Noise suppression and optical sectioning by non-phase-recording interferometry

It has long been known that image plane holography with low-coherence illumination achieves optical sectioning of a volume object. A method is analyzed that is similar to image plane holography, but the interferometric arrangement utilizes the interference between two object-bearing beams instead of the basic object and reference beams.     

Wide-band suppression of laser intensity noise

A simple and efficient laser intensity stabilization system was constructed based on an electro-optical modulator (EOM) and a photodetector. It is capable of reducing the laser intensity fluctuations to the shot noise limit within the range of Fourier frequencies from a few tens of hertz to a few megahertz. The achieved bandwidth of the laser control system is limited by the light handling capacity of the photodetector and spurious resonances of the EOM. We discuss the general approach to the design of the laser intensity stabilization system and its noise properties.     

Speckle suppression in scanning laser display

The theory of speckle noise in a scanning beam is presented. The general formulas for the calculation of speckle contrast, which apply to any scanning display, are obtained. It is shown that the main requirement for successful speckle suppression in a scanning display is a narrow autocorrelation peak and low sidelobe level in the autocorrelation function of the complex amplitude distribution across a scanning light beam. The simple formulas for speckle contrast for a beam with a narrow autocorrelation function peak were obtained. It was shown that application of a diffractive optical element (DOE) with a Barker code phase shape could use only natural display scanning motion for speckle suppression. DOE with a Barker code phase shape has a small size and may be deposited on the light modulator inside the depth of the focus of the reflected beam area, and therefore, it does not need an additional image plane and complicated relay optics.     

变压器的噪声及其降低

本文简术了变压器噪声的特点及产生机理,并介绍了降低变压器噪声的主要措施。     

基于可变形空腔的起落架舱体噪声抑制研究EI北大核心CSCD

提出了一种基于可变形空腔的起落架舱体结构,通过机械装置调节舱体底板及后壁倾斜角度,不需要额外增加舱体体积,使用声学有限元法探讨了该结构在低马赫数下的噪声抑制效果。研究发现:随着舱体后壁倾斜角度的增大,舱体内部及外部的噪声明显减小,同时模态频率逐渐增大,有助于避免舱体结构发生共振破坏;舱体后壁倾斜一个较小的角度就能有效地改善内部的声反射环境,进而抑制舱体内部的高频模态噪声、总声压级。当后壁倾斜角度大于某个临界值时,继续增大倾斜角度对于舱体内部高频模态噪声以及总声压级的抑制效果不再明显,在当前的仿真条件下,舱体后壁最佳倾斜角度范围为10°~16°。     

X-射线工业CT射束硬化校正中噪声的抑制

X-射线工业CT中,传统的线性化(ConventionalLinearization,CL)校正法去除射束硬化影响时,会放大投影的噪声,进而降低CT图像的信噪比(SNR)。为克服该点不足,提出了一种改进的线性化(ImprovedLinearization,IL)校正法。该方法基于投影噪声的特点,设计了一种自适应滤波器;利用该滤波器从原始投影数据中提取其噪声,求得由噪声引起的校正误差;根据该误差对校正结果进行修正以抑制投影噪声的放大。通过不同管电压和被检材质,对IL校正法进行了验证。实验结果表明,IL校正法能去除射束硬化的影响,并能有效地抑制校正过程中投影噪声的放大。与CL校正法相比,IL校正的CT图像SNR提高了约6倍。     

闪烁噪声的自相关函数及其表征

闪烁噪声是一种非平稳随机过程,其功率谱密度函数在频率低端（ｆ＝０）发散,无法直接利用Ｗｉｅｎｅｒ－Ｋｈｉｎｔｃｈｉｎｅ关系得到它的自相关函数,本文采用“降阶－积分”方法,得到了频率源中闪烁调频噪声的自相关函数的解析表达式。根据这一结果,分析了频率稳定度的表征原理,指出了频率稳定度表征的实质是将非平稳过程转化为平稳过程并求取时间平均。最后,通过时域分析得到了闪烁噪声的Ａｌｌａｎ方差表达式。     

纺织厂噪声控制技术与应用

对纺织厂织布车间,细纱车间的噪声情况做了调研与测试.分析了目前纺织厂噪声的现状及对生产和工人身心健康的影响,提出了降噪综合控制方法,并通过实验测试得以验证.     

