Simple method for the temporal characterization of amplified spontaneous emission in femtosecond terawatt Ti:sapphire lasers

We have analyzed the temporal characteristics of amplified spontaneous emission (ASE) in femtosecond terawatt Ti:sapphire lasers by using a simple method based on fast photodiodes. Instead of measuring ASE directly with fast photodiodes, we created a narrow gap in the spectrum of seed pulses and, after amplification, detected the pure ASE signal through the gap by using a fast photodiode covered with a bandpass filter with high transmission at the gap. Because the detected ASE signal was completely separated from amplified main pulses, preceding and even trailing ASEs could be characterized quantitatively in a single-shot measurement. We believe that our method is a good alternative or a complement to conventional methods for ASE measurements.     

Effect of nanocarbides and interphase hardness deviation on stretch-flangeability in 998 MPa hot-rolled steels

By performing various laboratory hot-rolling simulation tests, effective rolling conditions for steel with a composition (mass %) of 0.06C–2.0Mn–0.5Si–0.5Cr–0.2(Ti + Nb + V) was studied. Very remarkable mechanical properties were obtained when the coiling simulation temperature was 650 °C: a hole-expanding ratio of 65%, total elongation of 19%, and ultimate tensile strength of 998 MPa were achieved in the hot-rolled state. These results were associated with the Ti–Nb–V multi-microalloyed carbides (10–50 nm in size) precipitated in the ferrite matrix, and with the minimized deviation in interphase hardness.     

Effects of Pd(P) Thickness on the Microstructural Evolution Between Sn-3Ag-0.5Cu and Ni(P)/Pd(P)/Au Surface Finish During the Reflow Process

The microstructural evolution between Sn-3Ag-0.5Cu (SAC305) solder and Ni(P)/Pd(P)/Au finish during the reflow process was investigated for various Pd(P) thicknesses (0?μm to 0.6?μm). The reflow process was carried out in a belt-conveying reflow oven with peak temperature of 260°C. In the early stages of the reflow process, the Pd(P) layer either dissolved or spalled in the form of (Pd,Ni)Sn₄ into the molten solder, leaving behind an Ni₂SnP/Ni₃P bilayer on the Ni(P) layer. From the dissolution of the spalled (Pd,Ni)Sn₄ particles during the reflow process, the solubility of Pd in the molten SAC305 solder in the reflow process was estimated to be 0.18?wt.% to 0.25?wt.%. Regardless of the ratio of solder volume to pad opening size, the Ni₂SnP layer that formed in the early stage of reflow had a significant influence on the subsequent formation and growth of (Cu,Ni)₆Sn₅ at the solder interface. As the Ni₂SnP layer became thicker with increasing Pd(P) thickness, the formation of (Cu,Ni)₆Sn₅ became increasingly sluggish and occurred only at locations where the Ni₂SnP layer was locally thin or discontinuous, leading to a discontinuous morphology of (Cu,Ni)₆Sn₅. This was attributed to the Ni₂SnP layer that became an increasingly effective barrier to Ni diffusion with increasing thickness. Based on the experimental results, this study suggests detailed mechanisms underlying the effects of the Pd(P) thickness on the morphology and growth of the (Cu,Ni)₆Sn₅ formed during the reflow process.     

A monolithic 5 Gb/s p-i-n/HBT integrated photoreceiver circuit realized from chemical beam epitaxial material

The authors report on a high performance monolithic photoreceiver fabricated from chemical beam epitaxy (CBE) grown InP/InGaAs heterostructures, incorporating a p-i-n photodetector followed by a transimpedance preamplifier circuit configured from heterojunction bipolar transistors (HBTs). The optoelectronic integrated circuit (OEIC) was fabricated on a semi-insulating Fe-doped InP substrate. Microwave on-wafer measurements of the frequency response of the transistors yielded unity current gain cutoff frequencies of 32 GHz and maximum oscillation frequencies of 28 GHz for collector currents between 2 and 5 mA. The photoreceiver was operated up to 5 Gb/s, at which bit rate a sensitivity of -18.8 dBm was measured at a wavelength of 1.5 mu m. The results demonstrate that the CBE growth technique is suitable for high performance HBT-based OEICs.<>     

Long-wavelength InGaAsP/InP distributed feedback lasersincorporating gain-coupled mechanism

Successful operation of long-wavelength InGaAsP low-threshold-current gain-coupled DFB lasers was demonstrated by using an InGaAsP quaternary grating that absorbs the DFB (distributed feedback) emission. The amount of gain (loss)-coupling is controlled by the composition (bandgap) and thickness of the grating quaternary and the InP-spacer layer between the grating and the active layer. With optimally designed lasers, CW (continuous-wave) threshold currents were 10-15 mA (250-μm cavity, as-cleaved), slope efficiency was ~0.4 mW/mA (both facets) and SMSR (side-mode suppression ratio) was as high as 52 dB. The laser operated in the same DFB mode with SMSR staying ~50 dB throughout the entire current range. At 100°C, the CW threshold current stayed low, ~50 mA, and SMSR was ~40 dB. Results also indicate that the presence of gain-coupling removes the degeneracy in lasing wavelength     

Microstructure and mechanical properties of SiC-platelet reinforced Al2O3/SiC-particle hybrid composites

