The coherence of a semiconductor laser subjected to a coherent optical feedback of greater than -30 dB will collapse, causing the laser¡¦s performance in many applications to be severely degraded. This research investigated the feasibility of suppression the coherence collapse with orthogonal-polarization optical feedback. In the experiments, we first studied the characteristics of coherence collapse state of semiconductor laser. Under an operational condition of 63.9 mA, 21.9¢J and -11 dB of coherent optical feedback, a Hitachi HLP 7806G semiconductor laser would be conducted from its solitary single-mode state into a multimode oscillation state with a very broad linewidth. At the same time, the relative intensity noise measured from the RF spectrum analyzer drastically raised about 15 dB. When a -16 dB and a -23 dB orthogonal-polarization optical feedback were fed back into the laser¡¦s cavity. The coherence collapse could be effectively suppressed, while the laser was conducted back to its solitary single-mode state stably. This result will greatly increase the application of the laser. Some experiments were also reported to explore the interaction between the laser and the orthogonal-polarization feedback light.

Semiconductor has electronic bandgap because of the periodic potential barriers. Similarly, as shown in Yablonovitch and John¡¦s original idea in 1987, and the optical bandgap can be formed by arranging the dielectric material periodically, named photonic crystal. The innovation promotes vigorous development in the last twenty years. Many applications were discovered by using the idea of photonic crystal, such as waveguide, left-hand material, slow light, optical register, etc. Conventional fibers guide light in the core by the total internal reflection principle, but Russel and co-workers demonstrated fibers with a so-called photonic crystal cladding in 1996, and these fibers guide light by a new physical mechanism different from traditional fibers. Photonic crystal fibers can be simply divided into two groups, one is index guiding fiber and the other is photonic bandgap fiber. Both of them have 2D periodic structures with designed defect structure in the center. Hence light can be confined and guided by special defect modes. We have successfully demonstrated microstructured fibers which have 2D periodic structure by LHPG method. During the fabrication processes, capillaries may collapse due to the surface tension. We discuss the hole-collapse issue and our solution. Besides, the quality of fiber extremely depends on the stability of laser power of the LHPG system, so we design an efficient feedback control to improve it. We also discuss the fibers¡¦ SEM images and optical properties. Finally the future work refers to the drawing of 3D photonic crystal fiber and improving the sharp thermal gradient by using a sapphire tube.

NO ENGLISH ABSTRACT.

The first instability of circular tubes compressed into the plastic range is axisymmetric wrinkling, which is stable. Compressed further the wrinkle amplitude grows, leading to a limit load instability followed by collapse. The two instabilities can be separated by strain levels of a few percent. This work investigates whether a tube that develops small amplitude wrinkles can be subsequently collapsed by persistent cycling. The problem was first investigated experimentally using SAF 2507 super-duplex steel tubes with D/t of 28.5. The tubes are first compressed to strain levels high enough for mild wrinkles to form and then cycled axially under stress control about a compressive mean stress. This type of cycling usually results in accumulation of compressive strain; here it is accompanied by growth of the amplitude of the initial wrinkles. The tube average strain initially grows nearly linearly with the number of cycles, but as a critical value of wrinkle amplitude is approached, wrinkling localizes, the rate of ratcheting grows exponentially and the tube collapses. Similar experiments were then performed for tubes involving axial cycling under internal pressure and the combined loads cause simultaneous ratcheting in the hoop and axial directions as well as a gradual growth of the wrinkles. The rate of ratcheting and the number of cycles to collapse depend on the initial compressive pre-strain, the internal pressure, and the stress cycle parameters all of which were varied sufficiently to generate vii a sufficient data base. Interestingly, in both the pressurized and unpressurized cases collapse was found to occur when the accumulated average strain reaches the value at which the tube localizes under monotonic compression. A custom shell model of the tube with initial axisymmetric imperfections, coupled to the Dafalias-Popov two-surface nonlinear kinematic hardening model, are presented and used to simulate the experiments performed. It is demonstrated that when suitably calibrated this modeling framework reproduces the prevalent ratcheting deformations and the evolution of wrinkling including the conditions at collapse accurately for all experiments. The calibrated model is then used to evaluate the ratcheting behavior of pipes under thermal-pressure cyclic loading histories experienced by axially restrained pipelines. / text

