General IIR optical notch filter based on Michelson Gires–Tournois interferometer

A general IIR optical notch filter design is presented from a digital filter design perspective for Michelson Gires–Tournois Interferometer structure. Optical notch filter with arbitrary notch frequency, notch point number, and 3 dB rejection bandwidth can be designed easily. According to the spectral requirement of desired notch filter, in frequency domain we firstly calculate the transfer function of desired allpass filter. Then the numbers of reflectors in Gires–Tournois etalon can be determined. We calculate the transfer function of this multi-cavity Gires–Tournois etalon by using Z-transform. By making the transfer function of allpass filter in frequency domain equal to that of the multi-cavity Gires–Tournois etalon, the notch filter can be directly realized. Different design examples are given in detail in the paper. The change of output spectrum is also investigated for the reflectance of the reflectors and the distance between the reflectors deviating from the ideal value. The results show that the notch filter has the tunability of notch frequency and 3 dB rejection bandwidth. The chromatic dispersion characteristic of the notch filter is analyzed finally. It shows that the notch filter has excellent chromatic dispersion characteristic.     

A digital filtering scheme for SQUID based magnetocardiography

Considering the properties of slow change and quasi-periodicity of magnetocardiography (MCG) signal, we use an integrated technique of adaptive and low-pass filtering in dealing with two-channel MCG data measured by high $T_{\rm c}$ SQUIDs. The adaptive filter in the time domain is based on a noise feedback normalized least-mean-square (NLMS) algorithm, and the low-pass filter with a cutoff at 100Hz in the frequency domain characterized by Gaussian functions is combined with a notch at the power line frequency. In this way, both relevant and irrelevant noises in original MCG data are largely eliminated. The method may also be useful for other slowly varying quasi-periodical signals.     

空时多普勒频移域运动小目标的抗干扰探测方法

有源声呐使用多普勒敏感信号可在多普勒频移域分离杂波、混响等干扰和运动目标,但低信干比时强干扰的时间旁瓣泄露和多普勒旁瓣泄露会淹没弱目标.针对该问题,提出了一种基于自适应最小均方(LMS)算法和宽带模糊函数(WAF)的运动小目标抗干扰探测方法.首先在空间多普勒频移域上利用基于LMS的自适应陷波器抑制干扰,然后在快时间多普...     

Importance of temporal-envelope cues in consonant recognition

The role of different modulation frequencies in the speech envelope were studied by means of the manipulation of vowel-consonant-vowel (VCV) syllables. The envelope of the signal was extracted from the speech and the fine-structure was replaced by speech-shaped noise. The temporal envelopes in every critical band of the speech signal were notch filtered in order to assess the relative importance of different modulation frequency regions between 0 and 20 Hz. For this purpose notch filters around three center frequencies (8, 12, and 16 Hz) with three different notch widths (4-, 8-, and 12-Hz wide) were used. These stimuli were used in a consonant-recognition task in which ten normal-hearing subjects participated, and their results were analyzed in terms of recognition scores. More qualitative information was obtained with a multidimensional scaling method (INDSCAL) and sequential information analysis (SINFA). Consonant recognition is very robust for the removal of certain modulation frequency areas. Only when a wide notch around 8 Hz is applied does the speech signal become heavily degraded. As expected, the voicing information is lost, while there are different effects on plosiveness and nasality. Even the smallest filtering has a substantial effect on the transfer of the plosiveness feature, while on the other hand, filtering out only the low-modulation frequencies has a substantial effect on the transfer of nasality cues.     

A method for on-line background subtraction in frequency domain fluorometry

Time-resolved fluorescence determinations on biochemical samples are often complicated by contributions from background. In the time-correlated single-photon counting method, background subtraction is a routine procedure. A limitation of frequency domain fluorometry, however, arises from the difficulty of performing this operation. This limitation has become increasingly significant as frequency domain methods are being applied to evermore complex biological systems using the higher-frequency capabilities of modern instrumentation. We have devised a method for such a correction in the frequency domain, regardless of the complexity of the background decay, based on measurement of the background phasor and subsequent subtraction from the sample phasor. This method is applicable to both lifetime and dynamic polarization measurements, and it can be readily implemented on commercially available frequency domain fluorometers. Decay curves may be accurately recovered from samples containing background contributions ranging from less than 5% to greater than 90% of the total signal intensity.     

