Single-mode holey vertical-cavity surface-emitting laser with ultra-narrow beam divergence

We demonstrated oxide-confined 850-nm vertical-cavity surface-emitting lasers (VCSELs) with a two-dimensional petal-shaped holey structure composed of several annular-sector-shaped holes. Four types of devices with different hole numbers were designed and fabricated. The measured results showed that the larger hole number was beneficial to purifying the lasing mode, and realizing the single-mode operation. The side mode suppression ratio (SMSR) exceeded 30 dB throughout the entire drive current. Mode selective loss mechanism was used to explain the single-mode characteristic. The single-mode devices possessed good beam profiles, and the lowest divergence angle was as narrow as 3.2 degrees (full width at half maximum), attributed to the graded index profile and the shallow etching in the top distributed Bragg reflector (DBR).

Phase-locked ring-defect photonic crystal vertical-cavity surface-emitting laser

Phase-locked oxide-confined ring-defect photonic crystal vertical-cavity surface-emitting laser is presented. The coupled-mode theory is employed to illustrate the two supermodes of the device, in-phase and out-of-phase supermode. Experimental results verify the two supermodes by the characteristics of the spectra and the far field patterns. At the lower current, only the out-of-phase supermode is excited, whereas under the higher current, the in-phase supermode also appears at the shorter wavelength range. In addition, the measured spectral separation between the two supermodes agrees well with the theoretical result.

Comparing the Performance of Biomimetic Pattern Recognition with W8 and SVM on Prediction of Horizontal Gene Transfers in Bacteria Genomes

National Natural Science Foundation of China 60753001

Self-heating effect in 1.3 mu m p-doped InAs/GaAs quantum dot vertical cavity surface emitting lasers

The self-heating effect in 1.3 mu m p-doped InAs/GaAs quantum dot (QD) vertical cavity surface emitting lasers (VCSELs) has been investigated using a self-consistent theoretical model. Good agreement is obtained between theoretical analysis and experimental results under pulsed operation. The results show that in p-doped QD VCSELs, the output power is significantly influenced by self-heating. About 60% of output power is limited by self-heating in a device with oxide aperture of 5x6 mu m(2). This value reduces to 55% and 48%, respectively, as the oxide aperture increases to 7x8 and 15x15 mu m(2). The temperature increase in the active region and injection efficiency of the QDs are calculated and discussed based on the different oxide aperture areas and duty cycle.

