Third-Order Dispersion and Ultrafast-Pulse Propagation in Silicon Wire Waveguides

The dynamics of femtosecond (fs) pulse propagation in dispersion engineered silicon-on-insulator (SOI) photonic wires is investigated numerically. For fs pulses propagating in millimeter-long SOI waveguides, the interplay between nonlinear effects, group velocity dispersion (GVD), and the third-order dispersion (TOD) results in significant pulse reshaping. At the zero-GVD (ZGVD) wavelength, TOD causes strong pulse-shape asymmetry in both the temporal and spectral domains. In addition, in the anomalous GVD region near the ZGVD wavelength, soliton-like pulse dynamics is observed     

Interfacial Reactions of Sn, Sn-3.0Ag-0.5Cu, and Sn-9Zn Lead-Free Solders with Fe-42Ni Substrates

Interfacial reactions between Sn, Sn-3.0 wt.%Ag-0.5 wt.%Cu (SAC), and Sn-9 wt.%Zn (SZ) lead-free solders and Fe-42 wt.%Ni (alloy 42) substrates at 240°C, 255°C, and 270°C were investigated in this study. FeSn₂, (Fe,Ni, Cu)Sn₂, and (Ni,Fe)₅Zn₂₁ phases were formed, respectively, at the interface in the Sn/alloy 42, SAC/alloy 42, and SZ/alloy 42 couples. As the reaction time and temperature were increased, the layered intermetallic compound (IMC) assumed two distinct structures, i.e., a thicker layer and a pillar-shaped IMC, in all couples. The IMC thickness of these couples increased with the increase of reaction time and temperature. The IMC thickness was also proportional to the square root of the reaction time. The interfacial reaction mechanism of these couples was diffusion controlled.     

Development of a microelectromechanical system pressure sensor for rehabilitation engineering applications

Based on computer finite-element analysis ANSYS 5.3 and microelectromechanical systems (MEMS) technologies, a micropressure sensor was designed and fabricated. The sensor can be used to measure the distribution of normal stress between soft tissues on an above-knee amputee's skin and the contacting surface of a rehabilitation device. A square membrane with dimensions 2400 µm × 2400 µm × 80 µm is formed by backside photolithography and wet etching of an n-type ?100? monolithic silicon wafer. On the middle of the membrane edge, an X-shaped silicon wafer was implanted with boron ions and then enhanced by diffusion to form a piezoresistive strain gauge. In the design process, a finite-element method is used to analyse the effects of pressure sensitivity and its temperature coefficients. The developed micropressure sensors, which have smaller weight and volume than a conventional machine type, perform well and fit our design specifications.     

Dynamics analysis and controller synthesis forzero-voltage-transition PWM power converters

A small signal model for zero-voltage-transition pulse width modulation (ZVT-PWM) buck converters is proposed in this paper. It shows that the ZVT-PWM buck converter exhibits better dynamical behavior than the conventional PWM buck converter. Based on the derived model consisting of line voltage disturbances and load variations, μ-synthesis is applied for a robust controller design to achieve performance requirement. In addition, a classical controller and a sliding mode controller with modified integral variable structure are also designed for performance comparisons. Finally, simulation and experimental results are presented to verify the requirement for robust performance of ZVT-PWM converters     

An Investigation of Diffusion Barrier Characteristics of an Electroless Co(W,P) Layer to Lead-Free SnBi Solder

Diffusion barrier characteristics for eutectic SnBi solder/electroless Co(W,P) couples were investigated via liquid-state aging at 250°C and solid-state aging at 120°C. At the couple interface, CoSn₃ intermetallic compound (IMC) spallation was observed for the SnBi/amorphous Co(W,P) couple subjected to liquid-state aging. In contrast, no spallation of IMCs was observed for the SnBi/amorphous Co(W,P) couples subjected to solid-state aging. For the SnBi/polycrystalline Co(W,P) couple, a thick IMC layer was observed adjacent to a tungsten-enriched amorphous interfacial layer regardless of aging conditions. IMC formation in all samples indicated that Co(W,P) is essentially a sacrificial barrier to SnBi solder. However, amorphous Co(W,P) might also exhibit stuffed-type barrier behavior due to its relatively high phosphorus (P) content. Analytical results indicated that the P content in Co(W,P) is a crucial factor affecting the structural evolution at the SnBi/electroless Co(W,P) interface.     

