Pulsed acousto-optic imaging in dynamic scattering media with heterodyne parallel speckle detection

We present a new detection scheme for acousto-optic tomography based on pulsed-wave ultrasound and illumination combined with heterodyne parallel speckle detection. This setup can perform tomographies inside several-centimeter-thick scattering samples. Test experiments confirm the suitability of this method for performing tomographies inside various types of optically scattering media, including liquids.     

Blink speckle spectroscopy of scattering media

The speckle method of characterization of multiple scattering media by squared-pulse modulation of the probe light frequency is described. This method assumes the analysis of the decay in speckle modulation that is due to the superposition of two decorrelated blinking speckle patterns. Theoretical predictions based on the phenomenological model of formation of partially blurred speckles under the condition of frequency switching are in reasonable agreement with the corresponding experimental data.     

Average and most-probable photon paths in random media

Time-resolved experiments have revealed that, in contrast to the predictions of conventional diffusion theory, photons select certain curvilinear paths to travel between a source and a detector. Concepts of the average photon paths and Fermat paths are introduced on the basis of the non-Euclidean diffusion equation (NED) to explain experimental results. Comparison of the theory and the experiment demonstrates the potential of the NED to describe nondiffusive features of photon migration in the multiple-scattering regime.     

Attenuation and speckle modulation of laser light in dispersive dye-doped media: Competition of absorption and scattering processes

The effect of competition of laser light absorption and scattering in dispersive media (1 μm polystyrene microspheres in ethylene glycol) with added dye (rhodamine 6G) was studied under the condition of near-resonant absorption of probe light in the host medium (solution of rhodamine 6G in ethylene glycol). The parameters of speckle modulation of transmitted light (the average speckle intensity and the oscillation index) were applied for characterization of light transport in the probed scattering systems as a function of dye concentration. The increase in dye concentration does not cause the expected decay in the transmittance of the examined dispersive systems for 532 nm laser light, but results in the decrease of their turbidity. This effect is accompanied by the rise of the average intensity and the oscillation index of speckle-modulated light, and can be rationalized by partial matching of the real parts of refractive indices for the host medium and embedded scatterers. This interpretation was supported by statistical modeling of light transport through the examined scattering systems.     

Raman scattering in condensed media placed in photon traps

A new type of resonator cells (photon traps) has been worked out, which ensures the Raman opalescence regime (i.e., the conditions under which the relative Raman scattering intensity at the outlet of the cells increases significantly as compared to the exciting line intensity. The Raman scattering spectra of a number of organic and inorganic compounds placed in photon traps are studied under pulse-periodic excitation by a copper-vapor laser.     

Imaging through dynamic scattering media with stitched speckle patterns

Imaging through scattering media via speckle autocorrelation is a popular method based on the optical memory effect.However,it fails if the amount of valid information acquired is insufficient due to a limited sensor size.In this Letter,we reveal a relationship between the detector and object sizes for the minimum requirement to ensure image reconstruction by defining a sampling ratio R,and propose a method to enhance the image quality at a small R by capturing multiple frames of speckle patterns and piecing them together.This method will be helpful in expanding applications of speckle autocorrelation to remote sensing,underwater probing,and so on.     

First order statistics of speckle produced by weak scattering media

A weak scattering medium produces random phases in a transmitted or reflected wavefront distributed over less than 2π. The far-field speckle pattern has a central maximum. The statistics of intensity in this maximum are calculated and the applications of the results to surface roughness measurement are indicated.     

Motion imaging of objects in layers hidden by scattering media using low-coherence speckle interferometry

A novel method is proposed for motion imaging of objects in layers hidden by a scattering medium in a depth-resolved manner by means of the intensity difference in a temporal sequence of low-coherence speckle images. Depth-resolved motion imaging was demonstrated with a semi-transparent scattering plate and an aluminum diffusive plate behind Intralipid solution. Experimental results confirmed feasibility of the proposed method for imaging of moving objects by their relative velocity.     

基于蒙特卡罗模拟的散射介质中后向光散射模型及分析应用

构建了内光源模型探讨散射介质中的光散射现象,利用蒙特卡罗方法研究了逃逸出组织的后向散射光子数随光子在组织内部发生的散射次数的分布关系,探讨了光源照明方式、辐射强度、接收方式、调制等参数的变化对后向散射的影响,结果表明后向散射光子的数量随散射次数的分布并非简单的单调递增或递减,而是一条先增大后减小出现峰值的曲线. 峰值位置、峰值大小及曲线形状与光源、探测方式、组织光学特性参数等有关. 关键词： 医用光学与生物技术 散射介质 后向散射 蒙特卡罗模拟     

Characterization and imaging in optically scattering media by use of laser speckle and a variable-coherence source

We demonstrate the application of laser-speckle statistics formed by a variable-coherence source illuminating a scattering medium, for determining the scattering parameter mu;(s)>(?) of a diffusion model for the medium. Furthermore, we apply this technique to visualize laterally localized inhomogeneities embedded within a highly scattering sample.     

