Research on Michelson interference refractive index sensing technique based on double-core microfiber

We propose a novel refractive index sensor based on Michelson interferometer by using double-core microfiber. Through the reflection of the end and taper of double-core fiber (DCF), the Michelson interference spectrum is formed. Owing to the structure characteristic of double-core microfiber, this interferometer can achieve the measurement of refractive index (RI) and temperature. The experimental results show that the refractive index sensitivity of the interferometer is 2 377.80 nm/RIU at the diameter of the taper waist of 8.76 µm. In the temperature ranges from 30 °C to 60 °C, the temperature sensitivity is 0.070 48 nm/°C. This sensor has the advantage of high refractive index sensitivity.     

A Stable 0.2-THz Coaxial-Waveguide Gyrotron Traveling-Wave-Tube Amplifier with Distributed Losses

For high-power operation, a THz gyrotron traveling-wave-tube (gyro-TWT) amplifier must operate in a high-order waveguide mode to enlarge the transverse dimension of an interaction waveguide. However, a gyro-TWT amplifier operating in a high-order waveguide mode is susceptible to spurious oscillations. To improve the device stability, in this study, we investigate the possibility of using a coaxial waveguide with distributed losses as the interaction structure. For the same required attenuation, all threatening oscillating modes can be suppressed using different combinations of losses of inner and outer cylinders. This provides flexibility in designing distributed losses when considering the ohmic loading of the interaction structure. We predict that the 0.2-THz gyro-TWT can stably produce a peak power of 14 kW with an efficiency of 23 %, a 3-dB bandwidth of 3.5 GHz, and a saturated gain of 50 dB for a 20-kV 3-A electron beam with a 5 % velocity spread and 1.0 velocity ratio.     

Design of oversampled nonuniform filter banks with arbitrary rational frequency partitioning

Among the theory and design of oversampled nonuniform filter banks (NUFBs), most of the existing works only consider the cases with integer decimators, and up to now the issue with rational sampling factors has not been discussed yet. In this paper, we generalize the partial modulation technique to realize arbitrary rational frequency partitioning of oversampled NUFBs with highly desired linear-phase (LP) property. Further for the subbands with sampling factors violating the guard band restriction, a phase-modification scheme is derived to avoid uneliminable large aliasing and meanwhile preserving the LP characteristics of shifted analysis/synthesis filters. By using the proposed method, the design issue of LP oversampled NUFBs can be reduced to that of several prototypes, decreasing the design complexity largely. As illustrated by examples, the proposed algorithm is more general in terms of arbitrary rational decimation and thus has broad application prospects.     

基于子带脉冲压缩的雷达宽带接收方法

宽带信号的大容量高速数据率,使宽带信号实时处理受到软硬件各方面的限制。本文详细研究了宽带信号的子带脉冲压缩处理方法,给出了过采样子带脉冲压缩系统的基本结构,并对该方法的详细性能进行仿真分析,有效地解决了宽带信号的实时处理问题。将该系统应用于实际工程中,对实测数据的处理结果证明了系统的正确性和有效性。     

基于核函数法及马尔可夫链的节点定位算法

基于贝叶斯滤波框架,提出了基于核函数法及马尔可夫链的节点定位算法,该算法采用射频指纹匹配技术,使用核函数构建似然函数,充分利用观测与多个训练样本之间的相似性,避免使用先验确定型信号分布模型产生的误差.此外,为提高移动目标的定位精度和定位实时性,该算法还使用马尔可夫链,通过利用目标的历史状态和环境布局等信息对匹配定位的网格搜索空间进行限制,剔除目标移动过程中不可能发生的位置跳变.实验证明,与高斯分布模型相比,所提定位算法具有更高的定位正确率和定位精度.     

基于CZT的双基地SAR极坐标格式成像算法

与其它聚束式成像算法相比,极坐标格式算法(PFA)具有距离向数据率更低、与自聚焦兼容等优势,但同时也存在着由二维波数域插值而带来的插值误差和计算负担大等不足.本文利用Chirp-Z变换(CZT)的特性,提出了一种基于CZT的双基地极坐标格式成像算法(CZT-PFA),采用CZT来代替原来的插值处理,不仅有效的降低了算法计算量,而且避免了插值误差对成像结果的影响,提高了PFA的性能和实用性.     

