Mode mixing in asymmetric double trench photonic crystal waveguides

e investigate both experimentally and theoretically the waveguiding properties of a novel double trench waveguide where a conventional single-mode strip waveguide is embedded in a two dimensional photonic crystal (PhC) slab formed in silicon on insulator (SOI) wafers. We demonstrate that the bandwidth for relatively low-loss (50dB/cm) waveguiding is significantly expanded to 250nm covering almost all the photonic band gap owing to nearly linear dispersion of the TE-like waveguiding mode. The flat transmission spectrum however is interrupted by numerous narrow stop bands. We found that these stop bands can be attributed to anti-crossing between TE-like (positive parity) and TM-like (negative parity) modes. This effect is a direct result of the strong asymmetry of the waveguides that have an upper cladding of air and lower cladding of oxide. To our knowledge this is the first demonstration of the effects of cladding asymmetry on the transmission characteristics of the PhC slab waveguides.Comment: 7 pages, 6 figure

Electroslag remelting of wares at solid start with the use of exothermic fluxes

Експериментальними методами встановлено, що розроблений спосіб твердого старту з використанням екзотермічних флюсів, що проводять електричний струм у твердому стані, істотно знижує час наведення шлакової ванни необхідного обсягу, підвищуючи продуктивність процесу. Запропонований спосіб старту електрошлакового процесу із використанням екзотермічних електропровідних флюсів не здійснює негативного впливу на якість донної частини злитка. Розроблені технологічні процеси прискореного наведення шлакової ванни на твердому старті як за монофілярною, так і біфілярною, або трифазною схемою ведення процесу. Властивості литої електрошлакової сталі 5ХНМ задовольняють вимоги, які пред’являються до механічних властивостей кованого металу відкритої виплавки. Гранулометричний склад екзотермічного флюсу в основному впливає тільки на процес наведення шлакової ванни витратним електродом.The authors established that an effective way to improve performance of such electroslag processes as casting and remelting was the use of exothermic fluxes containing such components as: slag, ferro alloys and aluminum powder in quantities sufficient for exothermic reactions. In particular, the experimental techniques revealed that the presence of electrically conductive layer of exothermic flux allowed for electroslag process being carried out according to the monofilar, as well as bifilar or three-phase schemes at «solid» start. The conducted research testified that the use of exothermic alloyed flux does not result in lower quality of the metal on the bottom of an ingot. The macro-structural analysis revealed that the cast metal of the ingot-bottom was dense; cracks, cavities and other defects did not occur, and columnar crystals were directed almost in parallel to the center line of a templet; in the middle and upper parts a zone of columnar crystals is clearly revealed; in the central part there was a zone of equiaxed crystals; besides the content of sulfur in the bottom of the ingot under the proposed method of start was smaller than that in its middle and upper part. The micro- structural study of pollution of the stamping dice metal by nonmetallic inclusions demonstrated that the metal had low content of non-metallic inclusions, while the pollution of wrought metal was significantly higher than the pollution of electroslag cast metal. The authors proved that the properties of electroslag cast steel 5HNM met the requirements for mechanical properties of openly melted wrought metal. The component of exothermic flux was found to be worthy being used, which is a waste of rolling or forging and pressing industry with particle size of 0.1 ... 2.5 mm and alloying elements with particle size of 0.1 ... 6.0 mm. It has been determined experimentally that granulometric composition of exothermic flux mainly affected only the process of restoring the slag bath in the application of consumable electrode. As a result of comparative tests of the effective option of the developed method of start and the applied methods of solid and liquid starts of the electroslag remelting, the parameters were obtained, which testify that the developed method of start of electroslag remelting can improve the performance of electroslag remelting process and increase the production of useful metal by 2...7 %. In addition, the authors found that the melting of exothermic flux the component of slug is formed: aluminum oxide (Al2O3), the mass of which can contain up to 20% of the required mass of the working flux being melted