需求变更影响分析模型及相关技术研究

需求是后续开发活动的基准，早期的一些研究者认为应该在需求完全确定之后再进行后续开发，Royce所提出的瀑布模型就是这种思想的一个体现。但是，实践经验告诉我们，不管前期的需求分析做的多么完美，需求还是会发生变更。一方面是因为需求本身很复杂，对它的分析、理解和描述是一个循序渐进的过程，不可能一蹴而就；另一方面由于用户期望和偏好的改变、市场环境的转变、使用环境的日益复杂、技术的革新等都会使得原有的软件系统无法满足各涉众的利益。因此，需求变更是软件开发中固有的规律，是不可避免和普遍存在的。 需求变更通常会导致需求间及需求与后续工作产品间的不一致。因此，频繁的需求变更会造成产品质量下降、进度延期、成本超支等问题。变更影响分析(Change Impact Analysis)通过分析变更对象及其相关工作产品间的关系来评估变更造成的影响，从而控制变更。现有方法多数是从软件维护的角度，对代码的变更影响进行分析，过于细节和技术化，不能对需求变更影响分析提供有力支持。即使针对需求变更的影响分析方法也只是基于形式化需求规约，通过分析需求间的关系来识别影响范围，并未考虑需求变更对后续工作产品造成的影响，同时形式化需求规约的应用困难也限制了该方法的实用性。此外，随着需求和工作产品的规模与复杂性日益增加，使得手工建立和维护需求间及需求与工作产品间关系面临着不小的难度。 基于以上分析，本文提出了针对自然语言需求规约的需求变更影响分析模型RCIAM (Requirement Change Impact Analysis Model)，围绕着如何自动识别和筛选需求间的关系——横向需求跟踪关系(Horizontal Requirement Traceability)、如何自动识别和筛选需求与工作产品间的关系——纵向需求跟踪关系(Vertical Requirement Traceability)、如何较全面的进行需求变更影响计算和决策三个问题展开了研究。 本文的主要贡献有： (1) 提出了需求变更影响分析模型RCIAM 本文对RCIAM进行了形式化定义。该模型不但提供了需求变更影响分析算法和对决策的支持，还提供了自动识别横向和纵向需求跟踪关系的方法。RCIAM主要包含数据处理和数据分析两个层次。数据处理层采用了文本处理(Text Processing)技术实现了横向和纵向需求跟踪关系的自动识别，为数据分析层提供需求跟踪关系数据；数据分析层基于需求跟踪关系数据，在产生需求变更申请(Change Request)时，进行量化影响计算，并提供决策支持。 (2) 提出了横向需求跟踪关系的识别与筛选方法 在对自然语言需求规约文档进行深入分析后，我们发现了两种与需求变更影响密切相关的关系类型，并从文本相似性的角度将它们定义为相似跟踪关系和引用跟踪关系。在将需求项拆分为需求片段的基础上，利用信息检索技术(Information Retrieval，IR)计算需求片段间的文本相似度，并设计了相应的算法对相似跟踪关系和引用跟踪关系进行自动识别。最后，提出了“变更影响跟踪”的规则来辅助对候选跟踪关系的人工筛选。 (3) 提出了纵向需求跟踪关系的识别与筛选方法 已有的研究多采用IR技术来自动建立需求与工作产品之间的跟踪关系，但是却存在着精度不理想的问题。我们从查全率(Recall)和查准率(Precision)的角度，分析了应用IR技术自动建立需求与代码跟踪关系的方法中产生的错误关系，发现了造成精度问题的根源所在。依据这一发现，基于现有方法，本文方法加入了相关反馈(Relvant Feedback)辅助识别和代码注释信息辅助识别等改进措施，并提供了人工筛选策略。 (4) 提出了需求变更影响分析计算与决策方法 本文通过矩阵运算说明了需求变更影响通过需求跟踪关系传播到其它需求和工作产品的过程，并设计了相应的需求变更影响分析算法。该算法考虑变更发生在不同阶段时对不同类型工作产品的影响，采用变更类型和关系强度两个因子加权计算影响值，并提出了根据影响值来进行变更决策的方法。 (5) 应用研究 结合中科方德公司Qone平台的开发，对以上工作进行了应用研究和性能分析。在Qone平台的需求管理工具版本1.0的开发中，首先采用本文方法对横向和纵向需求跟踪关系进行了自动识别，然后对开发期间发生的十次需求变更申请进行了影响分析和决策。在项目完成后，设计了实验对横向和纵向需求跟踪关系的识别进行了性能分析。结果表明，本文方法能够有效辅助进行需求变更影响分析。

Experimental investigation on TBAB clathrate hydrate slurry flows in a horizontal tube: Forced convective heat transfer behaviors

Tetra-n-butyl-ammonium bromide (TBAB) clathrate hydrate slurry (CHS) is one kind of secondary refrigerants, which is promising to be applied into air-conditioning or latent-heat transportation systems as a thermal storage or cold carrying medium for energy saving. It is a solid-liquid two phase mixture which is easy to produce and has high latent heat and good fluidity. In this paper, the heat transfer characteristics of TBAB slurry were investigated in a horizontal stainless steel tube under different solid mass fractions and flow velocities with constant heat flux. One velocity region of weakened heat transfer was found. Moreover, TBAB CHS was treated as a kind of Bingham fluids, and the influences of the solid particles, flow velocity and types of flow on the forced convective heat transfer coefficients of TBAB CHS were investigated. At last, criterial correlations of Nusselt number for laminar and turbulent flows in the form of power function were summarized, and the error with experimental results was within 20%.

Chaotic behavior of pulsating heat pipes

Chaotic behavior of closed loop pulsating heat pipes (PHPs) was studied. The PHPs were fabricated by capillary tubes with outer and inner diameters of 2.0 and 1.20 mm. FC-72 and deionized water were used as the working fluids. Experiments cover the following data ranges: number of turns of 4, 6, and 9, inclination angles from 5 degrees (near horizontal) to 90, (vertical), charge ratios from 50% to 80%, heating powers from 7.5 to 60.0 W. The nonlinear analysis is based on the recorded time series of temperatures on the evaporation, adiabatic, and condensation sections. The present study confirms that PHPs are deterministic chaotic systems. Autocorrelation functions (ACF) are decreased versus time, indicating prediction ability of the system is finite. Three typical attractor patterns are identified. Hurst exponents are very high, i.e., from 0.85 to 0.95, indicating very strong persistent properties of PHPs. Curves of correlation integral versus radius of hypersphere indicate two linear sections for water PHPs, corresponding to both high frequency, low amplitude, and low frequency, large amplitude oscillations. At small inclination angles near horizontal, correlation dimensions are not uniform at different turns of PHPs. The non-uniformity of correlation dimensions is significantly improved with increases in inclination angles. Effect of inclination angles on the chaotic parameters is complex for FC-72 PHPs, but it is certain that correlation dimensions and Kolmogorov entropies are increased with increases in inclination angles. The optimal charge ratios are about 60-70%, at which correlation dimensions and Kolmogorov entropies are high. The higher the heating power, the larger the correlation dimensions and Kolmogorov entropies are. For most runs, large correlation dimensions and Kolmogorov entropies correspond to small thermal resistances, i.e., better thermal performance, except for FC-72 PHPs at small inclination angles of theta < 15 degrees.

