published_or_final_version / abstract / Electrical and Electronic Engineering / Master / Master of Philosophy

Thesis (M. Phil.)--University of Hong Kong, 2008. / Also available in print.

Thesis (M.Phil.)--Hong Kong University of Science and Technology, 2007. / Includes bibliographical references (leaves 84-88). Also available in electronic version.

Fiber optical parametric amplifier (FOPA) is undoubtedly one of the most thriving research topics about optical amplifiers during the past decades. The high optical gain, arbitrary gain regions and wavelength conversion with large frequency shift make FOPA outstanding in diverse application areas like the high-speed all-optical communication, wavelength-tunable laser sources and optical imaging systems. Special fiber gain medium and proper pump source are two essential elements in an FOPA setup. As the research interest on FOPAs has recently gradually extended from the conventional 1.5-_m region to the shorter wavelength band at 1 μm, photonic crystal fibers (PCFs) act as the gain media by virtue of their customized dispersion curve and nonlinearity in this band. And the 1-mum laser sources incorporating ytterbium-doped fiber (YDF) as the gain medium have been investigated as well. We prefer all-fiber laser as the pump source not only because of its high output quality but also its compatibility with other fiber systems like FOPA. However, compared with 1.5-_m range, fiber lasers in 1-_m wavelength window have not been fully developed. Most of the laser sources reported in this wavelength range are not all-fiber base. For those few all-fiber reports, the tuning range of the pulsing wavelength is not wide enough, which might limit the performance of the FOPA. In this thesis, we have investigated tunable fiber lasers aiming at becoming the promising pump sources for 1-μm FOPAs. All-fiber lasers with different techniques and operation schemes based on the YDF have been discussed. Tunable ytterbium (Yb) fiber lasers with short pulsed output are important for pulsed-pumped FOPAs in 1 _m. Passive and active mode locking techniques are both commonly employed in short pulse generation. Passive mode-locking laser cavity usually works at the fundamental frequency of the cavity (?MHz) and has the potential to generate ultra-short pulse (? fs) due to its fast recovery time. On the other hand, active mode locking is more agile in terms of the repetition rate, which is synchronized with the external electrical signal. It can be as high as tens of GHz, which is useful for high-speed optical communication, and also can be as low as tens of MHz, which can benefit applications that require high peak power. For an all-fiber mode-locked laser based on YDF, the self-starting of the passive mode locking in 1 _m is more difficult than in 1.5 μm due to the large value of the normal material dispersion in optical fibers in this shorter wavelength range. In this thesis, we have focused on the active mode-locking cavity. Two schemes of actively mode-locked fiber lasers have be demonstrated. One is with a high repetition rate of about 10-GHz at around 1030 nm. The 30-nm tuning range is beneficial to the development of the wavelength-division multiplexing (WDM) technology in the newly developed 1-μm communication band. And on the basis of this scheme, another actively mode-locked fiber laser with a wider tuning range (almost 50 nm) have been achieved by optimizing the length of the YDF inside the cavity. Considering the applications like fiber sensing or spectroscopy where high peak power is more essential and also due to the limitation of our 980-nm pump power, the repetition rate has been lowered down to around 300 MHz in the second scheme. Tunable continuous-wave (CW) fiber lasers in 1 _m have also been discussed. For an all-fiber ring laser cavity, a stable CW output without mode-hopping can be achieved by selecting out single frequency. Various experimental configurations have been proposed for single-longitudinal-mode (SLM) oscillation. We have combined the multiple-ring cavity (MRC) and the saturable absorber in the same fiber laser cavity to facilitate the SLM generation in 1 _m. The tunable CW SLM fiber laser has the potential to build a sweeping source with instantaneous narrow linewidth for optical coherence tomography (OCT) in this range. It can also be utilized as the pump source for CW FOPAs, which is more immune from the walk-off effect between the pulsed pump and the signal, as long as the stimulated Brillouin scattering (SBS) has been suppressed properly. / published_or_final_version / Electrical and Electronic Engineering / Master / Master of Philosophy

With the data explosion brought about by smartphones and tables during the past few years, how to keep these ever-increasing data in a stable, fast and green transmission and exchange environment is among the top problems for researchers in the communication field. As the backbone for the modern communication network, optical fiber communication is currently playing a key role in this on-going technology revolution. The optical amplifier is one of the most powerful tools of the optical communication system to cope with the data explosion. Distributed parametric amplification (DPA), with its potential green characteristics, i.e. noiseless, high-speed response, high power efficiency and wavelength flexibility, provides a promising amplification solution for the next generation of optical communication systems. As on specific type of optical parametric amplification (OPA), DPA is based on the combination of self-phase modulation (SPM), cross-phase modulation (XPM) and four-wave mixing (FWM) effects. DPA’s main difference from OPA lies in the amplification medium. As DPA utilizes the most commonly adopted transmission fiber, i.e. single-mode fiber (SMF) and dispersion-shifted fiber (DSF), the signal transmission can thus be fulfilled simultaneously with the parametric amplification in the same optical fiber: DPA’s configuration also brings another green feature, pump-power recycling, which further enhances the power efficiency of the communication system. As the fundamental study on DPA, first the gain spectrum is investigated. Both single- and two-pump DPAs are presented experimentally for WDM signals. In these experiments, residual pump power recycling is enabled by a concentrated photovoltaic (CPV) cells, Moreover, through experimental comparison with another important distributed amplification technology, distributed Raman amplification (DRA), DPA’s advantages over DRA are demonstrated. When considering similar performance levels, DPA needs much lower pump power than DRA, which in return improves the system power efficiency. The performance of DPA cannot be judged unless it is assessed in more advanced application scenarios. Thus more advanced studies on DPA are conducted. The modulation format transparency is first presented with both phase (differential phase-shift keying (DPSK)) and intensity (on-off keying (OOK)) modulation formats, and our experimental results show the superiority of DPSK over traditional OOK. Furthermore, from the perspective of wavelength flexibility, we have demonstrated, for the first time to the best of our knowledge, a DPA system at the 1.3μm telecommunication window, which provides a potentially green amplification scheme at this transmission band. All these experiments, to a certain extent, certify the feasibility of DPA to become a green optical fiber amplifier. Finally, to demonstrate DPA’s compatibility within a more complicated communication system, we propose a power–efficient UWB/DPA system for the “last mile”. After experiments on photonic UWB pulse generation and the supporting DPA system, the hybrid UWB/DPA system is demonstrated with preliminary simulation results. My research efforts presented in this thesis all aim at the practical application of the DPA scheme into the next-generation of green communication systems. If further armed with the phase-sensitive configuration, DPA’s potential as a green amplifier will be further augmented. / published_or_final_version / Electrical and Electronic Engineering / Doctoral / Doctor of Philosophy