Ocean-island flank collapses are amongst the most dangerous of all landslide related hazards in the world, as they have the potential to trigger ocean-wide tsunamis that can cause damage and loss of life to communities thousands of kilometres from their source of origin. The implications for landslide-induced tsunami originating from high volcanic islands in the Pacific are serious; and consequent hazards to life, infrastructure, and emergency management need to be constantly reviewed, monitored, and investigated. Ta’u, the easternmost inhabited island in the Samoa Islands volcanic chain, exhibits a series of down-faulted benches on its southern flank; believed to be the remnant of catastrophic collapse involving ~30km³. An historical map of Ta’u, charted during the first United States exploring expedition into the Pacific Ocean (Charles Wilkes Expedition), suggests that the event was recent; having occurred less than 170 years ago. A collapse event of this magnitude would have generated a locally devastating tsunami, with possible impacts experienced at the regional level. However, there exists no written or oral record of such an event. It appears that half the island, involving an estimated 30km³, disappeared off the map less than 170 years ago without anybody noticing it. A number of key questions thus emerged. Did this event actually happen within the last 170 years, and if so, how and why could it have gone unnoticed? Is the event much older than the impression obtained from the literature? More importantly, what is the likelihood of a future collapse and subsequent tsunami, and what would the hazard impacts be at the local and regional levels? These questions formed the research basis for this thesis. Specific aims were developed to address the issues identified, and a range of inter-disciplinary scientific techniques using innovative methods and new datasets were implemented to achieve them. The results demonstrate that the collapse most likely occurred more than 170 years ago, raising serious debate on the accuracy of observations made during the Charles Wilkes Expedition. The results also show that the eruptive-hazard at the site exists. Given that the nature and frequency of active volcanism in the area is uncertain, the risk of a future collapse and subsequent tsunami in the medium-term is considered high. The inter-disciplinary approach to landslide-tsunami hazard investigation on an oceanic island presented in this thesis, can be developed and applied by disaster managers to similar hazard investigations on other oceanic islands. Ultimately, the increase in knowledge-base can be used as a tool for developing safer and more resilient coastal communities.

No description available.

Under vintrarna 2006/2007 och 2007/2008 vittnade man i USA om storskaliga förluster avhonungsbin (Apis mellifera). Gemensamma symptom för dessa förluster var att arbetarbinförsvann från kolonierna fastän det fanns rikligt med mat och inga tecken på infektion av någonparasit. Fenomenet fick namnet Colony Collapse Disorder (CCD) eftersom det slutgiltigastadiet var att kolonierna kollapsade. Denna översiktstudie ämnar till att reda ut begreppetCCD och se över vilka potentiella orsaker den rådande forskningen föreslår och vad man harkommit fram till sedan utbrottet 2006. Resultaten indikerar att det antagligen inte är en ensamfaktor bakom CCD utan att flera faktorer samverkar. Det finns ett antal hot mot bina medsymptom som liknar CCD och som antas bidra till försvinnandet. Ett hot är det parasitiskakvalstret Varroa destructor som förutom att det suger binas hemolymfa även agerar vektor åtflera CCD-förknippade patogener som Nosema ceranae och Israeli acute paralysis virus(IAPV). Dessutom sänker V. destructor binas immunförsvar vilket gör att virus som överförsvia kvalstret replikeras snabbare och blir dödligare. Man har även visat att vanligt förekommandebekämpningsmedel kan göra bina mer mottagliga för sjukdomar. Där hittade man ettsignifikant samband med just N. ceranae vilket är i enlighet med teorin om att flera faktorersamverkar. I nuläget bidrar CCD fortfarande till bidöd men är inte lika akut som 2006/2007.Istället är det andra hot såsom biodlares svårighet att behandla invasion av varroakvalster,kalla vintrar och att bisamhällen svälter ihjäl för att de invintras i för små populationer, som ärett större hot än CCD.

