Evaluation of a compact sensor for backscattering and absorption

Seawater inherent optical properties (IOPs) are key parameters in a wide range of applications in environmental studies and oceanographic research. In particular, the absorption coefficient (a) is the typical IOP used to obtain the concentration of chlorophyll-a in the water-a critical parameter in biological oceanography studies and the backscattering coefficient (b(b)) is used as a measure of turbidity. In this study, we test a novel instrument concept designed to obtain both the absorption and backscattering coefficients. The instrument would emit a collimated monochromatic light beam into the water retrieving the backscattered light intensity as a function of distance from the center of illumination. We use Monte Carlo modeling of light propagation to create an inversion algorithm that translates the signal from such an instrument into values of a and b(b). Our results, based on simulations spanning the bulk of natural values of seawater IOP combinations, indicate that a 6.2 cm diameter instrument with a radial resolution of 1 cm would be capable of predicting b(b) within less than 13.4% relative difference and a within less than 57% relative difference (for 90% of the inverted a values, the relative errors fall below 29.7%). Additionally, these errors could be further reduced by constraining the inversion algorithm with information from concurrent measurements of other IOPs. Such a compact and relatively simple device could have multiple applications for in situ optical measurements, including a and b(b) retrievals from instrumentation mounted on autonomous underwater vehicles. Furthermore, the same methodology could possibly be used for an out-of-water sensor.     

Double-deformable-mirror adaptive optics system for phase compensation

Two kinds of algorithm for an adaptive optics (AO) system that uses two deformable mirrors (DMs), one with large stroke and the other with high spatial frequency, to correct different aberrations are described. The algorithms are based on modal method and direction-gradient method, respectively. Numerical simulations for the algorithms have been made. The simulation results indicate that the two DMs in the AO system can correct different aberrations with different characteristics, and the closed-loop performance of a double-DM AO system will be almost the same as that of an AO system that uses a single DM with an ideal stroke.     

Bulk wind estimation and prediction for adaptive optics control systems

We present a wind-predictive controller for astronomical adaptive optics (AO) systems that is able to predict the motion of a single windblown layer in the presence of other, more slowly varying phase aberrations. This controller relies on fast, gradient-based optical flow estimation to identify the velocity of the translating layer and a recursive mean estimator to account for turbulence that varies on a time scale much slower than the operating speed of the AO loop. We derive the Cramer-Rao lower bound for the wind estimation problem and show that the proposed estimator is very close to achieving theoretical minimum-variance performance. We also present simulations using on-sky data that show significant Strehl increases from using this controller in realistic atmospheric conditions.     

全相位Hv频响函数估计在MIMO随机振动

首先选用对噪声抑制效果较好的Hv估计方法,然后引入对泄露有明显抑制效果的全相位FFT频谱分析方法,构成全相位Hv频响函数估计方法,改善了频响函数估计效果,减小了频响估计误差对MIMO随机振动试验控制效果的影响.最后在钢质悬臂梁上做两输入两输的Hv估计和全相位Hv估计的对比试验以及两种估计结果在MIMO随机振动试验控制的应用效果,试验结果验证了全相位Hv频响函数估计对改善了随机振动试验控制效果的有效性.     

Analytic beam spread function for ocean optics applications

A discrete ordinates code is developed with which to compute the beam spread function (BSF) without invoking the small-angle scattering approximation or performing Monte Carlo calculations. The computed BSF is used to predict the response of a detector versus its distance to the origin of a highly collimated beam, its angle with respect to the beam, and the two local angles that specify the detector orientation. Numerical results have been obtained for water models that simulate a clear ocean, a coastal ocean, and a turbid harbor. Six orders of magnitude or more change in the detector response caused by scattered photons can be predicted for different detector locations while simultaneously obtaining small changes for different detector orientations. This capability is useful for assessment of the sensitivity of the detector response to the interpretation of time-independent underwater imaging systems or visibility models.     

Irradiance inversion algorithm for absorption and backscattering profiles in natural waters: improvement for clear waters

Our iterative inversion algorithm for retrieving absorption a(z) and backscattering b(b)(z) from profiles of upwelling and downwelling irradiance, on the basis of assuming a depth-independent phase function for the medium, was found to have unsatisfactory accuracy for b(b)(z) in clear waters. We modified the algorithm here by assuming a depth-independent phase function for the particles and then performing an additional iteration over the fraction of total scattering that is due to the water itself. The modified algorithm's accuracy is considerably improved over the original in clear waters and reduces to the original in waters for which the particle contribution to b(b)(z) is dominant.     

