Thesis (M.S.)--Oregon Graduate Center, 1988.

By utilising a CW mode-locked Nd:YAG pump laser an experimental study of self-phase modulation (SPM) and stimulated Raman scattering (SRS) in single mode optical fibres has been conducted. The dependence of the spectral broadening due to SPM upon the launched optical power was observed to obey a linear relationship in agreement with a simple theory. A deviation from this occurred for high input powers due to the onset of stimulated Raman scattering which caused a preferential depletion of the leading edge of the pump pulse and an increased spectral broadening to the long wavelength side of the spectrum. The pulses exiting the fibre were then compressed using a pair of holographic diffraction gratings, which were able to compensate for the linear part of the frequency chirp imposed on the pulse by SPM and the 1.06 ?m pulses were reduced in duration from ~ 100 ps to approximately 4 ps by this method. By making use of Raman generation in the fibre, a synchronously pumped fibre Raman oscillator was constructed. This enabled the generation of frequency tunable (1.07 - 1.12 ?m) near infrared pulses by the method of time dispersion tuning. By incorporating two fibre grating reflectors onto the ends of the optical fibre, an all-fibre device was constructed having the potential advantages of compactness and stability. The generation of mode-locked pulses around the 1.5 jim wavelength region was accomplished with the use of a colour centre laser based upon a stabilised F2+ centre in NaC1 or a thallium centre in KCl. Both of these lasers were examined, although to date the poor quality of our NaC1 laser crystals has meant that most of the work reported here was performed with KC1:T1. This laser produced pulses of ? 20 ps duration, tunable over 1.45 - 1.55 ?m with average powers ? 200 mW. A simple experiment to observe soliton propagation of these pulses in an optical fibre was conducted and this compressed the pulses to ? 0.8 ps, although this does not represent the optimum compression that could be achieved. Using nonlinear pulse propagation in an optical fibre, the mode-locked characteristics of the colour centre laser were dramatically improved with the duration of the pulses from the laser being reduced to ? 200 fs. This enhancement was achieved by the use of a nonlinear external cavity containing the optical fibre, which reinjected the pulses back into the main laser cavity, with an increased spectral bandwidth due to SPM. It was initially thought that the explanation to this effect was due to soliton formation within the control cavity, however experimental evidence is presented here which shows that the mode-locking enhancement phenomena is in fact quite general and does not rely on dispersion in the control cavity.

In this thesis, novel regimes of mode locking in quantum dot semiconductor laser diodes have been investigated by exploiting the unique features offered by quantum dots. Using an unconventional approach, the role of excited state transitions in the quantum dots was exploited as an additional degree of freedom for the mode locking of experimental quantum dot lasers. For the first time, passive mode locking via ground (1260nm) or excited state (1190nm) was demonstrated in a quantum dot laser. Picosecond pulses were generated at a repetition rate of 21GHz and 20.5GHz, for the ground and excited states respectively, with average powers in excess of 25mW. Switching between these two states in the mode-locking regime was achieved by changing the electrical biasing conditions, thus providing full control of the operating spectral band. A novel regime for mode locking in a quantum-dot laser was also investigated, where the simultaneous presence of cw emission in the excited-state band at high injection current levels, dramatically reduced the duration of the pulses generated via the ground state, whilst simultaneously boosting its peak power. This represents a radically different trend from the one typically observed in mode-locked lasers. From this investigation, it was concluded that the role of the excited state can not be neglected in the generation of ultrashort pulses from quantum-dot lasers. Stable passive mode locking of a quantum-dot laser over an extended temperature range (from 20ºC to 80ºC) was also demonstrated at relatively high output average powers. It was observed that the pulse duration and the spectral width decreased significantly as the temperature was increased up to 70ºC. The process of carrier escape in the absorber was identified as the main contributing factor that led to a decrease in the absorber recovery time as a function of increasing temperature which facilitated a decrease in the pulse durations. These results are shown to open the way for the ultimate deployment of ultra stable and uncooled mode-locked semiconductor diode lasers.

