Calculation of Photon Path Distribution Based on Photon Behavior Analysis in a Scattering Medium

We calculated the photon path distribution (PPD) in a scattering medium based on a theoretical analysis, which utilizes the relationship between the photon intensity and photon pathlength. This PPD is defined by local photon pathlengths for photons having total pathlengths of l between the light input points and detection points. The PPD of photons that consist of the impulse response at time t (= l/c) was calculated for a 2-D model. Precise analysis of photon migration in the scattering medium is essential in order to carry out image reconstruction of diffuse optical tomography (DOT). We show the PPD at time t (the total pathlength l = ct) and demonstrate its effectiveness. Our method for describing photon migration is intuitive and allows finding weight functions in DOT.     

Modeling of Raman-Scattering Signals in Biological Tissues by Direct and Two-Step Approaches

A search for an effective method of modelling of the Raman-spectroscopy problem in turbid (scattering) media has been performed taking into account the corresponding parameters of the detector and sample volume. A solution of the radiative-transfer equation by Monte-Carlo method underlies the proposed model. Two fundamental approaches to numerical modeling of Raman scattering are considered: the direct transport problem of Rayleigh and Raman photons at each point of the medium and the two-step model, in which a photon flux in the medium is calculated in the first stage, followed by generation of the corresponding number of Raman photons at each point.     

Statistical characteristics of photon distributions in a semi-infinite turbid medium: Analytical expressions and their application to optical tomography

The paper focuses on the determination of statistical characteristics of photon distributions in a semi-infinite turbid medium, specifically the photon average trajectory and the root-mean-square deviation of photons from the average trajectory, with an approach based on the diffusion approximation to the radiative transfer equation. We show that the Dirichlet and Robin boundary conditions used for this purpose give close results. We derive exact analytical expressions for the case of the Dirichlet boundary condition. To demonstrate the practical value of our results we consider approximate solution of the inverse problem of time-domain diffuse optical tomography with the flat layer transmission geometry. The problem is solved with the method of photon average trajectories which are constructed with analytical expressions derived for a semi-infinite medium.     

The numerical and experimental study of photon diffusion inside biological tissue using boundary integral method

In this study, the diffusion of photons in turbid media, like biological tissue has been studied. Due to scattering and absorption of photons in such media, the study of photon propagation in biological tissue is complicated. The several numerical methods have been presented to simulate the behavior of diffused photons. Recently, Boundary Integral Method (BIM) has been offered to simulate photon migration inside biological tissues. This method has advantage, e.g. lower computational time in compared with other numerical methods. In this study, the accuracy and precision of BIM compares with another numerical method like Monte Carlo technique and finite difference method, and also the calculated results obtained by BIM and Monte Carlo method evaluate with measured results. Furthermore, the effects of scattering and absorption coefficient of tissue on the measured signal are studied.     

Imaging in turbid media using quasi-ballistic photons

We study by means of experiments and Monte Carlo simulations, the scattering of light in random media, to determine the distance up to which photons travel along almost undeviated paths within a scattering medium, and are therefore capable of casting a shadow of an opaque inclusion embedded within the medium. Such photons are isolated by polarisation discrimination wherein the plane of linear polarisation of the input light is continuously rotated and the polarisation preserving component of the emerging light is extracted by means of a Fourier transform. This technique is a software implementation of lock-in detection. We find that images may be recovered to a depth far in excess of that predicted by the diffusion theory of photon propagation. To understand our experimental results, we perform Monte Carlo simulations to model the random walk behaviour of the multiply scattered photons. We present a new definition of a diffusing photon in terms of the memory of its initial direction of propagation, which we then quantify in terms of an angular correlation function. This redefinition yields the penetration depth of the polarisation preserving photons. Based on these results, we have formulated a model to understand shadow formation in a turbid medium, the predictions of which are in good agreement with our experimental results.     

Light transport model in a n-layered mismatched tissue

The steady-state and time-domain analytical solutions of the diffusion equation for photon migration through a highly scattering n-layered mismatched medium have been obtained. The effect of the refractive-index mismatch is taken into account, and the extrapolated boundary condition has been considered. A Monte Carlo code for photon migration through an n-layered mismatched medium has also been developed. Comparisons with the results of Monte Carlo simulations showed that the analytical solutions correctly describe the photon migration, while reflectance is observed.     