Least-squares phase estimation with multiple parameters in phase-shifting electronic speckle pattern interferometry

We have developed an accurate and robust phase-estimation method in phase-shifting electronic speckle pattern interferometry. Unlike other methods that assume a constant phase within a fitting window, our method treats the phase variation with a gradient. A cost function that can utilize the information of pixel positions is formulated on the basis of a least-squares criterion. Powell's iteration method is applied to it to derive the phase and its gradient. An automatic consistency-checking routine and an algorithm that improves the initial guess of the iteration are developed for severe situations with large noise and steep phase variations.     

Liquid phase epitaxy and magneto-optical properties of garnet films for integrated optics

Films of different compositions have been obtained by liquid phase epitaxy on 〈111〉 GGG substrates; their magnetic and optical properties have been investigated at 1.152 μm in view of their future utilization in integrated optics. Conditions of growth and results of characterization are given for films of the following compositions: (Sc, Ga): YIG, (Ga,Ga):YIG, Bi:YIG, (Pr,Ga):YbIG and (Pr,Bi,Ga):YbIG.     

跟骨超声背散射系数与新生儿相关参数的相关性

采用超声背散射法检测新生儿跟骨的超声背散射系数(Ultrasonic Backscatter Coefficient,BSC),并分析BSC与出生胎龄、体重、身长和头围等因素之间的关系,从而评估BSC在评价新生儿松质骨状况中的作用。对122例新生儿的临床测试分析结果表明新生儿跟骨中的BSC与出生时的胎龄、体重和身长有良好的相关性。说明超声背散射信号及其BSC可用于评价新生儿的骨质量。     

Geometric phase in optics and angular momentum of light

Abstract

The Physical mechanism of the geometric phase in terms of angular momentum exchange is elucidated. It is argued that the geometric phase arising out of the cyclic changes in the transverse mode space of Gaussian light beams is a manifestation of the cycles in the momentum space of the light. The apparent non-conservation of orbital angular momentum in the spontaneous parametric down conversion for the classical light beams is proposed to be related to the geometric phase.     

Radiance-irradiance inversion algorithm for estimating the absorption and backscattering coefficients of natural waters: homogeneous waters

A full multiple-scattering algorithm for inverting upwelling radiance (L(u)) or irradiance (E(u)) and downwelling irradiance (E(d)) profiles in homogeneous natural waters to obtain the absorption (a) and backscattering (b(b)) coefficients is described and tested with simulated data. An attractive feature of the algorithm is that it does not require precise knowledge of the scattering phase function of the medium. For the E(u)-E(d) algorithm, tests suggest that the error in the retrieved a should usually be ?1%, and the error in b(b)?10-20%. The performance of the L(u)-E(d) algorithm is not as good because it is more sensitive to the scattering phase function employed in the inversions; however, the error in a is usually still small, i.e., ?3%. When the algorithm is extended to accommodate the presence of a Lambertian-reflecting bottom, the retrievals of a are still excellent, even when the presence of the bottom significantly influences the upwelling light field; however, the error in b(b) can be large.     

Irradiance inversion algorithm for estimating the absorption and backscattering coefficients of natural waters: Raman-scattering effects

We modify an algorithm for retrieving the absorption (a) and backscattering (b(b)) coefficient profiles in natural waters by inverting profiles of downwelling and upwelling irradiance so as to include the presence of Raman scattering. For a given wavelength of interest, lambda, the light field at the appropriate Raman excitation wavelength lambda(e) is first inverted to obtain the Raman source function at lambda. Starting from estimates of the inherent optical properties at lambda, the contribution to the irradiances at lambda from Raman scattering is then estimated and subtracted from the total irradiances to obtain the elastically scattered irradiances. We then inverted the elastically scattered irradiances to find new estimates of a and b(b) using our original method [Appl. Opt. 37, 3886 (1998)]. The algorithm then operates iteratively: The new estimates are used with the Raman source function to derive a new estimate of the Raman contribution, etc. Sample results are provided that demonstrate the working of the algorithm and show that the absorption and scattering coefficients can be retrieved with accuracies similar to those in the absence of Raman scattering down to depths at which the light field is significantly perturbed by it, e.g., with ~90% of the upwelling light field originating from Raman scattering.     

Amplitude and phase representation of monochromatic fields in physical optics

The conservation equation for a monochromatic field with arbitrary polarization propagating in an inhomogeneous transparent medium is expressed in terms of amplitude and phase variables. The expressions obtained for linearly polarized fields are compared with the results obtained in the eikonal approximation. The electric field wave equation is written in terms of intensity and phase variables. The transport equations for the irradiance and the phase are shown to be particular cases of these derivations. The conservation equation arising from the second-order differential wave equation is shown to be equivalent to that obtained from Poynting's theorem.     