This research focuses on improvements and application of 1D finite elements for nonlinear dynamic analysis of reinforced concrete frames under extreme loadings. The concept of force interpolation is adopted for the element formulation and a solution scheme developed based on a total secant stiffness approach that provides good convergence characteristics. The geometrical nonlinearities including 2nd order P-Delta effects as well as catenary action are considered in the element formulation. It is shown that geometrical nonlinearities may have a significant effect on member (structure) response within extreme loading scenarios. In the analysis of structures subjected to extreme loadings, accurately modelling of the post peak response is vital and, in this respect, the objectivity of the solution with softening must be maintained. The softening of concrete under compression is taken into account, and the objectivity preserved, by adopting a nonlocal damage model for the compressive concrete. The capability of nonlocal flexibility-based formulation for capturing the post-peak response of reinforced concrete beam-columns is demonstrated by numerical examples. The 1D frame element model is extended for the modelling of 3D framed structures using a simplified torque-twist model that is developed to take account of interaction between normal and tangential forces at the section level. This simplified model can capture the variation of element torsional stiffness due to presence of axial force, bending moment and shear and is efficient and is shown to provide a reasonable degree of accuracy for the analysis of 3D reinforced concrete frames. The formulations and solution algorithms developed are tested for static and dynamic analysis of reinforced concrete framed structures with examples on impact analysis of beams, dynamic analysis of frames and progressive collapse assessment of frames taken from the literature. The verification shows that the formulation is very efficient and is capable of modelling of large scale framed structures, under extreme loads, quickly and with accuracy.

Ocean-island flank collapses are amongst the most dangerous of all landslide related hazards in the world, as they have the potential to trigger ocean-wide tsunamis that can cause damage and loss of life to communities thousands of kilometres from their source of origin. The implications for landslide-induced tsunami originating from high volcanic islands in the Pacific are serious; and consequent hazards to life, infrastructure, and emergency management need to be constantly reviewed, monitored, and investigated. Ta’u, the easternmost inhabited island in the Samoa Islands volcanic chain, exhibits a series of down-faulted benches on its southern flank; believed to be the remnant of catastrophic collapse involving ~30km³. An historical map of Ta’u, charted during the first United States exploring expedition into the Pacific Ocean (Charles Wilkes Expedition), suggests that the event was recent; having occurred less than 170 years ago. A collapse event of this magnitude would have generated a locally devastating tsunami, with possible impacts experienced at the regional level. However, there exists no written or oral record of such an event. It appears that half the island, involving an estimated 30km³, disappeared off the map less than 170 years ago without anybody noticing it. A number of key questions thus emerged. Did this event actually happen within the last 170 years, and if so, how and why could it have gone unnoticed? Is the event much older than the impression obtained from the literature? More importantly, what is the likelihood of a future collapse and subsequent tsunami, and what would the hazard impacts be at the local and regional levels? These questions formed the research basis for this thesis. Specific aims were developed to address the issues identified, and a range of inter-disciplinary scientific techniques using innovative methods and new datasets were implemented to achieve them. The results demonstrate that the collapse most likely occurred more than 170 years ago, raising serious debate on the accuracy of observations made during the Charles Wilkes Expedition. The results also show that the eruptive-hazard at the site exists. Given that the nature and frequency of active volcanism in the area is uncertain, the risk of a future collapse and subsequent tsunami in the medium-term is considered high. The inter-disciplinary approach to landslide-tsunami hazard investigation on an oceanic island presented in this thesis, can be developed and applied by disaster managers to similar hazard investigations on other oceanic islands. Ultimately, the increase in knowledge-base can be used as a tool for developing safer and more resilient coastal communities.

Proefschrift Amsterdam.