A photonic analog-to-digital converter (ADC) based on a robust symmetrical number system (RSNS) was constructed and tested. The analog signal to be converted is used to amplitude modulate an optical pulse from a laser using three Mach-Zehnder interferometers (MZI). The Mach-Zehnder interferometers fold the input analog signal for a three-channel RSNS encoding. The folding waveforms are then detected and amplitude-analyzed by three separate comparator banks, the outputs of which are used to determine a digital representation of the analog signal. This design uses the RSNS preprocessing to encode the signal with the fewest number of comparators for any selected bit resolution. In addition to the efficiency of its use of comparators, the RSNS encoding has inherent Gray-code properties making it particularly attractive for eliminating any possible encoding errors. The RSNS encoding is combined with an optical infrastructure that offers high bandwidth and low insertion loss characteristics. A full implementation was constructed and tested. The lack of a high-speed data acquisition device limited the results to examining the preprocessing and digital processing separately. With the system integration of a data acquisition device, a wideband direct digital antenna architecture can be demonstrated.

As a type of nonlinear system with complexity, mode-locked fiber lasers are known for their complex behaviour. It is a challenging task to understand the fundamental physics behind such complex behaviour, and a unified description for the nonlinear behaviour and the systematic and quantitative analysis of the underlying mechanisms of these lasers have not been developed. Here, we present a complexity science-based theoretical framework for understanding the behaviour of mode-locked fiber lasers by going beyond reductionism. This hierarchically structured framework provides a model with variable dimensionality, resulting in a simple view that can be used to systematically describe complex states. Moreover, research into the attractors' basins reveals the origin of stochasticity, hysteresis and multistability in these systems and presents a new method for quantitative analysis of these nonlinear phenomena. These findings pave the way for dynamics analysis and system designs of mode-locked fiber lasers. We expect that this paradigm will also enable potential applications in diverse research fields related to complex nonlinear phenomena.

Solid-state lasers are capable of providing versatile output characteristics with greater flexibility compared to other popular laser systems. Lasing action has been achieved in many hundreds of solid-state media, but Nd-ion doped gain media are widely used to reach high power levels with short pulses. In this work, commercially available Nd:KGW crystal served as a gain medium to achieve pulsed operation at 1067 nm. This laser crystal offers large stimulated emission crosssection and gain bandwidth which facilitates generation of high peak power pulses in the picosecond regime. The KGW crystal is monoclinic and biaxial in structure, and anisotropic in its optical and thermal properties. Due to poor thermal conductivity, this crystal can be operated within a limited power range before crystal fracture takes place. To reduce the amount of heat deposited in the gain media, we introduced a new pumping wavelength of 910 nm which reduces the quantum defect by more than 45%. Continuous-wave laser operation was optimized to operate in mode-locked regime. In order to achieve short light pulses from the continuous-wave laser, one of the end mirrors was replaced by a semiconductor saturable absorber mirror (SESAM) to generate 2.4 ps pulses at a repetition rate of 83.8 MHz. An average output power of 87 mW was obtained at lasing wavelength of 1067 nm and the beam was nearly diffraction limited with M^2 < 1.18. The peak power of the generated pulses was 427 W and energy of each pulse was >1 nJ. Pumping the crystal at longer wavelength (910 nm) reduced the thermal lensing of the crystal by half when compared to conventional pumping at shorter wavelength (808 nm). To the best of our knowledge, this is the first time passive mode-locking of a Nd:KGW laser was explored using the pump wavelength at 910 nm. / February 2017

