大口径光学元件中频波前的检测

大口径光学元件中频波前的准确评价已成为高功率激光系统中关注的焦点,元件中频波前均方根值是重要评价指标之一。根据波前中频检测频段及波前检测设备频响特性,将波前的中频区域分为两个检测频段,分别采用干涉仪和光学轮廓仪实现了中频波前均方根值的检测。采用大口径干涉仪可实现全口径波前中频区域低频段波前的检测,通过比对大口径干涉仪和采用小口径干涉仪结合分块融合平均方法的检测结果,提出采用分块融合平均方法也可检测相应频段全口径波前均方根。采用光学轮廓仪通过离散采样的方法检测大口径元件中频区域高频段波前均方根,针对不同离散采样方式的实验结果表明：33的采样方式能满足对410 mm410 mm口径元件中频区域高频段波前均方根的检测。     

A hybrid approach for predicting the distribution of vibro-acoustic energy in complex built-up structures

Finding the distribution of vibro-acoustic energy in complex built-up structures in the mid-to-high frequency regime is a difficult task. In particular, structures with large variation of local wavelengths and/or characteristic scales pose a challenge referred to as the mid-frequency problem. Standard numerical methods such as the finite element method (FEM) scale with the local wavelength and quickly become too large even for modern computer architectures. High frequency techniques, such as statistical energy analysis (SEA), often miss important information such as dominant resonance behavior due to stiff or small scale parts of the structure. Hybrid methods circumvent this problem by coupling FEM/BEM and SEA models in a given built-up structure. In the approach adopted here, the whole system is split into a number of subsystems that are treated by either FEM or SEA depending on the local wavelength. Subsystems with relative long wavelengths are modeled using FEM. Making a diffuse field assumption for the wave fields in the short wave length components, the coupling between subsystems can be reduced to a weighted random field correlation function. The approach presented results in an SEA-like set of linear equations that can be solved for the mean energies in the short wavelength subsystems.     

A combined parabolic-integral equation approach to the acoustic simulation of vibro-acoustic imaging

This paper aims to model ultrasound vibro-acoustography to improve our understanding of the underlying physics of the technique thus facilitating the collection of better images. Ultrasound vibro-acoustography is a novel imaging technique combining the resolution of high-frequency imaging with the clean (speckle-free) images obtained with lower frequency techniques. The challenge in modeling such an experiment is in the variety of scales important to the final image. In contrast to other approaches for modeling such problems, we break the experiment into three parts: high-frequency propagation, non-linear interaction and the propagation of the low-frequency acoustic emission. We then apply different modeling strategies to each part. For the high-frequency propagation we choose a parabolic approximation as the field has a strong preferred direction and small propagation angles. The non-linear interaction is calculated directly with Fourier methods for computing derivatives. Because of the low-frequency omnidirectional nature of the acoustic emission field and the piecewise constant medium we model the low-frequency field with a surface integral approach. We use our model to compare with experimental data and to visualize the relevant fields at points in the experiment where laboratory data is difficult to collect, in particular the source of the low-frequency field. To simulate experimental conditions we perform the simulations with the two frequencies 3 and 3.05 MHz with an inclusion of varying velocity submerged in water.     

Pyrotechnic shock response predictions combining statistical energy analysis and local random phase reconstruction

Numerous pyrotechnic devices are used on satellites to separate structural subsystems, deploy appendages, and activate on-board operating subsystems. The firing of these pyrotechnic mechanisms leads to severe impulsive loads which could sometimes lead to failures in electronic systems. The objective of the present investigation is to assess the relevance of a method combining deterministic calculations and statistical energy analysis to predict the time overall shock environment of electronic equipment components. The methods are applied to the low- and high-frequency ranges, respectively, which may be defined using a modal parameter based on the effective transmissibility. Initially, in order to address the problem of the low-frequency content of the mechanical shock pulse, the linear dynamic response of the equipment was calculated using direct time integration of a finite element model of the structure. The inputs in the form of the accelerations measured in all three directions at each of the four bolted interfaces were injected into the model. The high-frequency content of the shock response is taken into account by considering the intrinsic dynamic filtering of the equipment. This frequency filter magnitude is extrapolated from the transfer function given by statistical energy analysis between the different imposed accelerations and the response accelerations. Their associated phases are synthesized by considering pseudo-modal phase variations around the group velocity of the structural flexural waves. Combining the effects of the high-frequency filter outputs and the low-frequency finite element calculations yields good predictions of the equipment shock time response over the whole frequency range of interest.     

