Microring resonator channel dropping filters

Microring resonators side coupled to signal waveguides provide compact, narrow band, and large free spectral range optical channel dropping filters. Higher order filters with improved passband characteristics and larger out-of-band signal rejection are realized through the coupling of multiple rings. The analysis of these devices is approached by the novel method of coupling of modes in time. The response of filters comprised of an arbitrarily large dumber of resonators may be written down by inspection, as a continued fraction. This approach simplifies both the analysis and filter synthesis aspects of these devices     

Chemistry of hydrogen sulphide under coal liquefaction conditions: 1. Modelling hydrogen transfer catalysis

The work reported here represents initial attempts to develop a complete kinetic and mechanistic understanding of the reaction chemistry of H₂S under coal liquefaction conditions, using both model systems and coal. Hydrogen sulphide was found to promote/catalyse the transfer of hydrogen from tetralin to 2-hydroxyquinoline (2-HOQ). The presence of H₂S can increase the rate of hydrogen transfer from tetralin to 2-HOQ by a factor of 10 compared with the same reaction run in the absence of H₂S. The energy of activation for hydrogen transfer was found to decrease by ≈5 kcal mol⁻¹ in the presence of H₂S. The presence of H₂S was also found to promote loss of oxygen from 2-HOQ to form small amounts of quinoline. No evidence of CC or CN bond cleavage in 2-HOQ was noted under any of the reaction conditions studied. These results suggest that the presence of H₂S reduces the temperatures necessary to promote effective hydrogen transfer from tetralin by 50–75 °C. Moreover, they imply that similar effects occur in H₂S-promoted coal liquefaction.     

Stress and strain state of concrete during freezing and thawing cycles

The objective of this work is to calculate the pressures, stresses, and strains induced into moist concrete during freezing and thawing. The applied theory is based on thermodynamics and the linear theory of elasticity. If no additional salts are dissolved in the pore water the inputs needed in the theory are relative humidity and temperature measured in the sample chamber and inside concrete and evaporable water amount in the pore structure. Theoretical results were compared with the test results made with two concretes cured under water or at 96% relative humidity. One of the concretes was air entrained and in the comparison concrete no air-entraining agents were used. In the test cylinders cured under water the largest tensional stresses in freezing occurred on the surface of the test cylinders both in the axial and tangential direction. The largest tensional stress was 2.2 MPa, both in air-entrained and in non air-entrained concretes. The largest tensional stresses in the warming phase took place at the end of the thawing period when the chamber temperature was around +5 °C. Then the maximum tension occurred in the middle of the concrete cylinder in the axial direction of the cylinder. This maximum tensional stress was over 2.5 MPa in the air-entrained concrete cured in the relative humidity of 96%. The thermodynamic pumping effect at the end of the thawing phase in every cycle can increase the pore water amount remarkably if free water or moisture is available on the surface of the structure or in the environment vapor. The thermodynamic pumping effect seems to be remarkably greater and more dangerous in air-entrained concretes.     

Reaction engineering studies in a polytropic fixed-bed reactor over a highly active and selective catalyst for oxidative methane coupling

The oxidative coupling of methane (OCM) was carried out in a polytropic fixed-bed reactor applying a Zr/La/Sr catalyst developed by the Neste company. Over this catalyst the OCM reaction follows a complex reaction scheme which includes primary parallel reaction steps to CO, CO₂ and C₂H₆ and consecutive reactions of ethane to ethylene or CO_x. Yield of higher hydrocarbons C₂₊ obtained with this catalyst strongly depended on reaction conditions, i.e. low partial pressures of methane and oxygen obtained by diluting the feed gas with nitrogen and high reaction temperatures promoted C₂₊ selectivity and yield. The maximum yield amounted to 21.4% (20 Vol.-% CH₄, 9 Vol.-% O₂, 71 Vol.-% N₂, T = 860°C; X_CH4 = 41.8%, S = 52.5%). This result belongs to the highest yields reported in the open literature.     

