scan rate of over 100 MHz. In summary, this thesis presents advanced temporal imaging systems as powerful tools for the study of intriguing dynamics in various research fields. In addition, the identified new applications further demonstrate the potential of temporal imaging techniques.