Mode-selective amplification in a waveguide free electron laserwith a two-sectioned wiggler

One important issue in waveguide free electron lasers (FELs) involves an interaction of the electron beam with one waveguide mode at two different resonant frequencies. Since the low-frequency mode often has a higher pain, the usually preferred high-frequency mode is suppressed as a result of mode competition. In this paper, possible control of this mode competition is considered using a nonstandard wiggler magnet consisting of two cascaded wiggler sections with different periods and field strengths. It is demonstrated that with an appropriate differentiation between the two wiggler sections the high-frequency mode may be amplified preferentially. This mode-selective amplification may be used to suppress the low-frequency mode. A small signal gain formulation is developed for a waveguide FEL with such a two-sectioned wiggler arrangement and numerical examples are used to demonstrate its applicability to mode control in waveguide FELs. Effects of wiggler field errors and electron energy spread are also considered. It is shown that the requirement for wiggler field errors and electron energy spread in the two-sectioned wiggler arrangement is similar to that in the usual straight wiggler configuration     

A hybrid finite element–energy finite element method for mid-frequency vibrations of built-up structures under multi-distributed loadings

In this study, a hybrid method combining finite element analysis and energy finite element analysis is developed to predict the vibrations of built-up structures in mid-frequency, and the associated general formulations are derived. The interactions between the structural components are modeled using a global matrix of the system and the reverberant blocked force. To validate the proposed method, three examples of different built-up structural systems, subjected to different types of excitations, are analyzed and discussed. These types of excitations are single point, multipoint and distributed forces on stiff member or flexible member. The results predicted by the presented method show good agreements with the dense finite element model, and the detailed local energies of the whole system are acquired under the different regional loadings. These results indicate that the proposed method can be utilized for the prediction of vibrations in the mid-frequency range.     

HIGH-FREQUENCY RADIATION OF L-SHAPED PLATES BY A LOCAL ENERGY FLOW APPROACH

This paper describes a local energy approach to predict the high-frequency radiation of structures. This general approach is applied to a two-dimensional problem of the radiation of two joined semi-infinite, baffled plates, below their critical frequency. The first step consists in determining how energy travels through subsystems and, for this purpose, elementary waves are studied. Then some canonical problems are solved to describe the behaviour of waves impinging on boundaries. Below the critical frequency, flexural waves are subsonic and their radiation is due to diffraction phenomena. According to the locality principle, the corresponding canonical solutions are used in the whole system resolution; power sources are introduced on boundaries to account for energy exchanges and, with the assumption that waves are uncorrelated, the energy contributions of all sources are simply summed.     

Numerical and experimental validation of a hybrid finite element-statistical energy analysis method

The finite element (FE) and statistical energy analysis (SEA) methods have, respectively, high and low frequency limitations and there is therefore a broad class of "mid-frequency" vibro-acoustic problems that are not suited to either FE or SEA. A hybrid method combining FE and SEA was recently presented for predicting the steady-state response of vibro-acoustic systems with uncertain properties. The subsystems with long wavelength behavior are modeled deterministically with FE, while the subsystems with short wavelength behavior are modeled statistically with SEA. The method yields the ensemble average response of the system where the uncertainty is confined in the SEA subsystems. This paper briefly summarizes the theory behind the method and presents a number of detailed numerical and experimental validation examples for structure-borne noise transmission.     