by Kwok Chi Wing. / Thesis (M.Phil.)--Chinese University of Hong Kong, 1993. / Includes bibliographical references. / Abstract / Acknowledgements / Chapter 1. --- Introduction --- p.1 / Chapter 2. --- Basic Concepts of Lasers and Simple Survey of Laser Theories --- p.4 / Chapter 2.1 --- Introduction --- p.4 / Chapter 2.1.1 --- Basic Structure of a Laser --- p.4 / Chapter 2.1.2 --- "Concepts of "" Mode"" arid "" Mode-Locking""" --- p.6 / Chapter 2.2 --- Brief Review of Laser Theories --- p.9 / Chapter 2.3 --- Other Simple Models --- p.12 / Chapter 2.4 --- Review of the Maxwell-Bloch Equations --- p.17 / Chapter 2.4.1 --- Derivation of Maxwell-Bloch Equations --- p.17 / Chapter 2.4.2 --- Continuous-Wave Operation --- p.23 / Chapter 2.4.3 --- Mean-Field Approximation and Lorenz-Haken Instability --- p.24 / Chapter 2.4.4 --- Adiabatic Elimination of Fast Variables --- p.26 / Chapter 2.4.5 --- Thin-Sheet-Gain Approximation for Multimode Lasers --- p.30 / Chapter 2.4.6 --- Self-Mode-Locking Predicted by Using Maxwell-Bloch Equations --- p.33 / Chapter 2.4.7 --- Hysteresis Phenomena in Switching the Cavity Detuning --- p.35 / Chapter 3. --- "Moving-Mirror ""Mode-Locking""" --- p.41 / Chapter 3.1 --- Conventional Laser Mode-Locking --- p.41 / Chapter 3.1.1 --- Preliminaries： What is Mode-Locking (Conventional) ？ --- p.41 / Chapter 3.1.2 --- Active Mode-Locking and Passive Mode-Locking --- p.43 / Chapter 3.1.3 --- Spectra of Conventional Mode-Locked Lasers --- p.49 / Chapter 3.2 --- Moving-Mirror Mode-Locking --- p.50 / Chapter 3.2.1 --- Historical Notes --- p.50 / Chapter 3.2.2 --- Previously Proposed Explanations --- p.54 / Chapter 3.3 --- MMML Mechanism: our Proposal --- p.59 / Chapter 3.3.1 --- Relation between MMML Lasers and FSFC Lasers --- p.60 / Chapter 3.3.2 --- Concept of Moving Modes --- p.62 / Chapter 3.3.3 --- How are the Moving Modes Locked ？ --- p.64 / Chapter 3.4 --- Numerical Simulations ´ؤ Method and Results --- p.68 / Chapter 3.4.1 --- Description of Our Numerical Model --- p.68 / Chapter 3.4.2 --- Tests on the Simulation Method --- p.71 / Chapter 3.4.3 --- Ultrashort Pulses Generation of a MMML Laser --- p.73 / Chapter 3.4.4 --- Modulation of the Pulses --- p.74 / Chapter 3.4.5 --- Broadband or Discrete Spectra ？ --- p.75 / Chapter 3.4.6 --- Different Operation Regimes in MMML Lasers --- p.79 / Chapter 3.4.7 --- Why Period-T/2 Pulses --- p.84 / Chapter 3.4.8 --- Auto-Correlation Function of the Electric Field --- p.86 / Chapter 3.4.9 --- FSFC Laser with Injection Signal --- p.87 / Chapter 3.4.10 --- MMML in Class C Laser: d = 1.0 --- p.88 / Chapter 3.4.11 --- Exciting the Relaxation Oscillation Resonance --- p.89 / Chapter 4. --- Discussion and Conclusion --- p.92 / Chapter 4.1 --- Limitation of (Conventional) Thin-Sheet-Gain Approximation --- p.92 / Chapter 4.1.1 --- Problem with the Conventional Thin -Sheet-Gain Approximation --- p.92 / Chapter 4.1.2 --- Modified Thin-Sheet-Gain Approximation --- p.93 / Chapter 4.2 --- Concluding Remarks; Possibilities of Further Research --- p.97 / References and Notes / Appendix: Source Codes of the Fortran Program

