Experimental study of an ultra narrow linewidth fiber laser by injection locking

An ultra narrow linewidth fiber laser is presented, which is composed of a ring cavity laser and a linewidth-narrowing module. The fiber laser is introduced in detail and the single mode operation of the laser is verified. Using a revised self-heterodyne detection method, experimental data of the laser linewidth against the pump power are presented. The narrowest linewidth is measured to be 109 Hz, whereas the output power is 1.1 mW.     

大电流纳秒级脉宽激光二极管驱动电路的设计

脉冲激光雷达探测的性能直接与激光束的质量和能量有关,而激光驱动电路则直接决定了脉冲激光的功率和脉冲宽度。基于半导体激光雷达探测系统的要求,设计出应用于激光雷达发射系统的大电流纳秒级脉冲半导体激光二极管的驱动电路。介绍了电路的系统结构和驱动级电路的模型及其理论分析,推导其电流数学模型计算公式,并对关键器件的选择进行了总结;运用SPICE仿真程序对驱动电路进行了仿真。制作了印刷电路板,得到峰值电流25A,脉宽15ns的脉冲电流,满足设计要求。     

一种新型的多孔太阳墙采暖房

长江中下游地区很多建筑没有安装专门的供暖设备,传统的空调采暖增加了建筑能耗。利用太阳能和建筑物自身进行采暖,具有重要的意义和价值。本文提出了一种新型的多孔太阳墙采暖房,其核心部分多孔太阳墙是由多孔陶瓷构成。在晴朗的冬季,对多孔太阳墙采暖房的采暖性能进行了实验研究;研究重点检测了不同外界环境(主要是指太阳辐射照度和环境温度)下,多孔太阳墙采暖房的采暖及储热性能。研究表明:多孔太阳墙采暖房在白天大部分时间都具有较好的采暖效果,且采暖房内的温差较小;同时具有较好的储热性能。以实验测得的武汉地区1月某天的实验数据为例,相比环境温度,当天采暖房内最高温升为19.9℃。在07:00～16:00之间,采暖房的平均温升为11.5℃;在辐射强度较弱的16:00～18:00,采暖房的平均温升为6.6℃。在07:00～18:00之间,采暖房内最大温差为1.37℃,出现在12:40。     

Terahertz photonic states in semiconductor-graphene cylinder structures

We propose a semiconductor-graphene cylinder that can serve as a terahertz (THz) photonic crystal. In such a structure, graphene plays a role in achieving a strong mismatch of the dielectric constant at the semiconductor-graphene interface due to its two-dimensional nature and relatively low value of the dielectric constant. We find that when the radius of the outer semiconductor layer is about ρ(1)~100 μm, the frequencies of the photonic modes are within the THz bandwidth and they can be efficiently tuned via varying ρ(1). Furthermore, the dispersion relation of the photonic modes shows that a semiconductor-graphene cylinder is of excellent light transport properties, which can be utilized for the THz waveguide. This study is pertinent to the application of graphene as THz photonic devices.     

Analysis of absorption performances of anechoic layers with steel plate backing

Rubber layers with air-filled cavities or local resonance scatters can be used as anechoic coatings. A lot of researches have focused on the absorption mechanism of the anechoic coatings. As the anechoic coatings are bonded to the hull of submarine, the vibration of the hull should not be neglected when the analysis of the absorption characters is carried out. Therefore, it is more reasonable to treat the anechoic coating and the backing as a whole when the acoustic performance is analyzed. Considering the effects of the steel plate backing, the sound absorption performances on different models of anechoic coatings are investigated in this paper. The Finite Element Method is used to illustrate the vibrational behaviors of the anechoic coatings under the steel backings by which the displacement contours is obtained for analysis. The theoretical results show that an absorption peak is induced by the resonance of the steel slab and rubber layer. At the frequency of this absorption peak, the steel plate and the coating vibrates longitudinally like a mass-spring system in which the steel slab serves for mass and the coating layer is the spring. To illuminate the effects of the steel slab backing on the acoustic absorption, the thicknesses of the steel slab and the anechoic layer are discussed. Finally, an experiment is performed and the results show a good agreement with the theoretical analysis.     