Optimal design of engine mount using an artificial life algorithm

When designing fluid mounts, design parameters can be varied in order to obtain a desired notch frequency and notch depth. The notch frequency is a function of the mount parameters and is typically selected by the designer to occur at the vibration disturbance frequency. Since the process of choosing these parameters can involve some trial and error, it seems to be a great application for obtaining optimal performance of the mount. Many combinations of parameters are possible to give us the desired notch frequency, but the question is which combination provides the lowest depth? Therefore, an automatic optimal technique is needed to optimize the fluid mount.In this study, the enhanced artificial life algorithm (EALA) is applied to minimizing transmissibility of a fluid mount at the desired notch frequency, and at the notch and resonant frequencies. The present hybrid algorithm is the synthesis of a conventional artificial life algorithm with the random tabu search (R-tabu) method and then, the time for searching optimal solution could be reduced from the conventional artificial life algorithm and its solution accuracy became better.The results show that the performance of the optimized mount by using the hybrid algorithm has been better than that of the conventional fluid mount.     

Quantitative MRS: comparison of time domain and time domain frequency domain methods using a novel test procedure

For quantitative analysis of in vivo MR spectra, a state-of-the-art time domain method was compared with a recently reported time domain frequency domain method which uses wavelets for background characterization. The comparison was made on the basis of results for simulated test problems that were constructed by combining measured and simulated MRS data at different signal-to-noise ratios in order to simultaneously reflect real world difficulties, in particular the overlapping background problem, and to allow for quantitative judgment of a method's accuracy. Incorporating prior knowledge was also considered. The results obtained give insight into the accuracy of the methods when applied to measured MRS data. Due to the improved background characterization, the time domain frequency domain method outperformed the time domain method in some of the test cases. Both methods were also applied to serial brain MR spectra of a healthy volunteer on 10 occasions.     

天光背景下混浊大气中成像质量的分析方法

调制传递函数(MTF)定量描述混浊介质对图像质量的影响,它是混浊介质的固有光学特性,利用等效原理可以获得MTF从低频到高频的完整特征.在实际应用中,混浊介质中的图像质量不仅取决于介质的MTF,还与图像的背景辐射密切相关.本文从混浊大气中图像退化机理出发,理论分析了空间频域中天光背景下图像质量的退化过程.参考等效原理,提出了一种考虑天光背景的表观MTF,得到了表观MTF与介质MTF和天光背景的定量关系,从而得到了一种分析背景辐射下混浊介质中图像质量的有效便捷方法.针对图像质量优化方法,从空间频域的角度提出了一种评价原则.     

基于滤波差分的双阈值弱划痕提取算法

高质量光学元件表面缺陷中存在一些深度较浅或者宽度较窄的划痕,在暗场成像检测中,该类划痕产生的散射光灰度值很低,甚至淹没在背景光中,很难被目视或常规机器视觉识别,造成划痕缺陷的漏检。针对该问题,以既有的疵病检测系统为基础,根据划痕灰度的等级特征,提出双阈值法分类处理划痕缺陷。在低阈值的弱划痕处理中,根据弱划痕和背景的频率特征以及空间对比度特征,设计了频域滤波及背景差分算法。通过空间域以及频率域的滤波处理,排除高频噪声以及高亮度噪声,根据几何特征等提取弱划痕图像中的复杂背景。经差分处理后,提取弱划痕并增强对比度,最后与正常灰度级划痕信息一同通过高阈值(正常阈值)进行后续划痕的特征提取,即得到所有的划痕信息,为划痕缺陷总长度计算以及最大长度的分级判定奠定基础。实验结果表明,该算法避免了过低二值化阈值引入的背景等不规则噪声,使得划痕与背景的对比度大大增强。目前该算法已经应用于惯性约束聚变系统中大口径光学表面划痕的定量检测,并且使长度计量的准确度已提升到约80%。     

Large RF susceptibility of transverse domain walls

Changes in domain wall resistance under radio-frequency (RF) irradiation are experimentally studied for transverse walls. An original experimental technique is applied to the measurement in a permalloy nano-stripe with a notch, where the walls are found to provide a largely enhanced resistive response as compared to saturated domains. Their susceptibility is found to be an order of magnitude larger than that of the domains in a frequency range between 5 and 20 GHz. We argue that the RF fields induce an internal distortion of the magnetization profile that depends on the shape of the domain wall.     

