盲波束形成算法仿真及水下试验

盲波束形成技术是针对通信信号的统计性质和确定性性质所构成的一类新的波束形成技术.文中首先对高阶累积量法、基于累积量和特征空间的方法以及最陡下降恒模算法这三种盲波束形成算法进行了详细阐述,然后运用这些算法对单信号源均匀线阵、双信号源均匀线阵、单信号源随机扰动阵和双信号源随机扰动阵四种情况进行仿真.结果表明,三种算法可以进行有效的波束形成,基于累积量和特征空间的方法具有最高的输出信噪比.此外,还通过处理实际水下试验数据验证了这三种算法在实际应用中的有效性.     

Straightness measurements with a reflection confocal optical system-an experimental study

Straightness measurement is an important technique in the field of mechanical engineering. We previously proposed a novel optical method for measuring straightness of motion using reflection confocal optics. The advantage of this method in comparison with the transmission optical systems of others [Opt. Laser Technol. 6, 166 (1974)] is that the lateral displacements in the two axes perpendicular to the optical axis and the rotation angles around all three axes can be measured simultaneously. We demonstrate straightness measurements using reflection confocal optics and show these measurements to be in good agreement with the theory.     

High bandwidth underwater optical communication

We report error-free underwater optical transmission measurements at 1 Gbit/s (10(9) bits/s) over a 2 m path in a laboratory water pipe with up to 36 dB of extinction. The source at 532 nm was derived from a 1064 nm continuous-wave laser diode that was intensity modulated, amplified, and frequency doubled in periodically poled lithium niobate. Measurements were made over a range of extinction by the addition of a Mg(OH)(2) and Al(OH)(3) suspension to the water path, and we were not able to observe any evidence of temporal pulse broadening. Results of Monte Carlo simulations over ocean water paths of several tens of meters indicate that optical communication data rates >1 Gbit/s can be supported and are compatible with high-capacity data transfer applications that require no physical contact.     

Crack determination from boundary measurements—Reconstruction using experimental data

In this work we assess the effectiveness of Electrical Impedance Tomography for determining the presence and the location of an interior crack from boundary measurements. Electrical Impedance Tomography uses boundary voltages and currents to image the interior of a region. We collect the data needed for this nondestructive evaluation technique by laboratory experiments and apply two numerical inversion algorithms to the data. Our experiments show that the data collected are sufficient to give good estimates of crack locations and crack sizes.     

Thin-film optical function acquisition from experimental measurements of the reflectance and transmittance spectra: a case study

Journal of Materials Science: Materials in Electronics - The determination of the spectral dependence of thin-film optical functions from experimental measurements of the optical response,...     

On the optical theory of underwater vision in humans

Defocus changes the visual contrast sensitivity function, thereby creating a complex curve with local dips and peaks. Since underwater vision in humans is severely defocused, we used optical theory and the phenomenon of spurious resolution to predict how well humans can see in this environment. The values obtained correspond well with experimental measurements of underwater human acuity from earlier studies and even point to an opportunity for humans with exceptional contrast sensitivity to see better underwater than the children in those studies. The same theory could be useful when discussing the visual acuity of amphibious animals, as they may use pupil constriction as a means of improving underwater vision.     

Parity measurements in quantum optical metrology

We investigate the utility of parity detection to achieve Heisenberg-limited phase estimation for optical interferometry. We consider the parity detection with several input states that have been shown to exhibit sub-shot-noise interferometry with their respective detection schemes. We show that with parity detection, all these states achieve the sub-shot-noise limited phase uncertainty. Thus making the parity detection a unified detection strategy for quantum optical metrology. We also consider quantum states that are a combination of a NOON state and a dual-Fock state, which gives a great deal of freedom in the preparation of the input state, and is found to surpass the shot-noise limit.     

Non-Markovianity: initial correlations and nonlinear optical measurements

By extending the response function approach developed in nonlinear optics, we analytically derive an expression for the non-Markovianity in the time evolution of a system in contact with a quantum mechanical bath, and find a close connection with the directly observable nonlinear optical response. The result indicates that memory in the bath-induced fluctuations rather than in the dissipation causes non-Markovianity. Initial correlations between states of the system and the bath are shown to be essential for a correct understanding of the non-Markovianity. These correlations are included in our treatment through a preparation function.     

Chemometric analysis of frequency-domain photon migration data: quantitative measurements of optical properties and chromophore concentrations in multicomponent turbid media

Frequency-domain photon migration (FDPM) is a widely used technique for measuring the optical properties (i.e., absorption, micro(a), and reduced scattering, micro(s)', coefficients) of turbid samples. Typically, FDPM data analysis is performed with models based on a photon diffusion equation; however, analytical solutions are difficult to obtain for many realistic geometries. Here, we describe the use of models based instead on representative samples and multivariate calibration (chemometrics). FDPM data at seven wavelengths (ranging from 674 to 956 nm) and multiple modulation frequencies (ranging from 50 to 600 MHz) were gathered from turbid samples containing mixtures of three absorbing dyes. Values for micro(a) and micro(s)' were extracted from the FDPM data in different ways, first with the diffusion theory and then with the chemometric technique of partial least squares. Dye concentrations were determined from the FDPM data by three methods, first by least-squares fits to the diffusion results and then by two chemometric approaches. The accuracy of the chemometric predictions was comparable or superior for all three dyes. Our results indicate that chemometrics can recover optical properties and dye concentrations from the frequency-dependent behavior of photon density waves, without the need for diffusion-based models. Future applications to more complicated geometries, lower-scattering samples, and simpler FDPM instrumentation are discussed.     

