Radiation build-up in laminar and turbulent regimes in quasi-CW Raman fiber laser

We study the radiation build-up in laminar and turbulent generation regimes in quasi-CW Raman fiber laser. We found the resulted spectral shape and generation type is defined by the total spectral broadening/narrowing balance over laser cavity round-trip, which is substantially different in different regimes starting from first round-trips of the radiation build-up. In turbulent regime, the steady-state is reached only after a few round-trips, while in the laminar regime the laser approaches the equilibrium spectrum shape asymptotically.

Gain dependent pulse regimes transitions in a dissipative dispersion-managed fibre laser

For the first time, we demonstrate the possibility to switch between three distinct pulse regimes in a dissipative dispersion-managed (DM) fibre laser by solely controlling the gain saturation energy. Nonlinear Schrödinger equation based simulations show the transitions between hyper-Gaussian similaritons, parabolic similaritons, and dissipative solitons in the same laser cavity. It is also shown that such transitions exist in a wide dispersion range from all-normal to slightly net-normal dispersion. This work demonstrates that besides dispersion and filter managements gain saturation energy can be a new degree of freedom to manage pulse regimes in DM fibre lasers, which offers flexibility in designing ultrafast fibre lasers. Also, the result indicates that in contrast to conservative soliton lasers whose intensity profiles are unique, dissipative DM lasers show diversity in pulse shapes. The findings not only give a better understanding of pulse shaping mechanisms in mode-locked lasers, but also provide insight into dissipative systems.

Revealing spatio-temporal regimes of generation in Raman fiber lasers

We present here experimental observation of different spatio-temporal generation regimes in quasi-CW Raman fiber laser in the most simple experimental configuration. The generation regimes depend on pump power and range from partial mode-locking to turbulent, and a generation of short-lived pulses. While in temporal domain transitions could be described in quantitative way, in spatio-temporal domain they represent qualitative change in observed dynamics.

Optimised phase disposition pulse-width modulation strategy for hybrid-clamped multilevel inverters using switching state sequences

This study describes an optimised modulation strategy based on switching state sequences for the hybrid-clamped multilevel converter. Two key control variables defined as 'phase shift angle' and 'switching state change' for a five-level hybrid-clamped inverter are proposed to improve all switches' operation, and by changing their values, different control methods can be obtained for modulation optimisation purposes. Two example methods can solve the voltage imbalance problem of the dc-link capacitors and furthermore avoid two switches' simultaneous switching transitions and improve the inverter's performance as compared with the traditional phase disposition pulse-width modulation strategy. A 6 kW prototype inverter is developed and a range of simulation and experiments are carried out for validation. It is found that simulation and experimental results are in a good agreement and the proposed modulation strategy is verified in terms of low-order harmonic reduction.

Control of the in-cavity dynamics in mode-locked fibre lasers

At the level of fundamental research, fibre lasers provide convenient and reproducible experimental settings for the study of a variety of nonlinear dynamical processes, while at the applied research level, pulses with different and optimised features – e.g., in terms of pulse duration, temporal and/or spectral intensity profile, energy, repetition rate and emission bandwidth – are sought with the general constraint of developing efficient cavity architectures. In this talk, we review our recent progress on the realisation of different regimes of pulse generation in passively mode-locked fibre lasers through control of the in-cavity propagation dynamics. We report on the possibility to achieve both parabolic self-similar and triangular pulse shaping in a mode-locked fibre laser via adjustment of the net normal dispersion and integrated gain of the cavity [1]. We also show that careful control of the gain/loss parameters of a net-normal dispersion laser cavity provides the means of achieving switching among Gaussian pulse, dissipative soliton and similariton pulse solutions in the cavity [2,3]. Furthermore, we report on our recent theoretical and experimental studies of pulse shaping by inclusion of an amplitude and phase spectral filter into the cavity of a laser. We numerically demonstrate that a mode-locked fibre laser can operate in dif- ferent pulse-generation regimes, including parabolic, flattop and triangular waveform generations, depending on the amplitude profile of the in-cavity spectral filter [4]. An application of technique using a flat-top spectral filter is demonstrated to achieve the direct generation of sinc-shaped optical Nyquist pulses of high quality and of a widely tuneable bandwidth from the laser [5]. We also report on a recently-developed versa- tile erbium-doped fibre laser, in which conventional soliton, dispersion-managed soli- ton (stretched-pulse) and dissipative soliton mode-locking regimes can be selectively and reliably targeted by programming different group-velocity dispersion profiles and bandwidths on an in-cavity programmable filter [6]. References: 1. S. Boscolo and S. K. Turitsyn, Phys. Rev. A 85, 043811 (2012). 2. J. Peng et al., Phys. Rev. A 86, 033808 (2012). 3. J. Peng, Opt. Express 24, 3046-3054 (2016). 4. S. Boscolo, C. Finot, H. Karakuzu, and P. Petropoulos, Opt. Lett. 39, 438-441 (2014). 5. S. Boscolo, C. Finot, and S. K. Turitsyn, IEEE Photon. J. 7, 7802008 (2015). 6. J. Peng and S. Boscolo, Sci. Rep. 6, 25995 (2016).

