Reduction of the Polarization-Dependent Loss of Long-Period Fiber Gratings by Thermal Heating

The polarization-dependent loss of UV-written long-period fiber gratings was significantly reduced by increasing the pulse repetition rate of the writing UV beam. The effect is closely related with the enhanced photosensitivity induced by thermal heating.     

Scheduling real-time tasks for dependability

Real-time systems are increasingly used in applications whose failure may result in large economic and human costs. Since many such systems operate in environments that are non-deterministic, and possibly hazardous, it is extremely important that the systems be dependable, namely the deadlines of tasks must be met even in the presence of particular failures. In order to enhance the dependability of a real-time system, we study the problem of scheduling a set of real-time tasks to meet their deadlines in the presence of processor failures. We first prove that the problem of scheduling a set of non-preemptive tasks on more than two processors to tolerate one arbitrary processor failure is NP-complete even when the tasks share a common deadline. Heuristic algorithms are then proposed to solve this problem. The schedules generated by the heuristic algorithms can tolerate one arbitrary processor failure in the worst case. The analysis and experimental data show that the performance of the algorithms is near-optimal.     

Photothermal probe beam deflection measurement of thermal diffusivity of atmospheric air

We have used a modified photothermal probe beam deflection system with a back pumping configuration for the measurements of the temperature-dependent thermal diffusivity of atmospheric air. The results are consistent and reasonably well agree with the literature values. The measured thermal diffusivity values are the same for the same measuring temperature regardless of the beam offsets and the deflecting surface temperatures. For the gas with known temperature-dependent thermal diffusivity, this method can be used to deduce the temperature of a gas from the measured thermal diffusivity value.     

Modified Lensed Fiber for Improvement of Optical Performance in Free-Space Interconnection

We report a modified technique for fabrication of lensed fibers to improve optical performance such as working distance and longitudinal misalignment tolerance. Expanded coreless silica fiber with 200 μm-diameter is utilized to induce the larger expansion of mode field in a composite lensed fiber. Optical performance for free-space interconnection is also characterized through lensed fibers with various curvature radii.     

Dynamic analysis of directly modulated Fabry–Perot laser diodes using time-domain modeling with random noise fluctuations

Based on time-domain laser modeling, we have systematically evaluated multi-mode excitation and mode competition in directly modulated Fabry–Perot laser diodes biased with different current levels. In our simulation, random noise fluctuations depending on the bias current are included since they play an important role in determining the number of lasing modes. Multi-mode excitation and mode competition occurred at the biased current below the threshold current can be explained with the aid of frequency chirp at turn-on and gain difference in between '0' and '1' bits. The positive frequency chirp (25 GHz for lasers biased below the threshold current and 15 GHz for lasers biased above the threshold current) affects to enhance the probability of multi-mode excitation in low-frequency region during turn-on delay.     

An inquiry-oriented approach to undergraduate mathematics

To improve undergraduate mathematics learning, teachers need to recognize and value characteristics of classroom learning environments that contribute to powerful student learning. The broad goal of this special issue is to share such characteristics and the theoretical and empirical grounding for an innovative approach in differential equations called the Inquiry Oriented Differential Equations (IO-DE) project. We use the IO-DE project as a case example of how undergraduate mathematics can build on theoretical and instructional advances initiated at the K-12 level to create and sustain learning environments for powerful student learning at the undergraduate level. In addition to providing an overview of the five articles in this special issue, we highlight the theoretical background for the IO-DE project and provide a summary of two quantitative studies done to assess the effectiveness of the IO-DE project on student learning.     

Finite element analysis of filling stage in die‐casting process using marker surface method and adaptive grid refinement technique

The marker surface method and the adaptive grid refinement technique have been applied to the three‐dimensional (3‐D) finite element analysis of the filling stage in the die‐casting process. Especially, the marker surface plugging technique and the marker surface regeneration technique incorporated in the marker surface method have been proposed for the efficient analysis of 3‐D practical problems. Through the marker surface plugging technique, new parts of marker surface are effective lycreated in order to eliminate the gaps between the parts of marker surface or between the edge of marker surface and cavity wall. By using the marker surface regeneration technique, the marker surface including a great number of marker elements is recreated on the basis of its original shape in order to decrease the number of marker elements and computational time. A3‐D example used as the benchmark test and a typical industrial problem of the die‐casting process have been analysed. The numerical results have been in good agreement with the experimental results and the efficiency of the adaptive grid refinement technique has been verified. It has been shown that the proposed techniques incorporated in the marker surface method and the adaptive grid refinement technique can be effectively applied to general industrial problems. Copyright © 2004 John Wiley & Sons, Ltd.     

Molecular dynamics study on the field effect ion transport in carbon nanotube

We investigated potassium ion transport in the (5, 5) carbon nanotube by using classical molecular dynamics simulations under applying external force fields. This can be applied to nanoscale ionic-field-effect devices. As the applying external force field increases, the potassium ions rapidly go through the nanochannel. Under the low external force fields, the thermal fluctuation of the nanochannel affected on the tunneling of the potassium ions, whereas the effects of the thermal fluctuation were negligible under the high external force fields.     

Copper nanocluster diffusion in carbon nanotube

The diffusion of copper nanocluster in carbon nanotube was investigated using a classical molecular dynamics simulation and three empirical potential functions. The results indicated a growth mechanism of the copper-filled ultra-thin carbon nanotubes: the copper nanoclusters inserted into carbon nanotubes swiftly migrate along the tube axis, and then the copper nanowires grow in the ultra-thin carbon nanotubes. Periodic energy barriers in the carbon nanotubes induced the directional movement of copper nanoclusters in the carbon nanotubes. The diffusion speeds of copper nanocluster in the carbon nanotube showed the Arrherius relation.     

Synthesis of polyaniline/multiwall carbon nanotube composite via inverse emulsion polymerization

The composite of polyaniline (PANI) and multiwall carbon nanotube carboxylated through acid treatment (c‐MWCNT) was synthesized by chemical oxidative polymerization in an inverse emulsion system. The resultant composites were compared with products from aqueous emulsion polymerization to observe the improvements in electrical conductivity, structural properties, and thermal stability obtained by this synthetic method. Prior to the inverse emulsion polymerization, MWCNT was treated with a strong acid mixture to be functionalized with carboxylic acid groups. Carboxylic acid groups on surfaces induced selective dispersibility between polar and nonpolar solvents because of the increase of hydrophilicity. As the content of c‐MWCNT was increased, the electrical conductivity was increased by a charge transport function from the intrinsic electrical conductivity of MWCNT and the formation of a highly ordered dense structure of PANI molecules on the surface of c‐MWCNT. The images observed with electron spectroscopy showed the capping of c‐MWCNT with PANI. The growth of additional ordered structures of PANI/c‐MWCNT composite, which was observed through wide‐angle X‐ray diffraction patterns, supported the capping by PANI. It was observed that the doping of the composite had a significant relationship with the concentration of dodecylbenzenesulfonic acid (DBSA). The thermal stability of PANI composite was improved by the addition of c‐MWCNT; this was thought to be related with structure ordering by inverse emulsion polymerization. © 2008 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 46: 2255–2266, 2008