Ultrasonic scattering properties of three random media with implications for tissue characterization

Random media with different structural properties were used to simulate some of the differences in liver morphology that may occur with disease. First, a reference medium consisting of glass spheres in agar was studied to verify the accuracy and precision of the data obtained with our equipment and processing procedures. Then, studies were conducted on a pair of media comprised of graphite particles in gelatin, one of the pair with twice as many particles as the other. Finally, studies were carried out on a set of media composed of Sephadex particles in water. Three samples were employed, each with a different size of Sephadex. The average differential scattering cross section per unit volume sigma sd (v) of each media was obtained as a function of scattering angle v and frequency. The measured sigma sd were compared with predictions based on models of scattering from the media. The agreement between the measured and predicted sigma sd of the glass sphere medium was excellent. The graphite medium with twice the number of particles as the other was observed to scatter twice as much power as the other. The shape of the angular scattering pattern measured from each size of Sephadex followed the prediction reasonably well. The largest size exhibited marked variations in the shape of sigma sd as a function of frequency, while the absolute magnitude of sigma sd of the smallest size grade was extremely sensitive to frequency. Our results suggest that the dependence of sigma sd (v) on scatterer number density and size, both in absolute magnitude and shape, can provide reliable information which may be useful in the diagnosis of some diseases.     

A microscope for the Glasgow photon tagging spectrometer in Mainz

A 96-element plastic-scintillator detector array has been constructed to improve the energy resolution and tagged-photon flux over a moveable energy range of up to 60MeV in the Glasgow photon tagging spectrometer at Mainz. Test results are presented which demonstrate that this device improves the resolution by a factor of about 6 compared to the main detector array. It is also shown that it is possible to achieve accurate energy calibration by using electron beams of several different accurately known energies from the Mainz accelerator.     

Soft photon theorem in potential scattering: Non-local potentials

The soft-photon theorem is proven for non-relativistic bremsstrahlung from a charged particle in an external, non-local potential.     

Upgrade of the Glasgow photon tagging spectrometer for Mainz MAMI-C

The Glasgow photon tagging spectrometer at Mainz has been upgraded so that it can be used with the 1500MeV electron beam now available from the Mainz microtron MAMI-C. The changes made and the resulting properties of the spectrometer are discussed.     

Characterization and validation of the frequency-modulated continuous-wave technique for assessment of photon migration in solid scattering media

The frequency-modulated continuous-wave (FM-CW) technique, based on the beat signal of a Mach?CZehnder interferometer employing a frequency-ramped light source, is studied for solid scattering media applications. The method is used to evaluate the mean time-of-flight (MTOF) of light traveling in scattering media, specifically polystyrene foams. We assume that each the time-of-flight (TOF) time corresponds to different light scattering paths resulting in a different phase shift. The phase shift variations produce a speckle pattern, which together with the frequency leakage induced by the discrete Fourier transform (DFT) cause ??spikes?? in the power spectrum of the beat signal, thus decreasing the accuracy of the measured MTOF values in solid scattering media. For comparison, time-of-flight spectroscopy (TOFS) is also employed to evaluate the MTOF for the same samples, while the geometrical difference between these two techniques is compensated for by using diffusion theory. The MTOFs measured by the FMCW and TOFS techniques agree well, which demonstrates a great potential to develop a robust FMCW setup for simplified MTOF assessment.     

IMAGING: Focusing light in scattering media

Combining ultrasonic modulation and optical phase conjugation allows light to be tightly focused in a scattering medium, providing benefits for studies of photophysical, photochemical and photobiological processes.     

Ultrasonic flaw detection in NDE of highly scattering materials using wavelet and Wigner-Ville transform processing

In ultrasonic non-destructive evaluation of highly scattering materials the backscattering noise may attain peak values greater than the searched flaw pulse and the mean value of noise spectrum is very similar to the searched echo spectrum. Several specific methods have been proposed for the reduction of this type of noise, but the comparison of the performance of different methods is still an open problem. In this paper, we make a comparison among some methods based on simultaneous representations in time and frequency/scale domains of the ultrasonic traces. Synthetic and experimental traces are de-noised using a discrete wavelet processor with decomposition level-dependent threshold selection and a method that combines Wigner-Ville transform and filtering in the time-frequency domain. The results are comparatively evaluated in terms of signal to noise ratio and probability of detection.     

Resonances in electron scattering and photon absorption

A short survey is given of the development of ideas about resonances in atomic scattering processes and their connection with the theory of resonant states in nuclei, impurity resonances in solids, ion-atom scattering and recombination in plasmas. A detailed discussion of the experimental situation for atomic resonances is then given, followed by a review of the theory of resonance reactions as applied to them. Special attention is given to effective range and quantum defect methods, and to Fano's configuration interaction theory. Theoretical results for line positions, shapes and widths are compared with experimental data and the need for more angular distribution data is emphasized.