Temporal and spatio-temporal regimes of generation of Raman fibre lasers

Temporal dynamics of Raman fibre lasers tend to have very complex nature, owing to great cavity lengths and high nonlinearity, being stochastic on short time scales and quasi-continuous on longer time scales. Generally fibre laser intensity dynamics is represented by one-dimensional time-series, which in case of quasi-continuous wave generation in Raman fibre lasers gives little insight into the processes underlying the operation of a laser. New methods of analysis and data representation could help to uncover the underlying physical processes, understand the dynamics or improve the performance of the system. Using intrinsic periodicity of laser radiation, one dimensional intensity time series of a Raman fibre laser was analysed over fast and slow variation time. This allowed to experimentally observe various spatio-temporal regimes of generation, such as laminar, turbulent, partial mode-lock, as well as transitions between them and identify the mechanisms responsible for the transitions. Great cavity length and high nonlinearity also make it difficult to achieve stable high repetition rate mode-locking in Raman fibre lasers. Using Faraday parametric instability in extremely simple linear cavity experimental configuration, a very high order harmonic mode-locking was achieved in ò.ò kmlong Raman fibre laser. The maximum achieved pulse repetition rate was 12 GHz, with 7.3 ps long Gaussian shaped pulses. There is a new type of random lasers – random distributed feedback Raman fibre laser, which temporal properties cannot be controlled by conventionalmode-locking or Q-switch techniques and mechanisms. By adjusting the pump configuration, a very stable pulsed operation of random distributed feedback Raman fibre laser was achieved. Pulse duration varied in the range from 50 to 200 μs depending on the pump power and the cavity length. Pulse repetition rate scaling on the parameters of the system was experimentally identified.

The Polluter Profits Principle: A Note on the Grandfathering of CO2 Emissions Rights in Lax-Cap Trading Regimes

Grandfathering is currently the main principle for the initial allocation of tradable CO2 emission rights under the European cap-and-trade scheme. Furthermore, political feasibility often requires non-restrictive emission caps. Grandfathering under lax cap is unjust, biased and brings polluters unintended windfall profits. Still, in any post-Kyoto international CO2 regime, lax caps may be critical in coaxing binding emission targets out of more countries, especially those in the less-developed world. This paper argues that there is a certain quantity of emission rights between the initial and the optimal emissions, the grandfathering of which brings polluters zero windfall profits or zero windfall losses. Our theoretical concept of zero-windfall grandfathering can be used to demonstrate the windfall profits that have emerged at company level during the first EU trading period. It might thus encourage governments to embrace auctioning, and to combine it with grandfathering as a legitimate tool in the initial allocation of emission rights in later trading regimes.

Banking Contagion under Different Exchange Rate Regimes in CEE

The global crisis of 2008 caused both liquidity shortage and increasing insolvency in the banking system. The study focuses on credit default contagion in the Central and Eastern European (CEE) region, which originated in bank runs generated by non-performing loans granted to non-financial clients. In terms of methodology, the paper relies on one hand on review of the literature, and on the other hand on a data survey with comparative and regression analysis. To uncover credit default contagion, the research focuses on the combined impact of foreign exchange rates and foreign private indebtedness.