Anonymous authentication protocol based on elliptic curve Diffie–Hellman for wireless access networks

Anonymous channel tickets have been proposed as a way to provide user anonymity and to reduce the overhead of re‐authentication for authentication in wireless environments. Chen et al. proposed a secure and efficient protocol, based on a protocol proposed by Yang et al., which is resistant to guessing attacks on networks from which users’ secret keys are easy to obtain. However, their scheme is time‐consuming in the phases of ticket issuing and authentication. Furthermore, a malicious attacker can utilize the expired time, T_exp, to launch a denial of authentication (DoA) attack, which is a type of denial of service attack. Because T_exp is exposed to any user, it would be easy to launch a DoA attack that could make the scheme impractical. To resist against DoAs that the scheme of Chen et al. might suffer, we propose an improved scheme based on elliptic curve cryptography in this paper. Our scheme not only reduces time cost but also enhances security. The basis of the proposed scheme is the elliptic curve discrete logarithm problem. The operations of points of an elliptic curve are faster and use fewer bits to achieve the same level of security. Therefore, our scheme is more suitable for mobile devices, which have limited computing power and storage. Copyright © 2012 John Wiley & Sons, Ltd.     

Efficient Color-theory-based Dynamic Localization for Mobile Wireless Sensor Networks

Location information is critical to mobile wireless sensor networks (WSN) applications. With the help of location information, for example, routing can be performed more efficiently. In this paper, we propose a novel localization approach, Color-theory based Dynamic Localization (CDL), which is based on color theory to exploit localization in mobile WSNs. CDL makes use of the broadcast information, such as locations and RGB values, from all anchors (a small portion of nodes with GPS receivers attached), to help the server to create a location database and assist each sensor node to compute its RGB value. Then, the RGB values of all sensor nodes are sent to the server for localization of the sensor nodes. A unique feature of our color-theory based mechanism is that it can use one color to represent the distances of a sensor node to all anchors. Since CDL is easy to implement and is a centralized approach, it is very suitable for applications that need a centralized server to collect user (sensor) data and monitor user activities, such as community health-care systems and hospital monitoring systems. Evaluation results have shown that for mobile WSNs, the location accuracy of CDL (E-CDL, an enhanced version of CDL) is 40–50% (75–80%) better than that of MCL (Hu, L., & Evans, D. (2004). Localization for mobile sensor networks. In Proceedings of the 10th annual international conference on mobile computing and networking, pp. 45–57). In addition, we have implemented and validated our E-CDL algorithm on the MICAz Mote Developer’s Kit.     

Electrically tunable room-temperature 2/spl pi/ liquid crystal terahertz phase shifter

Tunable phase shift up to 360/spl deg/ at 1 THz is demonstrated using electrically controlled birefringence in a vertically aligned nematic liquid crystal (E7) cell, 1.83 mm in thickness. The driving voltage and corresponding field required for a phase shift of 360/spl deg/ at 1 THz are 100 V and 90.5 V/cm, respectively.     

Finding point correspondence using local similarity and global constraint under insignificant scaling and roll

Establishing point correspondences is an important research topic in computer vision. Proposed is an algorithm using local similarity and global constraint to obtain point correspondence. The point correspondences are obtained by comparing their associated colour codes, which are computed by image gradients, and using spatial relationships among neighbouring feature points. The approach assumes insignificant scaling and roll, and is suitable for real-time applications     

Low-Dose X-ray CT Reconstruction via Dictionary Learning

Although diagnostic medical imaging provides enormous benefits in the early detection and accuracy diagnosis of various diseases, there are growing concerns on the potential side effect of radiation induced genetic, cancerous and other diseases. How to reduce radiation dose while maintaining the diagnostic performance is a major challenge in the computed tomography (CT) field. Inspired by the compressive sensing theory, the sparse constraint in terms of total variation (TV) minimization has already led to promising results for low-dose CT reconstruction. Compared to the discrete gradient transform used in the TV method, dictionary learning is proven to be an effective way for sparse representation. On the other hand, it is important to consider the statistical property of projection data in the low-dose CT case. Recently, we have developed a dictionary learning based approach for low-dose X-ray CT. In this paper, we present this method in detail and evaluate it in experiments. In our method, the sparse constraint in terms of a redundant dictionary is incorporated into an objective function in a statistical iterative reconstruction framework. The dictionary can be either predetermined before an image reconstruction task or adaptively defined during the reconstruction process. An alternating minimization scheme is developed to minimize the objective function. Our approach is evaluated with low-dose X-ray projections collected in animal and human CT studies, and the improvement associated with dictionary learning is quantified relative to filtered backprojection and TV-based reconstructions. The results show that the proposed approach might produce better images with lower noise and more detailed structural features in our selected cases. However, there is no proof that this is true for all kinds of structures.