Interaction of linear waves with an array of infinitely long horizontal circular cylinders in a two-layer fluid of infinite depth

The linear water wave scattering and radiation by an array of infinitely long horizontal circular cylinders in a two-layer fluid of infinite depth is investigated by use of the multipole expansion method. The diffracted and radiated potentials are expressed as a linear combination of infinite multipoles placed at the centre of each cylinder with unknown coefficients to be determined by the cylinder boundary conditions. Analytical expressions for wave forces, hydrodynamic coefficients, reflection and transmission coefficients and energies are derived. Comparisons are made between the present analytical results and those obtained by the boundary element method, and some examples are presented to illustrate the hydrodynamic behavior of multiple horizontal circular cylinders in a two-layer fluid. It is found that for two submerged circular cylinders the influence of the fluid density ratio on internal-mode wave forces is more appreciable than surface-mode wave forces, and the periodic oscillations of hydrodynamic results occur with the increase of the distance between two cylinders; for four submerged circular cylinders the influence of adding two cylinders on the wave forces of the former cylinders is small in low and high wave frequencies, but the influence is appreciable in intermediate wave frequencies.

Periodic bubble emission and appearance of an ordered bubble sequence (train) during condensation in a single microchannel

Condensation of steam in a single microchannel, silicon test section was investigated visually at low flow rates. The microchannel was rectangular in cross-section with a depth of 30 pm, a width of 800 mu m and a length of 5.0 mm, covered with a Pyrex glass to allow for visualization of the bubble formation process. By varying the cooling rate during condensation of the saturated water vapor, it was possible to control the shape, size and frequency of the bubbles formed. At low cooling rates using only natural air convection from the ambient environment, the flow pattern in the microchannel consisted of a nearly stable elongated bubble attached upstream (near the inlet) that pinched off into a train of elliptical bubbles downstream of the elongated bubble. It was observed that these elliptical bubbles were emitted periodically from the tip of the elongated bubble at a high frequency, with smaller size than the channel width. The shape of the emitted bubbles underwent modifications shortly after their generation until finally becoming a stable vertical ellipse, maintaining its shape and size as it flowed downstream at a constant speed. These periodically emitted elliptical bubbles thus formed an ordered bubble sequence (train). At higher cooling rates using chilled water in a copper heat sink attached to the test section, the bubble formation frequency increased significantly while the bubble size decreased, all the while forming a perfect bubble train flowing downstream of the microchannel. The emitted bubbles in this case immediately formed into a circular shape without any further modification after their separation from the elongated bubble upstream. The present study suggests that a method for controlling the size and generation frequency of microbubbles could be so developed, which may be of interest for microfluidic applications. The breakup of the elongated bubble is caused by the large Weber number at the tip of the elongated bubble induced by the maximum vapor velocity at the centerline of the microchannel inside the elongated bubble and the smaller surface tension force of water at the tip of the elongated bubble.

Interaction of linear waves with infinitely long horizontal cylinders studied by boundary element method

The two-dimensional problems concerning the interaction of linear water waves with cylinders of arbitrary shape in two-layer deep water are investigated by use of the Boundary Integral Equation method (BIEM). Simpler new expressions for the Green functions are derived, and verified by comparison of results obtained by BIEM with these by an analytical method. Examined are the radiation and scattering of linear waves by two typical configurations of cylinders in two-layer deep water. Hydrodynamic behaviors including hydrodynamic coefficients, wave forces, reflection and transmission coefficients and energies are analyzed in detail, and some interesting physical phenomena are observed.

Scattering of linear water waves by a fixed and infinitely long rectangular structure parallel to a vertical wall in oblique seas

The scattering of linear water waves by an infinitely long rectangular structure parallel to a vertical wall in oblique seas is investigated. Analytical expressions for the diffracted potentials are derived using the method of separation of variables. The unknown coefficients in the expressions are determined through the application of the eigenfunction expansion matching method. The expressions for wave forces on the structure are given. The calculated results are compared with those obtained by the boundary element method. In addition, the influences of the wall, the angle of wave incidence, the width of the structure, and the distance between the structure and the wall on wave forces are discussed. The method presented here can be easily extended to the study of the diffraction of obliquely incident waves by multiple rectangular structures.