Sensitivity of the backscattering Mueller matrix to particle shape and thermodynamic phase

The Mueller matrix (M) corresponding to the phase matrix in the backscattering region (scattering angles ranging from 175 degrees to 180 degrees) is investigated for light scattering at a 0.532-microm wavelength by hexagonal ice crystals, ice spheres, and water droplets. For hexagonal ice crystals we assume three aspect ratios (plates, compact columns, and columns). It is shown that the contour patterns of the backscattering Mueller matrix elements other than M11, M44, M14, and M41 depend on particle geometry; M22 and M33 are particularly sensitive to the aspect ratio of ice crystals. The Mueller matrix for spherical ice particles is different from those for nonspherical ice particles. In addition to discriminating between spherical and nonspherical particles, the Mueller matrix may offer some insight as to cloud thermodynamic phase. The contour patterns for large ice spheres with an effective size of 100 microm are substantially different from those associated with small water droplets with an effective size of 4 microm.     

Instrumentation to measure the backscattering coefficient b(b) for arbitrary phase functions

A single detector instrument concept that collects scattered light over the full range of backscattering angles is described. Its light collection aperture is designed so as to introduce a sin θ factor into the collection probability. Hence, the instrument is exactly a b(b) meter; it directly measures b(b), not a proxy for it. For an infinitesimal aperture to the detector, the instrument would give b(b) exactly; for a finite aperture (e.g., 1.26 cm(2)), it would typically give b(b) to an accuracy of a few tenths of 1%. The instrumentation itself is as simple as that of the well-known fixed-angle meters-it projects a beam of light into the medium and collects backscattered light with a single detector; the differences are the position of the detector and the shape/orientation of the entrance aperture to the detector.     

Variation of the Fourier transform mass spectra phase function with experimental parameters

It has been known for almost 40 years that phase correction of Fourier transform ion cyclotron resonance (FTICR) data can generate an absorption-mode spectrum with much improved peak shape compared to the conventional magnitude-mode. However, research on phasing has been slow due to the complexity of the phase-wrapping problem. Recently, the method for phasing a broadband FTICR spectrum has been solved in the MS community which will surely resurrect this old topic. This paper provides a discussion on the data processing procedure of phase correction and features of the phase function based on both a mathematical treatment and experimental data. Finally, it is shown that the same phase function can be optimized by adding correction factors and can be applied from one experiment to another with different instrument parameters, regardless of the sample measured. Thus, in the vast majority of cases, the phase function needs to be calculated just once, whenever the instrument is calibrated.     

Bayesian inference for parameters estimation using experimental data

A finite element (FE) model is developed for a curved cable-stayed footbridge located in Terni (Umbria Region, Central Italy) which accounts for uncertainties in geometry, material properties, and boundary conditions as well as limited knowledge on the behavior of connections and other components. Ambient vibration tests (AVTs) are carried out to identify the main dynamic parameters which are used for model updating in the Bayesian framework. Sensitivity analysis is performed to identify the main mechanical parameters affecting natural frequencies and mode shapes to be used as updating parameters. Finally, the posterior probability distributions of the selected updating parameters is estimated and used to assess the accuracy of the FE-based model. The importance of using a proper informative reference data set in the updating framework is assessed using different observations together with the importance of reliable surrogate models able to reduce the computational costs related to the whole framework.     

Optical phase space, Wigner representation, and invariant quality parameters

Wigner's quasi probability and related functional and operator methods of quantum mechanics have recently played an important role in optics. We present an account of some of these developments. The symmetry structures underlying the ray and wave approaches to paraxial optics are explored in some detail, and the manner in which the Wigner phase-space representation captures the merits of both approaches is brought out. A fairly self-contained analysis of the second or intensity moments of general astigmatic partially coherent beams and of their behavior under transmission through astigmatic first-order optical systems is presented. Geometric representations of the intensity moments that render the quality parameters or polynomial invariants manifest are discussed, and the role of the optical uncertainty principle in assigning unbeatable physical bounds for these invariants is stressed. Measurement of the ten intensity moments of an astigmatic partially coherent beam is considered.     

Temporal autocorrelation function for a diffusing-wave spectroscopy experiment with a point source and backscattering detection

The autocorrelation function of the backscattered intensity in a diffusing-wave spectroscopy experiment that uses a point source is calculated by use of the diffusive-wave model. We show that in this approximation the calculated autocorrelation function decays faster than if the plane-source approximation were used. The design of a probe that implements this geometry is presented together with preliminary results that show the utility of the probe as a sizing tool in concentrated dispersions.