Single-shot measurement of revival structures in femtosecond laser-induced alignment of molecules

We present a single-shot detection technique for field-free molecular alignment. The method is based on probing the time-varying birefringence of an aligned sample by use of a chirped probe pulse, thus encoding the dependence of the alignment on time onto the spectral domain. The technique is applied to alignment of O2. The recorded signals are well described by an analytical formula.     

浅海双基地有源探测调频信号直达波消除

双基地有源探测常采用长脉宽探测信号以获取更高的时间增益,容易导致回波弱信号被强直达波信号掩蔽而影响回波检测。针对长调频信号直达波的干扰,提出基于分数阶傅里叶变换(FrFT)的直达波消除方法。该方法利用调频信号在时频分布上的稀疏性,通过对接收信号进行分数阶傅里叶变换分离直达波和回波信号然后在变换域对直达波信号进行去除。远程双基地有源探测的收发距离较远,直达波信号经过长距离传播,浅海多途信道的影响不可忽视,因此通过数值仿真分析了浅海声传播对FrFT消除直达波方法的影响。理论和仿真分析结果表明,FrFT对一定时频分布的调频类信号具有足够的分辨力,可以有效在浅海环境中分离和消除直达波干扰。利用2018年的一次海上试验数据,对基于FrFT的直达波消除方法进行了检验,结果表明该方法可有效消除有源探测中的直达波干扰。该方法计算复杂度低,受浅海频散信道影响小,具有很好的稳健性。     

基于时反算子分解的时反高分辨率定位技术研究

基于时反算子分解技术,提出了主动时间反转选择性定位方法。为提高此定位方法的分辨率,根据时反算子和协方差矩阵的相似性,提出了时反MUSIC和对角线加载时反MVDR定位技术。传统的获取时反算子的方法是通过单个阵元依次发射,该方法的缺点是每次只有单个元发射,导致输入信噪比较小,本文提出通过多个波束正交发射获取时反算子,能得到较大的输入信噪比,从而改善三种定位方法的定位性能。波导实验分别对三种定位方法和正交发射技术进行了验证,结果显示三种定位算法中,子空间基于的时反MUSIC方法具有较好的定位精度,时反MVDR技术具有较高的分辨率。     

基于声发射的光学元件损伤监测方法研究

光学元件损伤是限制激光通量水平提高的重要因素之一。为快速、准确地检测光学元件损伤是否产生,支撑光学元件循环修复策略的使用,研究并提出了基于声发射技术的光学元件损伤检测方法,通过研究光学元件损伤产生的声发射信号特征,判断光学元件是否发生损伤,使用一种基于二次相关和相关峰精确插值（FICP）的时延估计算法,通过仿真验证了该算法的可行性,结合时差定位原理建立了损伤位置求解方法,并通过实验进行了验证。研究结果表明:该方法能从监测信号中快速地获得损伤的位置估计,其平均定位误差为8.61 mm,计算时间为0.143 s/次,对大口径光学元件的损伤在线监测具有应用潜力。     

Laser detection of elastic waves diffraction by the crack’s edge

Diffraction effects and features of acoustic wave propagation in elastic media with the surface-breaking microcrack were investigated in detail for the pulse probing signals. The crack’s plane was oriented along the direction of longitudinal ultrasonic wave incidence in such a way that the detection of the crack with such an unfavorable spatial location was difficult by means of traditional acoustic techniques. Using laser Doppler interferometer a set of instantaneous pictures of acoustic field on the specimen’s surface, corresponding to the different moments of time was obtained. This allowed us to investigate and visualize the diffraction effects of acoustic field in dynamics. Using numerical modeling of diffraction processes of acoustic waves on the crack’s edge and top for pulse signals the origin of V-like structures on the snapshots of acoustic fields was explained and analyzed.     

Measurements of Optical Signals and Their Spectra in the Femtosecond Range with Additional Phase Modulation

A solution to the phase problem in optics is considered within the context of the analysis of signals changing in time and of the effect of an object under investigation on the probing radiation. The solution is based on the use of a twin-wave interferometer with phase modulation in one of the channels and a spectrometer. The analysis is concerned, in particular, with signals and processes of ultrashort duration. To shift the frequency in the interferometer channel, an electrooptical crystal of ferroelectric perovskite is used as the modulator. Errors of amplitude and phase structures determined are analyzed.     