Validation of the Climate-SAF surface broadband albedo product: Comparisons with in situ observations over Greenland and the ice-covered Arctic Ocean

This paper describes a validation study performed by comparing the Climate-SAF Surface Albedo Product (SAL) to ground truth observations over Greenland and the ice-covered Arctic Ocean. We compare Advanced Very High Resolution Radiometer (AVHRR)-based albedo retrievals to data from the Greenland Climate Network (GCN) weather stations and the floating ice station Tara for polar summer 2007. The AVHRR dataset consists of 2755 overpasses. The overpasses are matched to in situ observations spatially and temporally. The SAL algorithm presented here derives the surface broadband albedo from AVHRR channels 1 and 2 using an atmospheric correction, temporal sampling of an empirical Bidirectional Reflectance Distribution Function (BRDF), and a narrow-to-broadband conversion algorithm. The satellite product contains algorithms for snow, sea ice, vegetation, bare soil, and water albedo. At the Summit and DYE-2 stations on the Greenland ice sheet, instantaneous SAL RMSE is 0.073. The heterogeneous surface conditions at satellite pixel scale over the stations near the Greenland west coast increase RMSE to > 0.12. Over Tara, the instantaneous SAL RMSE is 0.069. The BRDF sampling approach reduces RMSE over the ice sheet to 0.053, and to 0.045 over Tara. Taking into account various sources of uncertainty for both satellite retrievals and in situ observations, we conclude that SAL agrees with in situ observations within their limits of accuracy and spatial representativeness.     

Overlap-freeness in infinite partial words

We prove that there exist infinitely many infinite overlap-free binary partial words containing at least one hole. Moreover, we show that these words cannot contain more than one hole and the only hole must occur either in the first or in the second position. We define that a partial word is $k$-overlap-free if it does not contain a factor of the form $xyxyx$ where the length of $x$ is at least $k$. We prove that there exist infinitely many 2-overlap-free binary partial words containing an infinite number of holes.     

Extension of the decidability of the marked PCP to instances with unique blocks

In the Post Correspondence Problem (PCP) an instance (h,g)$(h,g)$ consists of two morphisms h$h$ and g$g$, and the problem is to determine whether or not there exists a nonempty word w$w$ such that h(w)=g(w)$h\left(w\right)=g\left(w\right)$. Here we prove that the PCP is decidable for instances with unique blocks using the decidability of the marked PCP. Also, we show that it is decidable whether an instance satisfying the uniqueness condition for continuations has an infinite solution. These results establish a new and larger class of decidable instances of the PCP, including the class of marked instances.     

BOAR: an advanced HW/SW coemulation environment for DSP system development

The BOAR emulation system is targeted to hardware/software (HW/SW) codevelopment of advanced embedded DSP and telecom systems. The challenge of the BOAR system is efficient customization of programmable hardware, and dedicated partitioning routine to target applications and structures, which allows quite high overall system performance. The system allows multiple configurations for communication between processors and field programmable gate arrays (FPGAs) making the BOAR system an efficient tool for real-time HW/SW coverification. The reprogrammable hardware of the emulation tool is based on four Xilinx 4000-series devices, two Texas TMS320C50 signal processors and one Motorola MC68302 microcontroller. With current devices the BOAR hardware provides approximately 40–70 kgates of logic capacity in DSP applications. The emulation capacity can be expanded by connecting several similar boards in chain. The system has also a versatile internal reprogrammable test environment for test bench development, performance evaluations and design debugging. The logic development environment is based on the Synopsys synthesis tools and an automatic design management software, which performs resource mapping and performance-driven design partitioning between FPGAs. The emulation hardware is currently connected to logic and software development environments via an RS-232C bus. The BOAR emulation system has been found a very efficient platform for real-life prototyping of different types of DSP algorithms and systems, and validating correct functionality of a VHDL macro library.     

How much nitrogen is fixed by biological symbiosis in tropical dry forests? 2. Herbs

Although biological nitrogen fixation (BNF) is considered the main input of N in mature and regenerating native tropical vegetation, it has seldom been quantified. Biomass and N accumulation and fixation were determined for spontaneously occurring herbaceous species in caatinga areas in four regeneration stages (2, 17, 39 and >50?years after abandonment from agricultural use). BNF was estimated using the ¹⁵N natural-abundance method. The 2-year regeneration area had the highest total herb (6,355?kg?ha^?1) and legume (262?kg?ha^?1) biomass production, N stocks (82?kg?ha^?1) and fixed N (5.0?kg?ha^?1). N₂-fixing legumes (nine species in the sampled area) contributed over 97?% of legume biomass in all areas. Macroptilium gracile added the largest amount of N (3.9?kg?ha^?1 in the 2-year regeneration area) because of its large biomass production (205?kg?ha^?1), although it was not the species with the highest proportion of fixed N (76?%). All of the N₂-fixing species obtained large proportions of their N from symbiosis, most of them more than 50?%.However, the amounts of fixed N per unit area were relatively low (0.22?C5.00?kg?ha^?1) because the biomass of N₂-fixing species was always less than 5?% of the total herb biomass.