SiC-platelet reinforced Al₂O₃/SiC-particle nanocomposites were fabricated by hot-pressing the mixture through the conventional powder mixing process. The mechanical properties of Al₂O₃/SiC-particle/SiC-platelet hybrid composites were evaluated. Fracture toughness and work of fracture were increased by the incorporation of SiC-platelets into Al₂O₃/SiC-particle nanocomposites. The typical rising R-curve was shown during crack growth for these hybrid nanocomposites, whereas Al₂O₃/SiC-particle nanocomposites showed the constant K _R value and no rising R-curve. The further improvement of Al₂O₃/SiC-particle nanocomposites in the creep resistance was observed by the addition of SiC platelets. The relationship between the microstructure and mechanical properties for Al₂O₃/SiC-particle/SiC-platelet hybrid composites was discussed.     

New Charge Carrier Transport-Assisting Paths in Ultra-Long GaN Microwire UV Photodetector

GaN-based materials are the hottest research topic in UV photodetectors (PDs) because of their low operating voltage, small volume, long lifetime, high-temperature resistance, and low energy consumption. However, there are still fundamental issues to be overcome, and the most important issue is to get a photoconductive gain. In this paper, the following new approaches are provided to innovatively improve the photoconductive gain of UV PDs in GaN-based materials. First, the aspect ratio of the 1D GaN microwire (MW) structure is dramatically improved by analyzing the pulse growth mechanism using the metal-organic vapor deposition system. Second, the comprehensive strain behavior in the MW epitaxial growth system is successfully analyzed. Third, the fabricated metal-semiconductor-metal-based MW UV PD shows photoresponsivity and sensitivity of 28.365 A W⁻¹ and 93.16%, respectively, at the −2 V bias, which significantly outperforms the conventional structures in the UV region. Finally, a trap-assisted Poole–Frenkel effect-based energy bandgap mechanism, that allows the defect level formed by lattice mismatch between the substrate and GaN to be used as an electron carrier path, is newly defined. This study will present the direction of future UV PDs by providing a new MW structure based on GaN materials, a third-generation semiconductor.     

A node management scheme for stable P2P service in mobile ad-hoc networks

In a mobile ad-hoc network, multiple number of nodes can communicate with one another without the need for an infrastructure network. It is used in many different types of places, including military zones and disaster or hazardous areas. In a mobile ad-hoc network, each node acts both as a main agent of communication and a relay. Furthermore, each gives a weakness to the network or is subjected to vulnerabilities from malicious attacks due to their distinctive qualities, namely mobility and limited power. Accordingly, in order for stable P2P service, it is important to maintain the reliability and connectivity of the network at a high level. With existing schemes, it has often been the case that when defective nodes or malicious nodes are detected, it also causes damage to normal nodes. In scheme suggested in this paper, those nodes that have temporary defects but otherwise normal and can recovered are kept in the network but those that are defective or malicious are eliminated from the network, using trust values. The simulation was carried out to evaluate the performance of the proposed method. As a result, the message transmission rate even if the impact of a malicious node than the conventional method was demonstrated increased keeping the stable network topology.     

Efficient foreground extraction using RGB-D imaging

Image segmentation is one of the most important topics in the field of computer vision. As a result, many image segmentation approaches have been proposed, and interactive methods based on energy minimization such as GrabCut, have shown successful results. Automating the entire segmentation process is, however, very difficult because virtually all interactive methods require a considerable amount of user interaction. We believe that if additional information is provided to users in order to guide them effectively, the amount of interaction required can be reduced. Consequently, in this paper we propose an efficient foreground extraction algorithm, which utilizes depth information from RGB-D sensors such as Microsoft Kinect and offers users guidance in the foreground extraction process. Our approach can be applied as a pre-processing step for interactive and energy-minimization-based segmentation approaches. Our proposed method is able to segment the foreground from images and give hints that reduce interaction with users. In our method, we make use of the characteristics of depth information captured by RGB-D sensors and describe them using information from the structure tensor. Further, we show experimentally that our proposed method separates foreground from background sufficiently well for real world images.     

Reliability of MEMS packaging: vacuum maintenance and packaging induced stress

In this study, the dominant reliability issues of MEMS packaging that include vacuum maintenance and packaging induced stress, are discussed, and design considerations to improve the reliability are presented. The MEMS vibratory gyroscope sensor is fabricated with anodically bonded wafer level vacuum packaging followed by die-bonding and wire-bonding processes. The epoxy-molding compound (EMC) is applied to encapsulate the gyroscope sensor. Several methods to improve reliability of the vacuum packaging are suggested based on extensive work. The two major failure mechanisms of anodic vacuum packaging, namely leakage and outgassing are investigated. Leakage is effectively reduced by optimization of the bonding process. Outgassing inside the cavity can be minimized by application of Ti coating. Packaging induced stress is mainly caused by thermal expansion mismatch among the materials used to fabricate the package. During anodic bonding, thermo-mechanical stress is generated to the bonded wafer, which increases with temperature. The EMC encapsulation also produces package-induced stress. In order to increase robustness of the structure against deformation, a crab-leg type spring is replaced with a semi-folded spring. The results show that the frequency shift is greatly reduced after applying the semi-folded spring.