Investigation on the reflection at the boundaries for reconstruction laser-based imaging

Optical tomography is acknowledged as an economic and harmless probing technique for medical applications. Recent research tends to show that the use of long term photons, which have travelled a long time in the whole sample to be probed, carry more information in the image reconstruction process. Numerical simulations based on short-pulsed laser beam interaction with non-homogeneous matter are presented. The aim is to emphasize the effect of reflective boundary conditions since reflection enforces photons to stay for a longer period of time in the phantom. It is found that the quality of reconstruction is better when the boundaries are reflecting.     

Non‐invasive probing of pharmaceutical capsules using transmission Raman spectroscopy

We demonstrate how transmission Raman geometry can be effectively used for non‐invasive probing of the content of pharmaceutical capsules. This approach is particularly beneficial in situations where the conventional Raman backscattering method is hampered or fails because of excessive surface Raman or fluorescence signals emanating from the capsule shell material, which pollute the much weaker subsurface Raman signals with undesired noise. It is demonstrated that such interfering signals can be effectively suppressed by the transmission geometry. The ability to avoid surface fluorescence and Raman signals in conjunction with the superior, bulk‐probing properties of the transmission Raman geometry provides an analytical technique ideally suited for fast on‐line process control monitoring applications in pharmaceutical industry where rapid, chemically specific bulk analysis is required. Copyright © 2007 John Wiley & Sons, Ltd.     

Reconstruction of the shape of echo-sounded objects

The paper considers the problem of reconstructing the shape of echo-sounded objects under conditions when the well-known methods, e.g., the Boyarskii method, are inapplicable or inefficient. A variant of the method is proposed for reconstructing the shape of an object using the function from the Gaussian curvature of the surface. As well, it is shown that the main information on this function is carried by the initial region of the echo signal and that the results of reconstructing the shape depend on the correlation duration of probing signals. When ultra-wideband probing signals are applied, this method is fundamentally suitable for reconstructing the shape of silt-covered objects. Formally, the method is applicable only for convex surfaces, but it can be expanded to more complex surfaces under certain conditions.     

Some effects of optical alignment errors in a differential laser Doppler velocimeter

A formulation has been developed to treat the effects of alignment errors of the optical system used in a differential laser Doppler velocimeter. It is then applied to analyse the properties of output beat signals obtained by the velocimeter. The effects of optical alignment errors on the spectral width and signal-to-noise ratio of the output beat signals and on the probing volume are investigated in some detail. The spectral width is not affected very much by the deviation of the angle between two beams incident on the probing volume, but depends mainly on the number of interference fringes produced in that volume. The signal-to-noise ratio is very sensitive to the displacement of two incident beams on the probing volume and is also affected by the intensity and beam width differences of these two beams. The probing volume is evaluated in the case of two incident beams having different beam widths.     

Double Electron–Electron Resonance Between Trapped Electron and Hole in a Semiconductor

We demonstrate the spin interactions between dispersedly trapped electrons and holes in a semiconductor using the double electron–electron resonance (DEER) method of the pulsed electron paramagnetic resonance (EPR) techniques. An aluminum-doped titanium dioxide crystal is adopted as a spin system, in which optically generated electrons and holes are trapped, to reveal EPR signals that appear close to each other at a selected crystal orientation under an external magnetic field. We used the four-pulse DEER method by applying two microwave frequencies to a microwave cavity for pumping electrons and probing holes at the optimum temperature of 32 K. The dipolar modulation in the probed signal by pumping interacting spins was successfully detected. The observed non-oscillating decay shape indicates that the detected interaction is caused by widely distributed trapped electron and hole spins over long distances. We were able to extract a spin-pair distribution function by the first derivative of a background-corrected curve, referring to a previously reported method.     

A method for retrieval of the profiles of atmospheric backscattering coefficients by three-wavelength lidar and nephelometric measurements

A method for retrieval of the profiles of atmospheric aerosol backscattering coefficients β _a along horizontal and inclined paths from the results of three-wavelength probing by laser location systems based on a Nd: YAG laser is proposed. The method uses a power dependence between the spectral values of β _a and includes spectral nephelometric measurements of the scattering coefficients by an angle of 33°, which characterize the relative spectral dependence of aerosol extinction. The sensitivity of this method to spatial variations in the above coefficients, to the deviation of β _a from the dependence used, and to errors in calibration and measurement of signals is studied. The method is demonstrated to be stable both upon using the absolute calibration of the lidar and at an arbitrary position of the reference point.     

Abilities of nonlinear acoustic methods in locating gas bubbles in biological tissues

On the basis of a numerical solution to the equation of radial oscillations of a gas bubble in a liquid, the abilities of nonlinear acoustic methods in detecting gas bubbles in biological tissues with the use of pulsed acoustic location are analyzed. It is demonstrated that, with moderate amplitudes of the probing signal, it is possible to detect bubbles whose size is close to resonance. An improved version of the method of paired pulses is proposed, along with a method for detecting the bubbles and measuring their dimensions on the basis of a nonlinear excitation of their natural oscillations.     

