A matched filter technique for Auger depth profile sensitivity enhancement

A matched filter technique is described which provides near-optimal enhancement of the signal-to-noise ratio for Auger depth profiling. This technique provides effective background suppression and is easier to implement than other recently described techniques, but does not deal as efficiently with spectral interferences. A simple BASIC subroutine is sketched and is applied to simulated Auger data. Experimental data from a ramped AlGaAs structure profiled simultaneously by Auger and SIMS (secondary ion mass spectrometry) are also analyzed. As compared with the standard peak-to-peak method, sensitivity is typically increased by a factor of two to four. It is shown that standard smoothing techniques offer comparable, though somewhat inferior improvements in sensitivity. A useful estimate for the maximum obtainable signal-to-noise ratio is also described.     

Word recognition in noise at higher-than-normal levels: decreases in scores and increases in masking

Under certain conditions, speech recognition in noise decreases above conversational levels when signal-to-noise ratio is held constant. The current study was undertaken to determine if nonlinear growth of masking and the subsequent reduction in "effective" signal-to-noise ratio accounts for this decline. Nine young adults with normal hearing listened to monosyllabic words at three levels in each of three levels of a masker shaped to match the speech spectrum. An additional low-level noise equated audibility by producing equivalent masked thresholds for all subjects. If word recognition was determined entirely by signal-to-noise ratio and was independent of overall speech and masker levels, scores at a given signal-to-noise ratio should remain constant with increasing level. Masked pure-tone thresholds measured in the speech-shaped maskers increased linearly with increasing masker level at lower frequencies but nonlinearly at higher frequencies, consistent with nonlinear growth of upward spread of masking that followed the peaks in the spectrum of the speech-shaped masker. Word recognition declined significantly with increasing level when signal-to-noise ratio was held constant which was attributed to nonlinear growth of masking and reduced "effective" signal-to-noise ratio at high speech-shaped masker levels, as indicated by audibility estimates based on the Articulation Index.     

基于激光光解诱导荧光技术实现燃烧反应区及中间产物CH_3瞬态可视化

实现火焰反应区和不同中间组分的在线二维瞬态成像,在湍流燃烧的基础研究中具有十分重要的意义。用Nd∶YAG激光器的5倍频输出(212.8nm)作为光源,通过激光光解诱导荧光技术在甲烷/空气预混火焰中,成功实现了火焰反应区的瞬态成像,并首次采用该技术实现了CH_3的在线瞬态成像测量。分析了该方法同其他荧光标示物在反应区二维瞬态成像方法的优势,并研究了火焰燃烧过程中其他燃烧中间产物和不同燃空比对CH_3单脉冲成像的影响,讨论了现有条件下该技术的应用范围。根据实验结果,在燃空比Φ=1.2的条件下,在反应区我们获得了信噪比约为8的单脉冲成像,分析火焰中CH_3的单脉冲成像结果可知火焰燃空比在1.0~1.4之间时,或者火焰中CH_3的浓度大于9.3×1015 molecules·cm-3信噪比较好。该项技术在动力机械及其他研究领域的应用有十分重要的参考价值。     

Intracranial microvascular imaging at 7 T MRI with transceiver RF coils

Purpose

To investigate intracranial microvascular images with transceiver radio-frequency (RF) coils at ultra-high field 7 T magnetic resonance imaging (MRI).

Materials and methods

We designed several types of RF coils for the study of 7 T magnetic resonance angiography and analyzed quantitatively each coil's performance in terms of the signal-to-noise ratio (SNR) profiles to evaluate the usefulness of RF coils for microvascular imaging applications. We also obtained the microvascular images with different resolutions and parallel imaging technique.

Results

The overlapped 6-channel (ch) transceiver coil exhibited the highest performance for angiographic imaging. Although other multi-channel coils, such as 4- or 8-ch, were also suitable for fast imaging, these coils performed poorly in homogeneity or SNR for angiographic imaging. Furthermore, the 8-ch coil was poor in SNR at the center of the brain, while it had the highest SNR at the periphery.

Conclusion

The present study has demonstrated that the overlapped 6-ch coil with large-size loop coils provided the best performance for microvascular imaging or angiography with the ultra-high-field 7 T MRI, mainly because of its long penetration depth together with high SNR.     

1.55 μm相干激光测风雷达平衡式探测接收实验

研究了用于相干激光测风雷达信号接收的平衡式探测接收技术.推导出了平衡式相干探测过程的数学模型和信噪比表达式,并通过仿真软件对该模型进行了模拟仿真.在同等输入条件下,当平衡式相干探测系统的最佳分束比范围在[0.293,0.707]时,其信噪比比普通相干探测的信噪比高.此外,平衡式探测接收还具有判断风向的功能,最后通过实验验证了仿真结果.     