Changes in surface morphology and work function caused by corrosion in aluminum alloys

Corrosion behavior of a material is complicated and can affect strongly its surface physical and chemical properties. In this study, the effect of corrosion on surface morphology and electron work function (EWF) of various Al alloys was comparatively investigated using a scanning electron microscopy and a scanning Kelvin probe. The experimental results showed that the change in EWF with corrosion time was not linear, but firstly increased and then decreased. The theoretical treatments demonstrated that such a change in EWF did not only depend on the surface roughness, but also correlate with oxide layers formed at the surfaces of Al alloys. The present results can also be used to reasonably explain the correlation between surface roughness and EWF reported previously.     

Optimization of locally resonant acoustic metamaterials on underwater sound absorption characteristics

The study of acoustic metamaterials, also known as locally resonant sonic materials, has recently focused on the topic of underwater sound absorption. The high absorption occurs only within a narrow frequency band around the locally resonant frequency. Nevertheless, this problem can be addressed through a combination of several acoustic metamaterial layers that have different resonant frequencies. In this paper, an optimization scheme, a genetic and a general nonlinear constrained algorithm, is utilized to enhance the low-frequency underwater sound absorption of an acoustic metamaterial slab with several layers. Both the physical and structural parameters of the acoustic metamaterial slab are optimized to enlarge the absorption band. In addition, the sound absorption mechanism of the acoustic metamaterial slab is also analyzed. The result shows that each layer is found to oscillate as a nearly independent unit at its corresponding resonant frequency. The theoretical and experimental results both demonstrate that the optimized metamaterial slab can achieve a broadband (800–2500 Hz) absorption of underwater sound, which is a helpful guidance on the design of anechoic coatings.     

First-principles studies on the structural and electronic properties of Li-ion battery cathode material CuF2

The 3d transition metal binary compounds have been extensively investigated for a large multi-electron redox capacity through reversible electrochemical reactions. Here, the structural, electronic and magnetic properties of CuF₂ are studied by the first-principles calculations within both the generalized gradient approximation (GGA) and GGA+U frameworks. Our results show that the antiferromagnetic (AFM) configuration of CuF₂ is more stable than the ferromagnetic (FM) one, which is consistent with experiments. The analysis of the electronic density of states (DOS) shows that CuF₂ is a classic Mott–Hubbard insulator with a large d–d type band gap, which is similar to the case of FeF₃. Moreover, small spin polarizations were found on the sites of fluorin ions, which accords with a fluorin-mediated superexchange mechanism for the Cu–Cu magnetic interaction.     

Effects of time on the magnetic properties of terbium-doped LaMnO3

The magnetic properties of the perovskite form of LaMnO₃ have been shown strong interest in recent years due to its high potential for use in magnetic devices. In this paper, the magnetic properties of a 30% terbium-doped LaMnO₃ (LMTO) perovskite manganite synthesized by a conventional solid-state reaction were investigated. Data on these properties was recorded periodically via SQUID and VSM to reveal it to be best described magnetically as a spin glass system. Thus, the time effect must be taken into consideration in instantaneously determining this material’s spin glass state as well as the overall magnetic properties in the absence of a magnetic field. The results of this paper point to a more in-depth understanding of the change in magnetic properties associated with doped LaMnO₃.     

Two-dimensional electron transport in AlGaN/GaN heterostructures

We present a theoretical study of electron transport properties of two-dimensional electron gas in AlGaN/GaN heterostructures. By assuming a drifted Fermi–Dirac distribution and taking into account all major scattering mechanisms, including polar optical and acoustic phonons, background impurities, dislocation and interface roughness, the momentum- and energy-balance equations derived from Boltzmann equation are solved self-consistently. The dependence of the electron drift velocity and electron temperature as a function of the applied electric field are obtained and discussed.