Post-process thermal treatment for microwave power improvement of AlGaN/GaN HEMTs

The effects of post-process rapid thermal annealing (RTA) treatment after device fabrication on direct current, microwave and power performances of AlGaN/GaN high electron mobility transistors (HEMTs) with a gate-length of 0.2 μm were fully investigated. By 3 min post-process RTA treatment at 350 °C under N₂ atmosphere, the direct current (DC), radio frequency (RF) small signal and power performances of AlGaN/GaN HEMTs have been much improved. The output power, power gain and power added efficiency (PAE) of GaN HEMT device with gate wide of 1 mm increase from 37.09 dBm, 6.09 dB and 42.79% to 38.22 dBm, 7.22 dB and 67.3%. The post-process RTA after device fabrication has two merits. On the one hand, it improves passivation effect of SiN_x dielectric layer on AlGaN/GaN HEMT surface, suppressing RF current dispersion. On the other hand, it helps recover dry-etch damage at the Schottky metal/AlGaN interface, leading to reduction of reverse Schottky leakage current.     

ACAR: Adaptive Connectivity Aware Routing for Vehicular Ad Hoc Networks in City Scenarios

Multi-hop vehicle-to-vehicle communication is useful for supporting many vehicular applications that provide drivers with safety and convenience. Developing multi-hop communication in vehicular ad hoc networks (VANET) is a challenging problem due to the rapidly changing topology and frequent network disconnections, which cause failure or inefficiency in traditional ad hoc routing protocols. We propose an adaptive connectivity aware routing (ACAR) protocol that addresses these problems by adaptively selecting an optimal route with the best network transmission quality based on statistical and real-time density data that are gathered through an on-the-fly density collection process. The protocol consists of two parts: 1) select an optimal route, consisting of road segments, with the best estimated transmission quality, and 2) in each road segment of the chosen route, select the most efficient multi-hop path that will improve the delivery ratio and throughput. The optimal route is selected using our transmission quality model that takes into account vehicle densities and traffic light periods to estimate the probability of network connectivity and data delivery ratio for transmitting packets. Our simulation results show that the proposed ACAR protocol outperforms existing VANET routing protocols in terms of data delivery ratio, throughput and data packet delay. Since the proposed model is not constrained by network densities, the ACAR protocol is suitable for both daytime and nighttime city VANET scenarios.     

Thermoelectric Efficiency of a Quantum Dot in the Single-Electron Transistor Configuration

The thermoelectric efficiency of a single-dot (e.g., nanocrystal or molecule) device has been studied theoretically. The transport coefficients, the power factor, and the figure of merit (ZT) of the quantum dot in the single-electron transistor configuration have been calculated in the sequential tunneling regime when Coulomb blockade is important. Very high values of ZT have been found in the quantum regime, and they have been explained by quantum confinement. An approximate analytical formalism has been derived for ZT as a function of the separation between the energy levels of the dot, ΔE, and the thermal energy, k _B T. The electron–phonon coupling has been included in the calculations, and it is shown that it is important for realistic prediction of the thermoelectric efficiency of a quantum-dot device. It has been found that the thermoelectric efficiency decreases due to electron–phonon coupling. Distinct behavior has been obtained for weak and for strong electron–phonon coupling. Quantum confinement is the dominant mechanism determining the magnitude of ZT for weak electron–phonon coupling. For strong electron–phonon coupling, the energy conversion efficiency is determined by the phonon spectrum. In all cases, it has been found that the thermoelectric efficiency decreases rapidly with increasing temperature.     

Structural Characterization of Doped GaSb Single Crystals by X-ray Topography

We characterized GaSb single crystals containing different dopants (Al, Cd, and Te), grown by the Czochralski method, using x-ray topography and high-angular-resolution x-ray diffraction. Lang topography revealed dislocations parallel and perpendicular to the crystal surface. Double-crystal GaSb 333 x-ray topography showed dislocations and vertical stripes that could be associated with circular growth bands. We compared our high-angular- resolution x-ray diffraction measurements (rocking curves) with findings predicted by the dynamical theory of x-ray diffraction. These measurements show that our GaSb single crystals have a relative variation in the lattice parameter (Δd/d) on the order of 10⁻⁵. This means that they can be used as electronic devices (e.g., detectors) and as x-ray monochromators.