Recognition of filtered words in noise at higher-than-normal levels: decreases in scores with and without increases in masking

To examine spectral effects on declines in speech recognition in noise at high levels, word recognition for 18 young adults with normal hearing was assessed for low-pass-filtered speech and speech-shaped maskers or high-pass-filtered speech and speech-shaped maskers at three speech levels (70, 77, and 84 dB SPL) for each of three signal-to-noise ratios (+8, +3, and -2 dB). An additional low-level noise produced equivalent masked thresholds for all subjects. Pure-tone thresholds were measured in quiet and in all maskers. If word recognition was determined entirely by signal-to-noise ratio, and was independent of signal levels and the spectral content of speech and maskers, scores should remain constant with increasing level for both low- and high-frequency speech and maskers. Recognition of low-frequency speech in low-frequency maskers and high-frequency speech in high-frequency maskers decreased significantly with increasing speech level when signal-to-noise ratio was held constant. For low-frequency speech and speech-shaped maskers, the decline was attributed to nonlinear growth of masking which reduced the "effective" signal-to-noise ratio at high levels, similar to previous results for broadband speech and speech-shaped maskers. Masking growth and reduced "effective" signal-to-noise ratio accounted for some but not all the decline in recognition of high-frequency speech in high-frequency maskers.     

Improvement of the bit error rate of a non-coherent OCDMA system for FTTH network applications

In this paper, an optical code division multiple access (OCDMA) system was analysed to improve the bit error rate (BER) performance at 10 Gbps. The improvements to this system were based on a modified double weight code and a NAND subtraction technique and aimed to support the large number of active users in the fibre-to-the-home network. The system performance was investigated through extensive theoretical and numerical simulation analyses. The theoretical and simulation results revealed that the new detection technique exhibits improved BER performance compared to conventional techniques, such as complimentary subtraction techniques. The system performance was characterised by the signal-to-noise ratio, the bit error rate (BER), and various transmit powers ( $P_{sr} - 10$  dBm). The results show that the proposed system, which is based on a new detection technique, can achieve the optimal BER with a high number of users and maintains the error floor transmission rate $(\le 10^{-9})$ .     

Perception of aperiodicities in synthetically generated voices

The purpose of this study was to investigate univariate relationships between perceived dysphonia and variation in pitch perturbation, amplitude perturbation, and additive noise. A time-domain, pitch-synchronous synthesis technique was used to generate sustained vowels varying in each of the three acoustic dimensions. A panel of trained listeners provided direct magnitude estimates of roughness in the case of the stimuli varying in pitch and amplitude perturbation, and breathiness in the case of the stimuli varying in additive noise. Very strong relationships were found between perceived roughness and either pitch or amplitude perturbation. However, unlike results reported previously for nonspeech stimuli, the subjective quality associated with pitch perturbation was quite different from that associated with amplitude perturbation. Results also showed that perceived roughness was affected not only by the amount of perturbation, but also by the degree of correlation between adjacent pitch or amplitude values. A strong relationship was found between perceived breathiness and signal-to-noise ratio. Contrary to previous findings, there was no interaction between signal-to-noise ratio and the amount of high-frequency energy in the periodic component of the stimulus: Stimuli with similar signal-to-noise ratios received similar ratings, regardless of differences in the spectral slope of the periodic component.     

Application of magnetic field over-modulation for improved EPR linewidth measurements using probes with Lorentzian lineshape

Magnetic field modulation in CW electron paramagnetic resonance (EPR) is used for signal detection. However, it can also distort signal lineshape. In experiments where the linewidth information is of particular importance, small modulation amplitude is usually used to limit the lineshape distortion. The use of small modulation amplitude, however, results in low signal-to-noise ratio and therefore affects the precision of linewidth measurements. Recently, a new spectral simulation model has been developed enabling accurate fitting of modulation-broadened EPR spectra in liquids. Since the use of large modulation amplitude (over-modulation) can significantly enhance the EPR signal, the precision of linewidth measurements is therefore greatly improved. We investigated the over-modulation technique in EPR oximetry experiments using the oxygen-sensing probe lithium octa-n-butoxy-substitued naphthalocyanine (LiNc-BuO). Modulation amplitudes 2-18 times the intrinsic linewidth of the probe were applied to increase the spectral signal-to-noise ratio. The intrinsic linewidth of the probe at different oxygen concentrations was accurately extracted through curve fitting from the enhanced spectra. Thus, we demonstrated that the over-modulation model is also applicable to particulate oxygen-sensing probes such as LiNc-BuO and that the lineshape broadening induced by oxygen is separable from that induced by over-modulation. Therefore, the over-modulation technique can be used to enhance sensitivity and improve linewidth measurements for EPR oximetry with particulate oxygen-sensing probes with Lorentzian lineshape. It should be particularly useful for in vivo oxygen measurements, in which direct linewidth measurements may not be feasible due to inadequate signal-to-noise ratio.     