Stochastic data assimilation of the random shallow water model loads with uncertain experimental measurements

This paper is concerned with the estimation of a parametric probabilistic model of the random displacement source field at the origin of seaquakes in a given region. The observation of the physical effects induced by statistically independent realizations of the seaquake random process is inherent with uncertainty in the measurements and a stochastic inverse method is proposed to identify each realization of the source field. A statistical reduction is performed to drastically lower the dimension of the space in which the random field is sought and one is left with a random vector to identify. An approximation of the vector components is determined using a polynomial chaos decomposition, solution of an optimality system to identify an optimal representation. A second order gradient-based optimization technique is used to efficiently estimate this statistical representation of the unknown source while accounting for the non-linear constraints in the model parameters. This methodology allows the uncertainty associated with the estimates to be quantified and avoids the need for repeatedly solving the forward model.     

Parametric uncertainty in nanoscale optical dimensional measurements

Image modeling establishes the relation between an object and its image when an optical microscope is used to measure the dimensions of an object of size comparable to the illumination wavelength. It accounts for the influence of all of the parameters that can affect the image and relates the apparent feature width (FW) in the image to the true FW of the object. The values of these parameters, however, have uncertainties, and these uncertainties propagate through the model and lead to parametric uncertainty in the FW measurement, a key component of the combined measurement uncertainty. The combined uncertainty is required in order to decide if the result is adequate for its intended purpose and to ascertain if it is consistent with other results. The parametric uncertainty for optical photomask measurements derived using an edge intensity threshold approach has been described previously; this paper describes an image library approach to this issue and shows results for optical photomask metrology over a FW range of 10 nm to 8 μm using light of wavelength 365 nm. The principles will be described; a one-dimensional image library will be used; the method of comparing images, along with a simple interpolation method, will be explained; and results will be presented. This method is easily extended to any kind of imaging microscope and to more dimensions in parameter space. It is more general than the edge threshold method and leads to markedly different uncertainties for features smaller than the wavelength.     

Sol-gel nanocomposites as metamaterials: preparation and optical measurements

Nanocomposites made of Ag nanowires imbedded in a sol-gel host have been morphologically and optically investigated. Sonication during solidification significantly improved nanowire dispersal. The data from the nanocomposites were compared to the data from pure sol-gels in order to determine the effects of the nanowires. Reflectometry data at 1064 nm show that the presence of ～5% nanowires (by volume) results in a decrease from 1.17 to ≈ 1.1 in the real part of the index of refraction accompanied by an increase in the imaginary part. Transmission loss in the pure sol-gel is mainly due to scattering from inhomogeneities, and the inclusion of nanowires (or the process of doing so) appears to result in a significant reduction of the scattering loss.     

A PC-based coherent sonar for experimental underwater acoustics

An important tool for experimental underwater acoustics research is a versatile sonar system for transmitting, receiving, anti recording acoustic waveforms. In this paper the design, implementation and performance of a variable-frequency (25 kHz to 500 kHz) coherent sonar are presented. The significance of the design lies in its simplicity and versatility. These attributes are a result of a sampling technique that provides quadrature samples of the complex envelope of a bandpass signal without analog demodulation using a single A/D converter. Implementation of the design is accomplished using an ISA bus personal computer (PC), a commercially available digital signal processing (DSP) board, and a small amount of custom electronics. Performance of the sonar, in terms of magnitude and phase errors, is determined from wideband noise characterization, a linear-system model, and transmit/receive measurements     

水下热扰动的光学成像失真问题研究

为了研究水下热扰动环境对光学成像的畸变、模糊等失真问题的影响,利用水下图像的灰度分布、结构相似性图像度量(SSIM)和归一化最大灰度梯度清晰度评价函数来评价目标图像在径向和轴向上的畸变和模糊等失真程度,得到水下热扰动对光学成像变化的规律。实验数据表明,随着成像系统与目标的轴向距离增加,图像的畸变和模糊程度越来越大。轴向距离L1=500 mm时,对应图像的SSIM值优于0.7,归一化清晰度值优于0.8;轴向距离L3=1500 mm时,对应图像的SSIM值低于0.2,归一化清晰度值不足0.6;此外,轴向距离L1时,成像在径向上,距离发热源越近,边缘漂移越大,即成像图像畸变越严重;最后,相同轴向和径向条件下,目标在不同时刻的图像SSIM和归一化清晰度值有优劣,该结论可为后续的水下图像复原提供参考。     

Critical current measurements: A compendium of experimental results

The results of a programme to evaluate the measurement of the critical current of relatively small (<600 A) practical superconductors are presented. Experimental data showing the effect of various parameters on the measurement are given. Specific areas covered are: experimental design and sample mounting; electric field and resistivity criteria; temporal and spatial variations in the field and current; and temperature and strain effects. The goal of the presentation is to describe the critical current measurement process and its pitfalls in sufficient detail to serve as a guide for those relatively new to the field of practical superconductors.