基于频域纹理消除的结构性纹理缺陷检测方法

针对金属加工表面等结构纹理表面图像缺陷检测问题,结构纹理的存在会对缺陷（比如划痕）检测带来干扰,该文开展在频率域中消除背景纹理的方法来进行缺陷检测的研究。首先基于傅里叶变换的图像复原技术,空间域图像中的结构性纹理对应傅里叶域中高能频率分量,使用最小二乘法直线拟合操作去除,并将这些能量设置为零,经傅里叶逆变换为空间域图像。在复原的图像中,原始图像中的结构纹理区域将变为近似的均匀灰度级,但其中缺陷部分将被保留下来。再使用统计过程控制来设置阈值的方法就能从复原图像中分离出缺陷。最后在一系列的结构性纹理图像上的实验证实了所提方法可行且有效。     

An adaptive filter-based method for robust, automatic detection and frequency estimation of whistles

This paper proposes an adaptive filter-based method for detection and frequency estimation of whistle calls, such as the calls of birds and marine mammals, which are typically analyzed in the time-frequency domain using a spectrogram. The approach taken here is based on adaptive notch filtering, which is an established technique for frequency tracking. For application to automatic whistle processing, methods for detection and improved frequency tracking through frequency crossings as well as interfering transients are developed and coupled to the frequency tracker. Background noise estimation and compensation is accomplished using order statistics and pre-whitening. Using simulated signals as well as recorded calls of marine mammals and a human whistled speech utterance, it is shown that the proposed method can detect more simultaneous whistles than two competing spectrogram-based methods while not reporting any false alarms on the example datasets. In one example, it extracts complete 1.4 and 1.8 s bottlenose dolphin whistles successfully through frequency crossings. The method performs detection and estimates frequency tracks even at high sweep rates. The algorithm is also shown to be effective on human whistled utterances.     

Detection and discrimination of spectral peaks and notches at 1 and 8 kHz

The ability of subjects to detect and discriminate spectral peaks and notches in noise stimuli was determined for center frequencies fc of 1 and 8 kHz. The signals were delivered using an insert earphone designed to produce a flat frequency response at the eardrum for frequencies up to 14 kHz. In experiment I, subjects were required to distinguish a broadband reference noise with a flat spectrum from a noise with either a peak or a notch at fc. The threshold peak height or notch depth was determined as a function of bandwidth of the peak or notch (0.125, 0.25, or 0.5 times fc). Thresholds increased with decreasing bandwidth, particularly for the notches. In experiment II, subjects were required to detect an increase in the height of a spectral peak or a decrease in the depth of a notch as a function of bandwidth. Performance was worse for notches than for peaks, particularly at narrow bandwidths. For both experiments I and II, randomizing (roving) the overall level of the stimuli had little effect at 1 kHz, but tended to impair performance at 8 kHz, particularly for notches. Experiments III-VI measured thresholds for detecting changes in center frequency of sinusoids, bands of noise, and spectral peaks or notches in a broadband background. Thresholds were lowest for the sinusoids and highest for the peaks and notches. The width of the bands, peaks, or notches had only a small effect on thresholds. For the notches at 8 kHz, thresholds for detecting glides in center frequency were lower than thresholds for detecting a difference in center frequency between two steady sounds. Randomizing the overall level of the stimuli made frequency discrimination of the sinusoids worse, but had little or no effect for the noise stimuli. In all six experiments, performance was generally worse at 8 kHz than at 1 kHz. The results are discussed in terms of their implications for the detectability of spectral cues introduced by the pinnae.     