On application of adaptive decorrelation filtering to assistive listening

This paper describes an application of the multichannel signal processing technique of adaptive decorrelation filtering to the design of an assistive listening system. A simulated "dinner table" scenario was studied. The speech signal of a desired talker was corrupted by three simultaneous speech jammers and by a speech-shaped diffusive noise. The technique of adaptive decorrelation filtering processing was used to extract the desired speech from the interference speech and noise. The effectiveness of the assistive listening system was evaluated by observing improvements in A-weighted signal-to-noise ratio (SNR) and in sentence intelligibility, where the latter was evaluated in a listening test with eight normal hearing subjects and three subjects with hearing impairments. Significant improvements in SNR and sentence intelligibility were achieved with the use of the assistive listening system. For subjects with normal hearing, the speech reception threshold was improved by 3 to 5 dBA, and for subjects with hearing impairments, the threshold was improved by 4 to 8 dBA.     

基于自适应准则的闪光照相图像重建

 针对闪光照相图像低信噪比的特点,提出了一种自适应的图像重建算法。该算法以代数法(ART)迭代重建为基础,在迭代重建过程中引入了两个权重矩阵,其中一个矩阵用来控制重建过程中的高频分量以保持重建结果的边界;另一个矩阵控制重建结果的平坦区域以提高重建结果的信噪比。通过在迭代过程中更新这两个矩阵来实现图像的自适应重建。数值模拟结果表明,相对于传统的单准则ART重建算法,该方法具有更好的抗噪能力和边界保持能力。     

基于自适应准则的闪光照相图像重建

针对闪光照相图像低信噪比的特点,提出了一种自适应的图像重建算法。该算法以代数法(ART)迭代重建为基础,在迭代重建过程中引入了两个权重矩阵,其中一个矩阵用来控制重建过程中的高频分量以保持重建结果的边界;另一个矩阵控制重建结果的平坦区域以提高重建结果的信噪比。通过在迭代过程中更新这两个矩阵来实现图像的自适应重建。数值模拟结果表明,相对于传统的单准则ART重建算法,该方法具有更好的抗噪能力和边界保持能力。     

Temperature measurement via oh spectrum in a flat C3H8/O2 flame by degenerate four-wave mixing

The degenerate four-wave mixing (DFWM) spectra method was employed to measure the temperature in a premixed propane-oxygen flame via OH radicals. The focus was to study the dependencies of the signal-to-noise ratio and linewidth of DFWM on the crossing angle. It was found that temperature measurements via DFWM could be made with crossing angles up to 13 °. Also, the DFWM signals were detectable for a crossing angle up to 25 ° with acceptable signal-to-noise ratio. The results suggest that it is possible to use DFWM two-dimensional imaging as a realistic diagnostic technique.     

Stochastic phase synchronization in the crayfish mechanoreceptor/photoreceptor system

The two light-sensitive neurons in the crayfish's abdominal sixth ganglion ("caudal photoreceptors," or CPRs), are both primary light sensors and secondary neurons in a mechanosensory pathway. Pei et al. (1996) demonstrated that light enhances the transduction of weak, periodic hydrodynamic stimuli (measured as an increase in the signal-to-noise ratio at the stimulus frequency in the power spectrum of the recorded neural spikes). This has been interpreted as a stochastic resonance effect, in which added light increases the noise intensity of the input to the photoreceptor (possibly through fluctuations in membrane potential), leading to an enhancement of the signal-to-noise ratio (SNR). Here, we discuss the recent demonstration (Bahar et al., 2002) of the correlation between a stochastic-resonance-like effect and an increase in stochastic phase synchronization between the neural response and a periodic mechanical stimulus. We also discuss a novel effect (Bahar et al., 2002) in which light increases the SNR of the second higher harmonic of a periodic input signal, effectively rectifying the input signal. This "second harmonic effect" can also be interpreted in terms of stochastic phase synchronization (Bahar et al., 2002). We review other recent results on the role of stochastic phase synchronization in mediating sensory responses in the crayfish nervous system.     

TNT detection with N NQR: Multipulse sequences and matched filter

Nuclear quadrupole resonance (NQR) has a distinct potential to verify the presence of nitrogen bearing substances based on the unequivocal signatures of their spectra. Therefore, this technique is especially suitable for remote detection of illicit substances and explosives. Unfortunately, the inherent signal-to-noise of the most abundant explosive trinitrotoluene (TNT) is very low. Here we present an NQR method with improved sensitivity for estimation of the probability of TNT presence in the investigated object. The method consists of a spin-lock spin-echo (SLSE) multipulse sequence for signal excitation and a time domain matched filter for signal detection. We find that the signal-to-noise increases by shortening the pulse spacings, even though this means a decrease in spectral resolution. In our case, the decrease of the pulse spacings from the typical to resulted in an increase of the signal-to-noise by . A theory describing this enhancement is presented and compared to experimental results on TNT. Issues related to temperature and polymorphism variations are also discussed.     