Banking Contagion under Different Exchange Rate Regimes in CEE

The global crisis of 2008 caused both liquidity shortage and increasing insolvency in the banking system. The study focuses on credit default contagion in the Central and Eastern European (CEE) region, which originated in bank runs generated by non-performing loans granted to non-financial clients. In terms of methodology, the paper relies on one hand on review of the literature, and on the other hand on a data survey with comparative and regression analysis. To uncover credit default contagion, the research focuses on the combined impact of foreign exchange rates and foreign private indebtedness.

The ineffectiveness of multilateral sanctions regimes under globalization: The case of Iraq

This dissertation examines the effectiveness and limits of multilateral sanctions regimes as instruments of foreign policy, particularly when trying to prevent the acquisition, development and proliferation of weapons of mass destructions. I hypothesize that globalization undermines the overall effectiveness of sanctions regimes. I analyze the agents and means of globalization. Agents are nation-states, corporations, non-state actors and organizations, and individuals. Means are the global import-export industry, global banking and investment, global corporate models, and global manufacturing industries. They all have contributed to vast increases in transnational economic activity and, furthermore, to more political tensions between nation-states, all of which jeopardize the implementation and enforcement of multilateral sanctions regimes. ^ To test this thesis, I examine how those factors impacted the multilateral sanctions regime imposed against Iraq from 1991 to 2002. This multilateral sanctions regime was conceived, approved and enforced by most nations in the United Nations. ^ Indeed, evidence collected for this dissertation suggests that Iraq did manage to consistently circumvent the UN sanctions regime, and that it did it by astutely utilizing the agents and means of globalization. Evidence also indicates that Iraq managed to rebuild parts of its military infrastructure, and that Iraq was on its way to rebuild its missile capability, for which it purchased large quantities of parts, components, technologies and manpower in the global market.^

Subtroprical wetland fish assemblages and changing salinity regimes: Implications for everglades restoration

During the 1960s, water management practices resulted in the conversion of the wetlands that fringe northeastern Florida Bay (USA) from freshwater/oligohaline herbaceous marshes to dwarf red mangrove forests. Coincident with this conversion were several ecological changes to Florida Bay’s fauna, including reductions in the abundances of top trophic-level consumers: piscivorous fishes, alligators, crocodiles, and wading birds. Because these taxa rely on a common forage base of small demersal fishes, food stress has been implicated as playing a role in their respective declines. In the present study, we monitored the demersal fishes seasonally at six sites over an 8-year time period. During monitoring, extremely high rainfall conditions occurred over a 3.5-year period leading to salinity regimes that can be viewed as “windows” to the area’s natural past and future restored states. In this paper, we: (1) examine the changes in fish communities over the 8-year study period and relate them to measured changes in salinity; (2) make comparisons among marine, brackish and freshwater demersal fish communities in terms of species composition, density, and biomass; and (3) discuss several implications of our findings in light of the intended and unintended water management changes that are planned or underway as part of Everglades restoration. Results suggest the reduction in freshwater flow to Florida Bay over the last several decades has reduced demersal fish populations, and thus prey availability for apex consumers in the coastal wetlands compared to the pre-drainage inferred standard. Furthermore, greater discharge of freshwater toward Florida Bay may result in the re-establishment of pre-1960s fauna, including a more robust demersal-fish community that should prompt increases in populations of several important predatory species.

The Transitioning Economic Dynamics of the Military in Communist Regimes: A Comparison of Cuba, China and Vietnam [Student's Paper Series]

Several factors can increase or decrease military-economic involvement in communist regimes. This anomalous form of military behavior, labeled as the Military Business Complex (MBC), emerged in various communist regimes in the 1980s. However, in early 2000s, the communist governments of China and Vietnam began to decrease the number of military-managed industries, while similar industries increased in Cuba. This paper explains why military industries in Cuba have increased over the last two decades, while they decreased in the Chinese and Vietnamese examples. This question is answered by comparatively testing two hypotheses: the Communist Party and the Bureaucratic-Authoritarian (BA) Hypotheses. The Communist Party hypotheses helps explain how the historical and current structures of Party oversight of the military have been lacking in strength and reliability in Cuba, while they traditionally have been more robust in China and Vietnam. The BA hypotheses helps explain how, due to the lack of a strong civilian institutional oversight, the Cuban military has grown into a bureaucratic entity with many political officers holding autonomous positions of power, an outcome that is not prevalent in the Chinese and Vietnamese examples. Thus, with the establishment of a bureaucratic military government and with the absence of a strong party oversight, the Cuban military has been able to protect its economic endeavors while the Chinese and Vietnamese MBC regimes have contracted.