Zigzag slabs for solid-state laser amplifiers: batch fabrication and parasitic oscillation suppression

We have developed a 100 W class Nd:YAG master oscillator power amplifier system based in part on an end-pumped zigzag slab power amplifier. This amplifier incorporates parasitic oscillation suppression by using roughened edges and achieves a small-signal gain coefficient (g(0)l) of 8.06. We describe a novel technique for suppression of parasitic oscillations using claddings on slab edges that significantly increases g(0)l to 11.63 and increases the single-pass extracted power in a power amplifier by 50%. Commercial use of these zigzag slab amplifiers has been limited by the time and cost of production. We describe a new batch fabrication technique that improves the quality and significantly reduces the cost of zigzag slabs.     

一种MEMS压控电容设计及其相位噪声分析

给出了一种MEMS压控电容的设计和相位噪声分析结果.在电容器结构中引入衬底金属层以达到控制寄生电容的目的;使用HFSS进行了上极板开孔对电容的影响的仿真,得到了不影响电容值的最佳尺寸是5微米.在恒寄生电容的假设下分析了MEMS压控电容的相位噪声,在SPICE模型基础上使用HSPICE进行了相位噪声仿真,分析发现布朗运动所产生的相位噪声的典型值是-176dBc/Hz,环境声波造成的相位噪声的典型值是-137dBc/Hz.     

Diffraction field of tightly focused laser beams and its influence to laser accelerator

In this paper, we have studied the dynamic characteristics of relativistic electron injected into stationary intense vacuum laser fields. We found the dynamic trajectories can basically be classified as three categories, namely Inelastic Scattering (IS), Capture and Acceleration Scenario (CAS) and Penetrate into Axial Region and Move (PARM) trajectory. The physical mechanism as to the three kinds of electrons have been examined. In particular, the PARM trajectory which we presented in this paper is different from the CAS and IS trajectory which we had already found in our previous work. We will show the PARM stems from the strong diffraction effect of a tightly focused laser field. In addition, the initial condition for the three kinds of electrons to emerge were detailed investigated. It has been found that there are four factors which chiefly decide the appearance of the three kinds of dynamics trajectories, namely the laser beam width w₀ and intensity a₀, the electrons incident angle θ and initial transversal momentum p_ti. The implication of the PARM electrons to the planned vacuum laser accelerators is illustrated.     

Noise characteristics of a Nd:YVO(4) laser pumped by laser diode modulated at high frequency

We report, for the first time to our knowledge, on the noise characteristics of a Nd:YVO(4) laser pumped by a laser diode modulated at high frequency. We have investigated noise characteristics of a Nd:YVO(4) laser pumped by a laser diode that is oscillating in a stable multilongitudinal mode because of modulation by a high-frequency (several hundred megahertz) current. As a result, low-noise operations of -130 dB/Hz above frequencies of 1 MHz have been achieved. This noise level is comparable to the level obtained when pumping a Nd:YVO(4) laser with a laser diode that is oscillating in a single longitudinal mode. However, a noise peak corresponding to relaxation oscillations of the Nd:YVO(4) laser has appeared around a frequency of several hundred kilohertz.     