Fringe pattern denoising via image decomposition

Filtering off noise from a fringe pattern is one of the key tasks in optical interferometry. In this Letter, using some suitable function spaces to model different components of a fringe pattern, we propose a new fringe pattern denoising method based on image decomposition. In our method, a fringe image is divided into three parts: low-frequency fringe, high-frequency fringe, and noise, which are processed in different spaces. An adaptive threshold in wavelet shrinkage involved in this algorithm improves its denoising performance. Simulation and experimental results show that our algorithm obtains smooth and clean fringes with different frequencies while preserving fringe features effectively.     

Research on improving the transverse resolution of binary optical element by digital-division-mask technique

We present the digital-division-mask technique for the first time to solve the problem of decline in transverse resolution, which is caused by using digital micro-mirror device (DMD) to make binary optical elements (BOEs). One high-frequency gray-tone mask can be divided into several low-frequency masks by fixed or variable low-frequency period sampling. And the superimposed lithography effect of these low-frequency masks in the spatial or temporal domain is the same as that of the original high-frequency gray-tone mask. The paper firstly analyzes the digital-division-mask technique in theory and describes the feasibility and advantages. Then taking the diffractive surface fabrication of refractive-diffractive hybrid lens as an example, we conclude that the digital-division-mask technique improves the edge sharpness of lithography pattern, and enhances the diffractive efficiency of BOEs by experiment.     

船尾伴流场-导管-螺旋桨互作用噪声预报研究

研究了导管螺旋桨低频离散谱噪声辐射机理和预报方法。依据线性声学原理,导管螺旋桨噪声场为螺旋桨直接辐射噪声与导管散射噪声之和,并利用速度势面元法分析流场,得到导管螺旋桨非定常力,将其作为FW-H方程的源项,求解得到螺旋桨直接辐射声。导管散射声通过Kirchoff积分方程求解获得。由于导管桨的导管是短导管,其算例分析计算表明,低频情况下导管散射声级远小于螺旋桨直接辐射声级。并将导管螺旋桨离散谱噪声级与测量所得的实桨离散谱噪声级进行了比较,证实导管螺旋桨离散谱噪声理论预报结果能够较合理的反映实桨离散谱噪声的量值。      

Surface and subsurface stress evaluation in case-carburised steel using high and low frequency magnetic barkhausen emission measurements

The effect of near-surface and subsurface stresses on the magnetic Barkhausen emission (MBE) profile has been studied in case-carburised and tempered En36 steel. The high- and low-frequency MBE measurements were made, on both tensile and compressive sides of the rectangular bar specimens with case-depth of 0.95 mm loaded in cantilever beam, under different stress levels as well as after unloading from different higher stress levels. The high-frequency MBE profile showed a single peak while the low-frequency MBE profile showed two peaks. Under applied elastic stresses, both types of measurement showed increase in MBE under tension and decrease in MBE under compression. But, the MBE profiles measured after unloading from higher stresses showed different behaviour. On the tensile side, the high-frequency MBE profile did not change significantly due to pre-stress. But, in the low-frequency MBE profile, the first peak increases and the second peak decreases with increase in pre-stress. On the compressive side, the peak height of high-frequency MBE profile decreased gradually with increase in pre-stress. The first peak of the low-frequency MBE profile also decreased gradually with increase in pres-stress level. But, the second peak of the low-frequency MBE profile decreased by about 10% at a pre-stress level of −1094 MPa and remained more or less the same even after unloading from −1783 MPa. The MBE behaviour has been correlated to the residual stress (RS)-depth profile measured using X-ray diffraction method. This correlation clearly indicates that the high-frequency MBE reflects only the changes in surface RS level. It does not indicate RS changes occurring at depths >20 μm below the surface. The low-frequency MBE profile reflects the changes in the RS distribution occurring in the near-surface as well as deeper subsurface layers.     

