一种双极性高精度半导体激光器温度控制系统

温度是影响半导体激光器（LD）寿命和输出特性的重要因素之一。为保证LD输出稳定的激光模式和功率,采用以ADC和DAC集成的微处理器芯片C8051F350和具有双极性输出电流的TEC驱动芯片MAX1968为控制核心,以积分分离和变速积分增量式相结合的数字PID算法为运算程序的自动温度控制系统(ATC)控制TEC驱动电流的方向和大小,实现对LD的加热或制冷,使其工作在恒定温度。实验证明,应用该系统,LD在0℃～40℃环境温度范围内能很快稳定在设定温度,且其不确定度为±0.03℃。     

Collaborative and Cognitive Network Platforms: Vision and Research Challenges

In this paper, we present a visionary concept referred to as Collaborative and Cognitive Network Platforms (CCNPs) as a future-proof solution for creating a dependable, self-organizing and self-managing communication substrate for effective ICT solutions to societal problems. CCNP creates a cooperative communication platform to support critical services across a range of business sectors. CCNP is based on the personal network (PN) technology which is an inherently cooperative environment prototyped in the Dutch Freeband PNP2008 and the European Union IST MAGNET projects. In CCNP, the cognitive control plane strives to exploit the resources to better satisfy the requirements of networked applications. CCNP facilitates collaboration inherently. Through cognition in the cognitive control plane, CCNP becomes a self-managed substrate. The self-managed substrate, in this paper, is defined as cognitive and collaborative middleware on which future applications run without user intervention. Endemic sensor networks may be incorporated into the CCNP concept to feed its cognitive control plane. In this paper, we present the CCNP concept and discuss the research challenges related to collaboration and cognition.     

Images of a First‐Order Spin‐Reorientation Phase Transition in a Metallic Kagome Ferromagnet

First‐order phase transitions, where one phase replaces another by virtue of a simple crossing of free energies, are best known between solids, liquids, and vapors, but they also occur in a wide range of other contexts, including even elemental magnets. The key challenges are to establish whether a phase transition is indeed first order, and then to determine how the new phase emerges because this will determine thermodynamic and electronic properties. Here it is shown that both challenges are met for the spin reorientation transition in the topological metallic ferromagnet Fe₃Sn₂. The magnetometry and variable temperature magnetic force microscopy experiments reveal that, analogous to the liquid–gas transition in the temperature–pressure plane, this transition is centered on a first‐order line terminating in a critical end point in the field‐temperature plane. The nucleation and growth associated with the transition is directly imaged, indicating that the new phase emerges at the most convoluted magnetic domain walls for the high temperature phase and then moves to self‐organize at the domain centers of the high temperature phase. The dense domain patterns and phase coexistence imply a complex inhomogenous electronic structure, which can yield anomalous contributions to the electrical conductivity.     

A comparative study of distributed frequency assignment algorithms for wireless sensor networks

Wireless sensor networks are composed of energy constrained nodes embedding limited transmission, processing and sensing capabilities. The main research efforts in this area sought to prolong the network lifetime by reducing energy consumption of network operations. Data gathering mechanisms such as clustering have been shown to achieve significant energy savings. However, such benefits can be obtained only if neighboring clusters operate on different frequencies (channels). As the salient characteristics of wireless sensor networks favor a distributed approach, we analyze the performance of several distributed frequency assignment algorithms with a focus on energy consumption. In this context, we find that a heuristic may achieve better results than backtracking-based algorithms.     

Identifying FMRI model violations with lagrange multiplier tests

The standard modeling framework in functional magnetic resonance imaging (fMRI) is predicated on assumptions of linearity, time invariance and stationarity. These assumptions are rarely checked because doing so requires specialized software, although failure to do so can lead to bias and mistaken inference. Identifying model violations is an essential but largely neglected step in standard fMRI data analysis. Using Lagrange multiplier testing methods we have developed simple and efficient procedures for detecting model violations such as nonlinearity, nonstationarity and validity of the common double gamma specification for hemodynamic response. These procedures are computationally cheap and can easily be added to a conventional analysis. The test statistic is calculated at each voxel and displayed as a spatial anomaly map which shows regions where a model is violated. The methodology is illustrated with a large number of real data examples.     