Magnetic soft x-ray microscopy of the domain wall depinning process in permalloy magnetic nanowires

Full-field magnetic transmission x-ray microscopy at high spatial resolution down to 20 nm is used to directly observe field-driven domain wall motion in notch-patterned permalloy nanowires. The depinning process of a domain wall around a notch exhibits a stochastic nature in most nanowires. The stochasticity of the domain wall depinning sensitively depends on the geometry of the nanowire such as the wire thickness, the wire width, and the notch depth. We propose an optimized design of the nanowire for deterministic domain wall depinning field at a notch.     

基于表面阻抗边界条件的等离子体薄涂层电磁散射的时域有限差分分析

基于表面阻抗边界条件时域有限差分(FDTD)方法研究了一维斜入射情况下非磁化等离子体薄涂层涂敷金属材料的电磁散射特性, 该方法忽略对薄层背景材料进行网格剖分, 大大减少了计算量. 首先推导了理想导体涂敷等离子体薄涂层的表面阻抗频域表达式, 然后代入边界条件并变换到时域, 再用分段线性递推卷积方法将时域表达式离散得到FDTD迭代式. 编程计算了垂直及斜入射情形下的平行极化和垂直极化反射系数, 通过验证算例与解析解对比, 结果表明该方法的准确性和有效性. 最后利用该方法分析了不同入射角对反射系数的影响.     

Performance limitation of chirped-fiber-Bragg-grating-based photonic microwave notch filter due to group delay and reflectivity ripples

We investigated the performance limitation of a frequency-tunable photonic microwave notch filter incorporating a chirped fiber Bragg grating (FBG) as a group delay line. In particular, we carried out theoretical and experimental studies to analyze the impact of the reflectivity ripple (RR) and group delay ripple (GDR) in the chirped FBG on the output notch performance. The results of experimental and theoretical analyses performed on a photonic microwave notch filter in which an S-bending-based linearly chirped FBG was incorporated, revealed that the notch depth and notch frequency were significantly affected by imperfections in the chirped FBG. The notch depth was found to be decreased by the RRs, whereas significant fluctuations in the notch frequency were caused by the GDRs. On the basis of the obtained results, we conclude that the RR and the GDR are the fundamental factors that degrade the performance of chirped FBG-based photonic microwave filters.     

A method to evaluate error correction ability of computer controlled optical surfacing process

Quantitatively investigating error correction ability in frequency domain is important for computer controlled optical surfacing (CCOS) process to correct different spatial frequency errors. Based on the mathematical model coherence between filtering and material removal process of CCOS, a method is proposed to quantitatively evaluate the correction ability of CCOS process. A generalized model named normalized smoothing spectral function (SSF) will be established combine convolution model of CCOS and power spectral density (PSD) function. A set of polishing experiments are performed to calculate SSF curves and validate SSF model. By comparing the results of SSF curve with PSD curve and surface figure, it reveals that SSF curve can quantitatively indicate the correction ability of CCOS process for different spatial frequency errors.     

在LiNbO3:Fe晶体中写入Y结孤子的实验研究

采用振幅掩模穿过部分空间非相干光的方法产生一暗迹,将暗迹成像在晶体前表面,得到入射暗迹半高宽为16.75 μm,入射光功率为400 μW.基于LiNbO3:Fe晶体的强光伏效应,在无背景光辐照时,在晶体内写入一对部分空间非相干Y结暗光伏孤子.实验结果表明：在无背景光辐射时可以在LiNbO3:Fe晶体内写入部分空间非相干Y结孤子.     

Baseline subtraction technique in the frequency–wavenumber domain for high sensitivity damage detection

This paper suggests a method for high-sensitivity damage detection. The method is based on pitch-catch measurements of Lamb waves combined with a baseline subtraction technique in the frequency–wavenumber domain. Small amplitude converted modes, generated during the interaction of propagating waves with damage, can thus be detected with minimal a priori information regarding their expected location in the frequency–wavenumber plane. This method is applied in the present paper to a case of notches with varied depth. Finite element simulations are carried out in the temporal domain to mimic results obtainable in real-world experiments. Two cases are studied, namely when each of the two pure fundamental modes are incident on a notch. The advantages of the method are detailed. The procedure to implement this method is described in the context of Structural Health Monitoring (SHM) or Non-Destructive Testing (NDT).