Seed‐mediated growth method for electroless deposition of AgNPs on glass substrates for use in SERS measurements

In this work, an electroless deposition method for silver nanoparticles (AgNPs) on glass substrates was developed for use in surface‐enhanced Raman scattering (SERS) measurements. To obtain evenly distributed AgNPs of suitable size on the glass substrates, a seeding procedure was utilized as a pretreatment before the electroless deposition of AgNPs. The AgNPs thus formed were affected by both the seeding and growing procedures. To optimize the procedures for preparation of SERS substrates, several factors, including reaction time, the concentration of silver ions, and the concentration of reducing agents (glucose) for seeding and growing procedures, were varied. The morphologies of the seeds and the resulting AgNPs on the glass substrates were characterized by field‐emission scanning electron microscopy (FE‐SEM) and correlated with the SERS signals from probing with para‐nitrothiophenol (pNTP). The results indicated that only the seeding time and the concentration of silver ions significantly influenced the distribution and sizes of the Ag seeds on the substrates. In the growing procedures, both the concentration of silver nitrate and the reducing agent affected the morphologies of the resulting AgNPs and, hence, the SERS signals. The substrates prepared using this newly developed method offer 2–5 times improvement of the SERS signals compared to substrates prepared without seed treatment. Also, the AgNPs prepared by this method can be easily controlled to designated sizes with even spatial and size distributions. Copyright © 2009 John Wiley & Sons, Ltd.     

An investigation of artificial ionospheric turbulence by the method of probing radio wave pulses

The experimental results of a study of artificial ionospheric turbulence by the method of sounding by short powerful pulses are presented. The characteristics of three types of signals scattered by artificial ionospheric turbulence are considered: time dependences and dependences on the heating power, the probing frequency, and the virtual height of scattering.Published from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 39, No. 2, pp. 203–209, February, 1996.     

Reversal of waves in forward-scattering location

Potentialities of the pulsed acoustic forward-scattering location with the use of time separation of the intense direct and weak scattered signals propagating at a small angle to one another are considered. Two types of interference are taken into account: nonstationary observation conditions under the effect of currents, which affect the arrival times of the direct and scattered signals, and nonstationary reverberation. It is shown that, in the presence of these interferences, the application of the correlation transformation equivalent to the time reversal of waves has an advantage over the conventional location technique based on the determination of the signal travel times.     

Non-destructive testing of tubes using a time reverse numerical simulation (TRNS) method

A method for the detection of defects in cylindrical structures and the determination of their positions and orientations is presented in this paper. The scattered field, which is generated by the interaction of excited guided waves with a defect, is evaluated with an approach named time reverse numerical simulation method (TRNS). Since the excited waves and the scattered field propagate along the sample, the time-consuming scanning of the whole tube can be eliminated. The scattered displacement field is measured in three dimensions over time with a laser vibrometer at different locations distributed equally around the circumference at a fixed axial coordinate far away from the defect. Instead of analyzing the complicated time signals directly, they are played back in time. If the recorded displacement histories of the scattered field are reversed in time and played back in an identical structure, the waves travel back the same path and interfere to a maximum at their origin. The result is an amplitude increase at the position of the defect where the scattered field was generated. Instead of playing back the recorded time signals in an experiment, this step is replaced by a numerical simulation. Only this enables the visualization and detection of the amplitude increase. As long as the simulation is of high accuracy, the position of the maximum interference corresponds exactly to the location of the defect in the experiment, although no defect is implemented in the simulation.     

Detection and Location of a Target in Layered Media by Snapshot Time Reversal and Reverse Time Migration Mixed Method

A target in layered medium can be located by the ridge-like distribution time reversal and reverse time migration(TR-RTM) mixed method. However, this method cannot distinguish between acoustic field distributions of the interface and target for the wider acoustic pulse signals, which may result in inaccurate location of the target. A snapshot TR-RTM mixed method is proposed to solve this problem. The principle of snapshot TR-RTM mixed method is first given. Experiments are then carried out, and a mountain-like acoustic field distribution is obtained by processing experimental data. The results show that the location of the peak is that of the target, and the ratio of the scattered signal and interface reflection signal(signal-to-interference ratio) is improved by about four times after processing. Furthermore, this method can effectively suppress the interface reflection signal and enhance the target scattering signal. Therefore, it can achieve effective detection and location of a target in a layered medium.