With the increased requirement on the ultrafast measurement technology, how to resolve the spectral dynamics has the top priority on the research list, since spectrum is an essential carrier for most of the physical or chemical phenomena. According to the Nyquist-Shannon sampling theorem, if a function 𝑥(𝑡) contains no frequencies higher than 𝐵 Hz, it is completely determined by giving its ordinates at a series of points spaced 1/(2𝐵) seconds apart. Since most of the conventional optical spectrum analyzers (OSAs) are operated with the sampling rate (or frame rate) of 5 Hz, it results in the resolvable bandwidth of the spectrum dynamic is less than 2.5 Hz. With the development of the space-time duality, the analogy transforms the conventional spatial dimension into the time axis, and the well-known spatial models inspire us in performing their counterparts in the time domain. As one of the most powerful tools in achieving ultrafast time axis information, time-lens plays a more and more important role in the single-pixel imaging system. By fully analyzing the diploma of the previous optical spectrum resolving mechanisms, in this thesis, for the first time, we raised up the concept the parametric spectrotemporal analyzer (PASTA), which is based on the time-lens focusing mechanism. Here the spectrum resolving frame rate is increased to 100 MHz, then the observable spectrum dynamic bandwidth could be 50 MHz, which is sufficient for most of the ultrafast phenomena. In the PASTA system, the time-lenses are implemented with the fiber optical parametric amplifier (FOPA) based parametric mixer, which provides higher conversion efficiency and repetition rate. On the other hand, the dispersion based dispersive Fourier transformation (DFT) technology generates the swept-pump for the FOPA, as well as the temporal dispersion medium. This research in this thesis is a fundamental study on the newly PASTA system, from its origin and the theoretical background, to the implementation techniques and operation performances. From its implementation, its principles are strongly related with the combination of the dispersion and the Kerr nonlinear effects, especially the swept-pump FOPA in the time-lens part. The DFT technique, in generating the fast swept-source, has also find its applications in the ultrafast serial time-encoded amplified microscopy (STEAM) and swept-source optical coherence tomography (SS-OCT) systems. Finally, the single-lens PASTA prototype is capable of resolving 5-nm wavelength range with 0.03-nm resolution under 100-MHz frame rate. Moreover, besides the singlelens PASTA, the telescope/wide-angle configurations have also been investigated experimentally to achieve the spectrum zoom in/out ratio as high as 17 times, here we have obtained the sharpest resolution of 5 pm (<1 GHz) with the telescope configuration, and the widest observation range of 9 nm with the wide-angle configuration. My research efforts presented in this thesis mainly leverage the ultrafast characteristics of the time-lens system, from theory to implementation, and achieve the real-time optical spectrum analysis – the PASTA system. PASTA is not only essential in observing some non-repetitive ultrafast phenomena, but also provides a potential solution for the frequency to time transformation in some ultrafast bio-medical imaging systems. / published_or_final_version / Electrical and Electronic Engineering / Doctoral / Doctor of Philosophy

Thesis (Ph. D.)--University of Missouri-Columbia, 2007. / The entire dissertation/thesis text is included in the research.pdf file; the official abstract appears in the short.pdf file (which also appears in the research.pdf); a non-technical general description, or public abstract, appears in the public.pdf file. Title from title screen of research.pdf file (viewed on November 29, 2007 Vita. Includes bibliographical references.

A FORTRAN program has been developed in order to investigate the process of modal parameter estimation and non-parametric system identification. The theory underlying the process of modal parameter estimation is reviewed and the decoupling of a MIMO system into several SISO systems is demonstrated. Modal filtering is shown to be useful in the field of non-parametric system identification and it is shown that it may also be of some use in the field of signal processing. The program is documented. It simulates the output of a n-th order system from which a smaller order subsystem can be decoupled. The modal parameters of a subsystem output signal and its first two derivatives and the modal parameters of a second subsystem output and its first derivative are calculated. The unit step response of the theoretical system and the subsystem are then calculated. The signals are then modal filtered to produce the periodic unit step response and the periodic unit square wave response. Finally, the discrete Fourier coefficients of the periodic unit step response are calculated. / Applied Science, Faculty of / Electrical and Computer Engineering, Department of / Graduate

published_or_final_version / Electrical and Electronic Engineering / Master / Master of Philosophy

Thesis (M. Phil.)--University of Hong Kong, 2009. / Includes bibliographical references. Also available in print.