by Chi Sang Wong. / Thesis (M.Phil.)--Chinese University of Hong Kong, 1999. / Includes bibliographical references. / Abstract also in Chinese. / Abstract --- p.ii / Acknowledgements --- p.vii / Table of Contents --- p.viii / Chapter 1 --- Introduction --- p.1 / Chapter 1.1 --- Background --- p.1 / Chapter 1.2 --- Organisation of Thesis --- p.4 / References --- p.6 / Chapter 2 --- Principles and Theories --- p.8 / Chapter 2.1 --- Device Concepts --- p.9 / Chapter 2.2 --- Picosecond Photoconductors --- p.14 / Chapter 2.3 --- Photoconducting Antennas --- p.18 / Chapter 2.4 --- Summary --- p.20 / References --- p.21 / Chapter 3 --- Self-mode-locked Ti:sapphire (Ti:Al203) Laser --- p.24 / Chapter 3.1 --- Introduction --- p.25 / Chapter 3.2 --- Self-mode-locked Ti:sapphire Laser Cavity --- p.26 / Chapter 3.3 --- Negative Dispersion Using Pairs of Prisms --- p.28 / Chapter 3.4 --- Kerr-lens Mode-Locked Model: Role of Space-time Effects --- p.33 / Chapter 3.5 --- Initiation of Self-mode-locked Pulses --- p.37 / Chapter 3.6 --- 39-fs Pulses from A Self-mode-locked Ti:sapphire Laser --- p.38 / Chapter 3.7 --- Summary --- p.42 / References --- p.43 / Chapter 4 --- Photoconductive Detection of Millimetre Waves Using LT-GaAs --- p.46 / Chapter 4.1 --- Introduction --- p.47 / Chapter 4.2 --- Devices Structures --- p.48 / Chapter 4.3 --- Experimental Setup --- p.52 / Chapter 4.4 --- Results and Discussion --- p.54 / Chapter 4.5 --- Summary --- p.57 / References --- p.58 / Chapter 5 --- Investigation of Other Materials for THz Detection --- p.60 / Chapter 5.1 --- Introduction --- p.61 / Chapter 5.2 --- Material Preparation --- p.62 / Chapter 5.3 --- Devices Structures --- p.64 / Chapter 5.4 --- Experimental Setup --- p.68 / Chapter 5.5 --- Results and Discussion --- p.69 / Chapter 5.6 --- Investigation of Other Materials --- p.72 / Chapter 5.7 --- Summary --- p.73 / References --- p.74 / Chapter 6 --- Characteristics of Millimetre Waves --- p.76 / Chapter 6.1 --- Introduction --- p.77 / Chapter 6.2 --- Experimental Setup --- p.78 / Chapter 6.3 --- Experimental Results --- p.80 / Chapter 6.4 --- Experimental Setup --- p.83 / Chapter 6.5 --- Experimental Results --- p.85 / Chapter 6.6 --- Summary --- p.86 / References --- p.87 / Chapter 7 --- Conclusion and Future Work --- p.88 / Chapter 7.1 --- Conclusion --- p.88 / Chapter 7.2 --- Future Work --- p.91 / Appendixes --- p.A-l / Chapter Appendix A: --- Hall Effect Measurement System --- p.A-l / Chapter Appendix B: --- Photography of Device Structures --- p.A-2 / Chapter Appendix C: --- Fast Fourier Transform Program --- p.A-3 / Chapter Appendix D: --- List of Publications --- p.A-4

Active mode locking is one of the simplest ways to generate picosecond pulses at gigahertz repetition rates. In my works, I demonstrate the generation of picosecond pulses with a center-wavelength spanning from 1489nm to 1589nm using a polarization maintaining fiber loop mirror filter (PMF-LMF) in a mode-locked semiconductor optical amplifier (SOA) ring laser. By applying the SOA gain shifting technique and with the help of the controllable transmission ratio of the PMF-LMF, the tuning range of the output wavelength can be extended. By applying the technique of dispersion tuning, electrical wavelength tuning can be achieved across a range of 100nm. / Compared to the active mode-locking method, the regenerative mode-locking is very convenient because it does not require any external source for modulation and is proved to be more robust against fluctuations in ambient temperature. We demonstrate a 10-GHz regeneratively mode-locked fiber laser using a PMF-LMF. The operating frequency is determined by the free-spectral-range of the PMF-LMF and the component is extracted optically from the ring laser output. / In addition, we also demonstrate a simple technique to generate wavelength tunable picosecond pulses at adjustable repetition rate without using electrical or optical RF filter to extract the radio frequency (RF). The RF signal for mode locking is generated from a Fabry-Perot laser diode (FP-LD) under optical injection. The output frequency can be varied by adjusting the biasing current of the FP-LD. (Abstract shortened by UMI.) / Picosecond optical pulse sources with broad tunability and various repetition rates are key elements for applications in wavelength- and time-division multiplexed optical transmission systems. Mode-locking is one of the main techniques for the generation of optical pulses with high repetition rate picosecond pulse trains. This thesis presents our research efforts in high repetition rate optical pulse generation using active and regenerative mode-locking techniques, and a self-starting approach. We also demonstrate the application of harmonic mode locking in all-optical clock recovery from NRZ data. / Tang Wing Wa. / "August 2005." / Adviser: C. T. Shu. / Source: Dissertation Abstracts International, Volume: 67-07, Section: B, page: 4015. / Thesis (Ph.D.)--Chinese University of Hong Kong, 2005. / Includes bibliographical references. / Electronic reproduction. Hong Kong : Chinese University of Hong Kong, [2012] System requirements: Adobe Acrobat Reader. Available via World Wide Web. / Electronic reproduction. [Ann Arbor, MI] : ProQuest Information and Learning, [200-] System requirements: Adobe Acrobat Reader. Available via World Wide Web. / Abstract in English and Chinese. / School code: 1307.