Essays on emissions trading markets

This dissertation is a collection of three economics essays on different aspects of carbon emission trading markets. The first essay analyzes the dynamic optimal emission control strategies of two nations. With a potential to become the largest buyer under the Kyoto Protocol, the US is assumed to be a monopsony, whereas with a large number of tradable permits on hand Russia is assumed to be a monopoly. Optimal costs of emission control programs are estimated for both the countries under four different market scenarios: non-cooperative no trade, US monopsony, Russia monopoly, and cooperative trading. The US monopsony scenario is found to be the most Pareto cost efficient. The Pareto efficient outcome, however, would require the US to make side payments to Russia, which will even out the differences in the cost savings from cooperative behavior. The second essay analyzes the price dynamics of the Chicago Climate Exchange (CCX), a voluntary emissions trading market. By examining the volatility in market returns using AR-GARCH and Markov switching models, the study associates the market price fluctuations with two different political regimes of the US government. Further, the study also identifies a high volatility in the returns few months before the market collapse. Three possible regulatory and market-based forces are identified as probable causes of market volatility and its ultimate collapse. Organizers of other voluntary markets in the US and worldwide may closely watch for these regime switching forces in order to overcome emission market crashes. The third essay compares excess skewness and kurtosis in carbon prices between CCX and EU ETS (European Union Emission Trading Scheme) Phase I and II markets, by examining the tail behavior when market expectations exceed the threshold level. Dynamic extreme value theory is used to find out the mean price exceedence of the threshold levels and estimate the risk loss. The calculated risk measures suggest that CCX and EU ETS Phase I are extremely immature markets for a risk investor, whereas EU ETS Phase II is a more stable market that could develop as a mature carbon market in future years.

Switching patterns and steady-state analysis of grid-connected and stand-alone single-stage boost-inverters for PV applications

Renewable or sustainable energy (SE) sources have attracted the attention of many countries because the power generated is environmentally friendly, and the sources are not subject to the instability of price and availability. This dissertation presents new trends in the DC-AC converters (inverters) used in renewable energy sources, particularly for photovoltaic (PV) energy systems. A review of the existing technologies is performed for both single-phase and three-phase systems, and the pros and cons of the best candidates are investigated. In many modern energy conversion systems, a DC voltage, which is provided from a SE source or energy storage device, must be boosted and converted to an AC voltage with a fixed amplitude and frequency. A novel switching pattern based on the concept of the conventional space-vector pulse-width-modulated (SVPWM) technique is developed for single-stage, boost-inverters using the topology of current source inverters (CSI). The six main switching states, and two zeros, with three switches conducting at any given instant in conventional SVPWM techniques are modified herein into three charging states and six discharging states with only two switches conducting at any given instant. The charging states are necessary in order to boost the DC input voltage. It is demonstrated that the CSI topology in conjunction with the developed switching pattern is capable of providing the required residential AC voltage from a low DC voltage of one PV panel at its rated power for both linear and nonlinear loads. In a micro-grid, the active and reactive power control and consequently voltage regulation is one of the main requirements. Therefore, the capability of the single-stage boost-inverter in controlling the active power and providing the reactive power is investigated. It is demonstrated that the injected active and reactive power can be independently controlled through two modulation indices introduced in the proposed switching algorithm. The system is capable of injecting a desirable level of reactive power, while the maximum power point tracking (MPPT) dictates the desirable active power. The developed switching pattern is experimentally verified through a laboratory scaled three-phase 200W boost-inverter for both grid-connected and stand-alone cases and the results are presented.