Detection of modulated molecular beams with electron bombardment detectors

By means of refinements in the modulated molecular beam technique the signal-to-noise ratio can be greatly improved, and differential cross-sections, for collision of molecuies of the same species, can be measured. This was accomplished by combining beam modulation and phase sensitive detection with very sharp turning on the front end of the lock-in-amplifier and long integration times on the output. In addition, the signal-to-noise ratio of the Ar-Ar system as a function of integration time was investigated using two different types of electron bombardment detectors an Aberth ion-source and a quadrupole mass filter. With an integration time of 40 min the estimated upper limit to the signal-to-noise ratio is 1500 to 1 for the Aberth ion-source. Using quadrupole mass filter with an integration time of 60 min the estimated upper limit to the signal-to-noise ratio is 5 × 10⁴ to 1. For chemical kinetics studies this ratio may be two orders of magnitude higher. Measurements were carried out at the Purdue University, Lafayette, Indiana, USA.     

Correspondence normalized ghost imaging on compressive sensing

Ghost imaging(GI) offers great potential with respect to conventional imaging techniques. It is an open problem in GI systems that a long acquisition time is be required for reconstructing images with good visibility and signal-to-noise ratios(SNRs). In this paper, we propose a new scheme to get good performance with a shorter construction time. We call it correspondence normalized ghost imaging based on compressive sensing(CCNGI). In the scheme, we enhance the signal-to-noise performance by normalizing the reference beam intensity to eliminate the noise caused by laser power fluctuations, and reduce the reconstruction time by using both compressive sensing(CS) and time-correspondence imaging(CI) techniques. It is shown that the qualities of the images have been improved and the reconstruction time has been reduced using CCNGI scheme. For the two-grayscale "double-slit" image, the mean square error(MSE) by GI and the normalized GI(NGI) schemes with the measurement number of 5000 are 0.237 and 0.164, respectively, and that is 0.021by CCNGI scheme with 2500 measurements. For the eight-grayscale "lena" object, the peak signal-to-noise rates(PSNRs)are 10.506 and 13.098, respectively using GI and NGI schemes while the value turns to 16.198 using CCNGI scheme. The results also show that a high-fidelity GI reconstruction has been achieved using only 44% of the number of measurements corresponding to the Nyquist limit for the two-grayscale "double-slit" object. The qualities of the reconstructed images using CCNGI are almost the same as those from GI via sparsity constraints(GISC) with a shorter reconstruction time.     

光锥与CCD耦合效率的理论分析

光锥与CCD的耦合技术是研制ICCD图像传感器的关键技术,本文主要从理论上分析并讨论了光锥与CCD耦合器件的耦合效率及其影响因素,进一步分析讨论了耦合效率对耦合器件信噪比的影响.证明了光锥与CCD的耦合效率不仅降低ICCD探测效率而且降低ICCD信噪比,并提出了提高光锥与CCD耦合效率的方法和途径.     

典型概率密度背景下四阶累积量波束形成的阵增益

在理论上零均值高斯分布噪声的四阶累积量恒等于零,而实际噪声和混响的概率密度是影响四阶累积量波束形成技术性能的关键问题。本文针对海洋环境噪声与海底混响的瞬时幅度分布,分别推导了四阶累积量波束形成阵增益函数;建立了阵增益与阵元数、海洋环境噪声和海底混响统计特性的四阶矩和二阶矩及输入信噪比的关系;确定了与常规波束形成阵增益之间存在临界信噪比,高于临界信干比时四阶累积量波束形成可以获得比常规波束形成更高的阵增益与分辨率。实验数据处理结果验证了四阶累积量波束形成阵增益和分辨率理论结果的一致性。     

Observation of stimulated Brillouin scattering in polymer optical fiber with pump-probe technique

Stimulated Brillouin scattering (SBS) in a perfluorinated graded-index polymer optical fiber (POF) with 120 μm core diameter was experimentally observed for the first time, to the best of our knowledge, at 1.55 μm wavelength with the pump-probe technique. Compared to spontaneous Brillouin scattering previously reported, the Brillouin gain spectrum (BGS) was detected with an extremely high signal-to-noise ratio, even with a short POF (1 m) and scrambled polarization state. We also investigated the BGS dependences on probe power and temperature, which indicate that SBS in a POF measured with this technique can be utilized to develop high-accuracy temperature sensing systems.