This thesis is organized into three body chapters: (1) the first use of naturally rotating tearing modes to diagnose intrinsic error fields is presented with experimental results from the EXTRAP T2R reversed field pinch, (2) a large scale study of locked modes (LMs) with rotating precursors in the DIII-D tokamak is reported, and (3) an in depth study of LM induced thermal collapses on a few DIII-D discharges is presented. The amplitude of naturally rotating tearing modes (TMs) in EXTRAP T2R is modulated in the presence of a resonant field (given by the superposition of the resonant intrinsic error field, and, possibly, an applied, resonant magnetic perturbation (RMP)). By scanning the amplitude and phase of the RMP and observing the phase-dependent amplitude modulation of the resonant, naturally rotating TM, the corresponding resonant error field is diagnosed. A rotating TM can decelerate and lock in the laboratory frame, under the effect of an electromagnetic torque due to eddy currents induced in the wall. These locked modes often lead to a disruption, where energy and particles are lost from the equilibrium configuration on a timescale of a few to tens of milliseconds in the DIII-D tokamak. In fusion reactors, disruptions pose a problem for the longevity of the reactor. Thus, learning to predict and avoid them is important. A database was developed consisting of 2000 DIII-D discharges exhibiting TMs that lock. The database was used to study the evolution, the nonlinear effects on equilibria, and the disruptivity of locked and quasi-stationary modes with poloidal and toroidal mode numbers m=2 and n=1 at DIII-D. The analysis of 22,500 discharges shows that more than 18% of disruptions present signs of locked or quasi-stationary modes with rotating precursors. A parameter formulated by the plasma internal inductance l_i divided by the safety factor at 95% of the toroidal flux, q_95, is found to exhibit predictive capability over whether a locked mode will cause a disruption or not, and does so up to hundreds of milliseconds before the disruption. Within 20 ms of the disruption, the shortest distance between the island separatrix and the unperturbed last closed flux surface, referred to as d_edge, performs comparably to l_i/q_95 in its ability to discriminate disruptive locked modes, and it also correlates well with the duration of the locked mode. On average, and within errors, the n=1 perturbed field grows exponentially in the final 50 ms before a disruption, however, the island width cannot discern whether a LM will disrupt or not up to 20 ms before the disruption. A few discharges are selected to analyze the evolution of the electron temperature profile in the presence of multiple coexisting locked modes during partial and full thermal quenches. Partial thermal quenches are often an initial, distinct stage in the full thermal quench caused by radiation, conduction, or convection losses. Here we explore the fundamental mechanism that causes the partial quench. Near the onset of partial thermal quenches, locked islands are observed to align in a unique way, or island widths are observed to grow above a threshold. Energy analysis on one discharge suggests that about half of the energy is lost in the divertor region. In discharges with minimum values of the safety factor above 1.2, and with current profiles expected to be classically stable, locked modes are observed to self-stabilize by inducing a full thermal quench, possibly by double tearing modes that remove the pressure gradient across the island, thus removing the neoclassical drive.