Analysis of three-dimensional turbo codes

Our paper presents a detailed study of the three-dimensional turbo code (3D TC). This code which combines both parallel and serial concatenation is derived from the classical TC by concatenating a rate-1 post-encoder at its output. The 3D TC provides very low error rates for a wide range of block lengths and coding rates, at the expense of an increase in complexity and a loss in convergence. This paper deals with the performance improvement of the 3D TC. First, we optimize the distance spectrum of the 3D TC by means of the adoption of a non regular post-encoding pattern. This allows us to increase the minimum hamming distance (MHD) and thereby to improve the performance at very low error rates. Then, we propose a time varying construction of the post-encoded parity in order to reduce the observable loss of convergence at high error rates. Performance comparisons are made between the 3GPP2 standardized TC and the corresponding 3D code. The different improvement stages are illustrated with simulation results, asymptotical bounds, and EXIT charts.     

WiMAX Wave 2(MIMO/STC)的单信道测量

WiMAX Wave 2增加了空时编码(STC)和多路输入/多路输出(MIMO)等多信道传输方案,通常人们认为,要想表征这些多信道传输的性能,必须具有多信道分析能力.实际上,许多重要和有用的测量,例如信道隔离度和频率响应,以及大部分传统射频参数都可以使用单信道分析仪来完成.尤其是大量的导频音和独特的导频结构使许多单信道测量得以实现.     

Transfer Hydrogenation in Mesoporous Palladium: Polar Hydrogen Engineering to Realize High Selectivity of Alkyne Semi-Hydrogenation

Semi-hydrogenation of alkynes to industrially important alkenes is earnestly desirable in the fine chemical industry but energetically unfavorable. Herein, it is reported that mesoporous palladium (meso-Pd) catalyst changes the hydrogenation pathways in ethanol with ammonium borane as the hydrogen source, realizing the high catalytic selectivity of ≈99% in semi-hydrogenation of alkynes. Mechanism studies reveal that the active polar hydrogen can be produced and reserved well in the electron-rich mesoporous channels of meso-Pd catalyst, resulting in a transfer hydrogenation pathway, which selectively semi-hydrogenates alkynes into alkenes without over-hydrogenating alkenes into alkanes. Moreover, it is demonstrated that the polar hydrogen engineering of meso-Pd catalyst is highly efficient in various alkyne semi-hydrogenation and chemoselective hydrogenation reactions. The results thus establish metal catalyst mesostructuring as an alternative route for engineering polar hydrogen in the transfer hydrogenation reactions, thus realizing the high catalytic selectivity in various selective catalysis.     

用户电路板中的抗干扰问题分析

用户电路板具有数字／模拟信号混合,电源种类多,地线种类多,元器件多密度大等特点,因而会带来一 系列干扰问题,正确合理的布局布线,对抑制其干扰起着重要作用。本文通过几个典型实例,分析其干扰现象和干扰原因,并给出了抑制其干扰的有效措施。最后提出了用户电路板中印制板电源,地线布线设计时应注意的问题。     

InGaAs-GaAs脊波导DBR激光器双模运行特性研究

对一种新型InGaAs-GaAs脊波导分布式Bragg反射(DBR)半导体激光器双模运行特性进行了研究.该激光器由1个普通增益区和2个DBR区组成,两DBR区采用均匀蚀刻的Bragg光栅,中间加入50～100 μm的分隔区,以减小两DBR区间的热作用影响.实验发现,在增益区偏置电流强度满足双模(双波长)运行条件的情况下,在分隔区施加一电流Ispace,会对双模调谐特性产生非常大的影响.结果表明:施加适当的Ispace可使双波长的可调谐间隔显著增加,并同时提高2个激射模的边模抑制比;在一定的范围内,较大的Ispace对应较好的双模运行参数.测量了3种几何尺寸样品在多种偏置电流下的双模运行特性,对分隔区长度、Ispace以及两者组合对双模调谐特性的影响作了深入研究.研究结果对改善双波长DBR半导体激光器双模运行特性给出了有用的信息.