An infrared and visible image fusion method based upon multi-scale and top-hat transforms

The high-frequency components in the traditional multi-scale transform method are approximately sparse, which can represent different information of the details. But in the low-frequency component, the coefficients around the zero value are very few, so we cannot sparsely represent low-frequency image information. The low-frequency component contains the main energy of the image and depicts the profile of the image. Direct fusion of the low-frequency component will not be conducive to obtain highly accurate fusion result. Therefore, this paper presents an infrared and visible image fusion method combining the multi-scale and top-hat transforms. On one hand, the new top-hat-transform can effectively extract the salient features of the low-frequency component. On the other hand, the multi-scale transform can extract highfrequency detailed information in multiple scales and from diverse directions. The combination of the two methods is conducive to the acquisition of more characteristics and more accurate fusion results. Among them, for the low-frequency component, a new type of top-hat transform is used to extract low-frequency features, and then different fusion rules are applied to fuse the low-frequency features and low-frequency background; for high-frequency components, the product of characteristics method is used to integrate the detailed information in high-frequency. Experimental results show that the proposed algorithm can obtain more detailed information and clearer infrared target fusion results than the traditional multiscale transform methods. Compared with the state-of-the-art fusion methods based on sparse representation, the proposed algorithm is simple and efficacious, and the time consumption is significantly reduced.     

An adaptive smooth unsaturated bistable stochastic resonance system and its application in rolling bearing fault diagnosis

An adaptive smooth unsaturated bistable stochastic resonance (ASUBSR) system for bearing fault signal detection is established. Based on the problem of output saturation and poor low-frequency suppression performance of classical bistable stochastic resonance (CBSR) system, an SUBSR with unsaturated characteristics is proposed. An ASUBSR system is designed by extracting the envelope spectrum of the input signal and resampling it to satisfy the adiabatic approximation condition, combining high-pass filter to filter out low-frequency interference, and using genetic algorithm to select the optimal system parameters. Through simulations and experiments, we found that the system can effectively suppress the interference of low-frequency and high-frequency, indicates that the system performs like a band-pass filter, and the output signal-to-noise ratio is better than that of the CBSR system. The proposed ASUBSR system has great application in the field of fault detection of rolling bearings.     

The flute instability of a plasma driven by the electrostatic high-frequency eigenmodes

The dispersion equation of the low-freguency flute instability driven by the high-frequency electrostatic eigenmode excited in a plasma slab is studied. If the amplitude of h.f. mode is small the equation is solved and the explicit formula for the maximum growth rate is obtained. As an example we investigate this problem numerically for a slab with the parabolic density profile. In our model we are able to consider the heating only in the frequency band stretching from the lower hybrid frequency to the electron cyclotron frequency. For this case it follows from our calculation that the danger of the low-frequency disruption of a plasma in the process ofheating is serious.     

Drift Lagrangian for a relativistic particle in an intense laser field

The Lagrangian and Hamiltonian functions describing the average motion of a relativistic particle under the action of a slightly inhomogeneous intense laser field are obtained. In weak low-frequency background fields, such a particle on average drifts with an effective relativistically invariant mass, which depends on the laser intensity. The essence of the proposed ponderomotive formulation is presented in a physically intuitive and mathematically simple form yet represents a powerful tool for studying various nonlinear phenomena caused by the interaction of currently available smooth ultraintense laser pulses with plasmas.     