深海立管涡激振动及其抑制方法研究

涡激振动是造成深海立管疲劳乃至失效的重要因素之一,会严重影响立管的使用寿命.现采用数值模拟的方法对深海顶张式立管的涡激振动及其抑制方法进行了研究.即利用有限元软件ABAQUS模拟了立管在不同流速下裸管和安装螺旋侧板的不同实例,得到其横向位移响应.结果表明,裸管条件下,当约化速度4.5相似文献   

混合型激光振荡-放大系统及其同步控制

介绍一种利用LD泵浦激光振荡-氙灯泵浦激光放大的混合型MOPA(MasterOscillatorPowerAmplifier)系统。实验中利用信号发生器SG产生频率和宽度可调的控制脉冲去控制LD电源,同时提供另一路延时可调脉冲信号去触发由两级氙灯泵浦的Nd:YAG激光器组成的放大级。从而将振荡-放大器电源同步,使LD泵浦的振荡级辐射激光经过氙灯泵浦放大级后获得同步放大。得到重复率为1Hz,脉冲能量为1.8mJ,脉宽为8ns的自调Q激光脉冲输出。将此激光束耦合入芯径为100μm的多模石英光纤中,观察到由背向受激布里渊散射(SBS)引起的位相共轭现象。     

群时延引起的受控小车二级摆失稳及其抑制

以受控小车一二级摆为对象,通过理论和实验方法讨论了数字滤波器群时延引起的系统失稳问题,并采用时滞LQ方法实现了对这类失稳的抑制.首先,建立小车-二级摆系统的动力学模型,测试其自由衰减振动后指出对其进行主动控制的必要性.然后,讨论了数字滤波器对受控系统稳定性的影响,其中低通滤波器用于去除控制输人信号中的强高频噪声.研究发现,滤波器指标设置过高时会因群时延量过大而引起系统失稳.由于滤波器群时延量仅与滤波器相频特性相关,可以事先得到,故含滤波器的系统适合于基于状态变换的时滞控制.最后介绍了一种离散的时滞LQ控制,并用其成功实现了对群时延造成的系统失稳的抑制.     

空调设计中的噪声及防治措施

指出了空调设计中的噪声问题并提出了控制噪声的几项措施.     

Passively Q-switched quasi-three-level laser and its intracavity frequency doubling

A passively Q-switched quasi-three-level Nd:YAG laser is intracavity frequency doubled to generate a blue laser. The 473-nm blue laser has a peak power of 37 W and a pulse width of 23 ns at a pumping power of 1.6 W. To model this laser numerically, we developed rate equations by taking into consideration both the quasi-three-level nature of the gain medium and the four-level nature of the saturable absorber. Good agreement was achieved between experimental and simulated results for both the fundamental and the second-harmonic output. The reabsorption loss of the gain medium is estimated under pulsed operation.     

Dynamic model of multipass ultrashort-pulse laser amplifiers and its application

We developed a dynamic model for evaluating the gain and amplified spontaneous emission of multipass ultrashort-pulse laser amplifiers. This model takes into account the time dependence of the population inversion that is due to the time-varying pump power and the evolution of the amplified spontaneous emission. For gain media of a short upper-state lifetime, a time-dependent analysis is essential for the model to extend beyond order-of-magnitude estimation. We verified the model with a femtosecond dye laser amplifier. The calculated output energy is accurate to within +/-5% of the experimental data. We utilized this model to optimize the conversion efficiency of the dye laser amplifier under low-energy pumping. An efficiency of 2.3% is achieved with an optimal pump energy of only 4 muJ.     

Amplitude-stabilized frequency-modulated laser diode and its interferometric sensing applications

A direct frequency-modulated (FM) laser diode light source without light power variation is developed. The amplitude variation of the FM laser diode is compensated by means of a feedback system with use of a superluminescent diode as an external light power controller. Output power greater than 1 mW is obtained at the modulation frequency to 5 kHz with a >10 stabilization factor. By use of the amplitude-stabilized FM laser diode, we measured subfringes with high accuracy in FM continuous wave interferometry, increased the dynamic range of the displacement measurement, and improved the stabilization factor in the laser diode feedback interferometer.