Eddy turbulence, the double mesopause, and the double layer of atomic oxygen

In this study, we consider the impact of eddy turbulence on temperature and atomic oxygen distribution when the peak of the temperature occurs in the upper mesosphere. A previous paper (Vlasov and Kelley, 2010) considered the simultaneous impact of eddy turbulence on temperature and atomic oxygen density and showed that eddy turbulence provides an effective mechanism to explain the cold summer and warm winter mesopause observed at high latitudes. Also, the prevalent role of eddy turbulence in this case removes the strong contradiction between seasonal variations of the O density distribution and the impact of upward/downward motion corresponding to adiabatic cooling/heating of oxygen atoms. Classically, there is a single minimum in the temperature profile marking the location of the mesopause. But often, a local maximum in the temperature is observed in the height range of 85–100 km, creating the appearance of a double mesopause (Bills and Gardner, 1993; Yu and She, 1995; Gusev et al., 2006). Our results show that the relative temperature maximum in the upper mesosphere (and thus the double mesopause) can result from heating by eddy turbulence. According to our model, there is a close connection between the extra temperature peak in the mesosphere and the oxygen atom density distribution. The main feature of the O density height profile produced by eddy turbulence in our model is a double peak instead of a single peak of O density. A rocket experiment called TOMEX confirms these results (Hecht et al., 2004). Applying our model to the results of the TOMEX rocket campaign gives good agreement with both the temperature and oxygen profiles observed. Climatology of the midlatitude mesopause and green line emission shows that the double mesopause and the double layers of the green line emission, corresponding to the double O density height profile, are mainly observed in spring and fall (Yu and She, 1995; Liu and Shepherd, 2006). Further observations of the oxygen atom densities and the double mesopause would improve our understanding of the impact of turbulence on critical mesospheric parameters

Use of low orbital satellite communications systems for humanitarian programs

Communication and information exchange play a decisive role in progress and social development. However, in many parts of the world the communication infrastructure is inadequate and the capacity for on-line exchange of information may not exist. This is true of underdeveloped countries, remote and relatively inaccessible regions, sites of natural disasters, and of all cases where the resources needed to create complex communication systems are limited. The creation of an inexpensive space communications system to service such areas is therefore a high priority task. In addition to a relatively low-cost space segment, an inexpensive space communications systems requires a large number of ground terminals, which must be relatively inexpensive, energy efficient (using power generated by storage batteries, or solar arrays, etc.), small in size, and must not require highly expert maintenance. The ground terminals must be portable, and readily deployable. Communications satellites in geostationary orbit at altitudes of about 36,000 km are very expensive and require complex and expensive ground stations and launch vehicles. Given current technology, it is categorically impossible to develop inexpensive satellite systems with portable ground terminals using such satellites. To solve the problem of developing an inexpensive satellite communications system that can operate with relatively small ground stations, including portable terminals, we propose to use a system with satellites in low Earth orbit, at an altitude of 900-1500 km. Because low orbital satellites are much closer to the Earth than geostationary ones and require vastly less energy expenditure by the satellite and ground terminals for transmission of messages, a system using them is relatively inexpensive. Such a system could use portable ground terminals no more complex than ordinary mobile police radios

Geolocation applications of the Gonets LEO messaging satellites

Geostationary satellites carry a majority of the international telecommunications traffic not carried by transoceanic cable. However, because the radio path links to and from geostationary satellites total at least 70,000 km and because of inherent on-board spacecraft power limitations, earth stations used in conjunction with geostationary satellites are usually large and expensive. This limits their installation to areas with a well-developed industrial and economic infrastructure. This reality helps perpetuate a chicken egg dilemma for the developing countries and isolated regions. Economic integration with the developed world requires being 'networked'. But for many developing entities, even the initial price of entry exceeds their modest resources. Exclusion from the global information highways virtually assures retardation of economic growth for developing nations, remote and isolated areas. Very Small Aperture Terminal (VSAT) earth stations are often thought of as a solution for networking developing regions. But economic considerations often forecloses this option. If VSAT size and cost is to be minimized, powerful spot beams from the satellite need to be focused on relatively small regions. This is not often feasible because of the high cost of the satellite itself. To dedicate a high power spot beam to a small region is usually not economically feasible