基于多尺度形态学滤波器的图像融合新方法

提出了一种保持图像细节和高抗噪性的图像融合新方法。这种方法首先对源图像进行多尺度形态学开闭滤波,得到源图像的低频平滑图像;然后应用多尺度Top-hat变换和Bottom-hat变换来提取小于相应尺度的图像细节特征。因为在较小的尺度特征中包含噪声颗粒的可能性较大,据此修正了Top-hat变换和Bottom-hat变换的相应系数;最后对以上两步骤得到的低频平滑图像和多尺度高频细节图像分别进行图像融合,应用形态学重建过程生成融合图像。实验表明,这种融合方法具有图像细节保持完整和噪声消除效果好的优点,处理效果优于传统的图像融合方法。     

A basic hybrid finite element formulation for mid-frequency analysis of beams connected at an arbitrary angle

When beams are connected at an arbitrary angle and subjected to an external excitation, both longitudinal and bending waves are generated in the system. Since longitudinal wavelengths are considerably longer than bending wavelengths in the mid-frequency region, the number of bending wavelengths in the beams is considerably larger than the number of longitudinal wavelengths. In this paper, plannar beams connected at arbitrary angles are considered. The energy finite element analysis (EFEA) is employed for modelling the bending behavior of the beams and the conventional finite element analysis (FEA) is utilized for modelling the longitudinal vibration in the beams. Thus, a basic hybrid FEA formulation is presented for mid-frequency analysis of systems that contain two types of energy. The bending vibration is associated with the long members in the system and the longitudinal vibration is associated with the short members. The long members are considered to have high modal overlap and to contain several wavelengths within their dimension, and uncertainty effects are present. The short members contain a small number of wavelengths, and exhibit a low modal overlap. Due to the low modal overlap the resonant frequencies are spaced far apart in the frequency domain, therefore the short members exhibit resonant or non-resonant behavior depending on the frequency of the excitation.In this work, the bending and the longitudinal vibration within the same beam member are treated as a long and as a short member, respectively. A hybrid joint formulation is developed between long and short members. Power reflection and transmission coefficients are derived for each joint. The distribution of the energy throughout the system demonstrates a strong dependency on the power transfer coefficients. Several systems are analyzed by the hybrid FEA and by analytical solutions, and good correlation between them is observed.     

Vibro-acoustic analysis of complex systems

A general method is presented for predicting the ensemble average steady-state response of complex vibro-acoustic systems that contain subsystems with uncertain, or random, properties. The method combines deterministic and statistical techniques to produce a non-iterative hybrid method that incorporates equations of dynamic equilibrium and power balance. The method is derived explicitly without reference to statistical energy analysis (SEA); however, it is seen that the wave approach to SEA can be viewed as a special case of the proposed method. The proposed method provides a flexible way to account for necessary deterministic details in a vibro-acoustic analysis without requiring that an entire system be modeled deterministically. The method therefore provides a potential solution to the mid-frequency problem (in which a system is neither entirely deterministic nor entirely statistical). The application of the method is illustrated with a numerical validation example.     

Development of an optimised, standard-compliant procedure to calculate sound transmission loss: design of transmission rooms

A numerical procedure to estimate the transmission loss of sound insulating structures is proposed based upon the technology of acoustic measurements and standards. A virtual laboratory (VL), namely, a numerical representation of a real laboratory consisting of two reverberation rooms meeting certain sound field quality criteria is designed. VL is to be used for the numerical simulation of standardised measurements under predefined, controlled, acoustic conditions. In this paper, the design and optimisation of VL is investigated. The geometry of the transmission rooms is designed following first principles, in order for diffuse field conditions and sufficiently smooth primary mode distribution in the low frequency to be achieved. A finite element-based optimisation procedure, introduced by the author in previous work, is extended to arbitrarily shaped rooms. It is used to predict the appropriate local geometric modifications so as for improved mode distribution and smoother sound pressure fluctuations of the transmission rooms in the low-frequency range to be achieved and low-frequency measurement reproducibility and accuracy to be increased. Steady-state acoustic response analysis is performed in order to quantify the acoustic field quality of the virtual transmission rooms in the frequency range of measurements. A method to calculate the total absorption, A, of the receiving room is introduced by simulation of the reverberation time measurement procedure using Transient acoustic response analysis. The acoustic performance of VL is overall considered and is shown to meet in a sufficient degree, relative laboratory measurement standards in the frequency range of 100÷704 Hz.