Temporal reflectance from a light pulse irradiated medium embedded with highly scattering cores

This paper presents a new approach to utilize ultrashort pulsed laser for optical diagnostics with numerical simulations. The method is based on the use of ultrafast pulses with a pulsewidth selected according to the probed medium's radiative property and/or size. Our previous work in nonhomogeneous media has shown that the resulting time-resolved reflectance signal will have a unique characteristic: it will show a direct correlation of ballistic photon travel time and interface location, which is in between different layers or nonhomogeneous regions. The premise is based on utilizing the medium's structural information carried by the ballistic and snake photons without being masked by the diffuse photons. In this study, the space-time correlation is further explored in the case of minimally scattered photons from a large scattering coefficient core region embedded within a less-scattering medium. Time-resolved reflectance signals of the single scattering core and multiple scattering cores within a three-dimensional medium demonstrate the concept and illustrate the additional effect due to the scattered photons from the core region. A unique temporal signal profile's correlation at various detector positions with respect to the scattering core is explained in detail. The result has important implications. This approach will lead to a much simpler and more precise determination of the probed medium's composition or structure. Due to the large computational requirement to obtain the physical details of the light pulse propagation inside highly scattering multi-dimensional media, the reverse Monte-Carlo method is used. The potential applications of the method include non-destructive diagnostics, optical imaging, and remote sensing of underwater objects.     

光子偏振度受大气散射影响的分析

采用矩阵形式描述光子的偏振态和大气散射理论, 分析了“BB84协议”中四个不同偏振光子经单次散射后光子的偏振度与前向散射角的关系。发现单次散射不改变偏振光子的总偏振度, 但改变偏振光子的线偏振度与圆偏振度, 尤其对垂直偏振光子的线偏振度与圆偏振度改变明显; 当前向散射角小于0.25 rad时, 四个不同偏振光子的线偏振度基本保持不变, 量子信息仍然保持; 同时分析了大气散射对不同波长的垂直偏振光子线偏振度的影响, 发现长波光子偏振度保持度高。     

多光子非线性Compton散射的能量转换

研究了多光子非线性Compton散射中电子与光子的能量转换及其转换效率.结果表明:散射光子频率随电子吸收光子数n的增大而增大,随碰撞非弹性成分ξ的增大而迅速减小.在超强激光场中,当极端相对论性电子与光子发生多光子非线性Compton散射且被光场俘获时,能量转换效率趋于无限大,即电子可以从超强激光场中获得巨大的加速能量.用高速电子束入射并与光子发生多光子非线性Compton散射,是提高非线性Compton散射能量转换效率的重要途径.     

Superfluid state of coherent light in nonlinear waveguides

We establish the photonic superfluid theory in waveguides made of self-defocussing polar crystals. In quantum theory it is shown that photons can sense an attractive effective interaction by exchange of virtual optical phonons. Such an interaction leads to the photonic superfluid state, in which a propagating photon pair consists of a combination of two photons with opposite transverse wave vector and spins. The most important property of the photonic superfluid state is that the system of photon pairs evolves without scattering attenuations. The traveling-wave superfluid state has the squeezing property and the soliton effect.     

Generation of photon pairs in dispersion shift fibers through spontaneous four wave mixing: Influence of self-phase modulation

Correlated signal and idler photon pairs with small detuning in the telecom band can be generated through spontaneous four-wave mixing in dispersion shift fibers. However, photons originated from other nonlinear processes in optical fibers, such as Raman scattering and self-phase modulation, may contaminate the photon pairs. It has been proved that photons produced by Raman scattering are the background noise of photon pairs. Here we show that photons induced by self-phase modulation of pump pulses are another origin of background noise. After studying the dependence of self-phase modulation induced photons in signal and idler bands, we demonstrate that the quantum correlation of photon pairs can be degraded by the self-phase modulation effect. The investigations are useful for characterizing and optimizing an all fiber source of photon pairs.     

Influence of substance characteristics on the interaction parameters of high-energy photons with free electrons

Various types of interaction of high-energy photons with free electrons in substances have been studied. It is shown that photon absorption by electrons, coherent photon scattering, noncoherent photon scattering, and electron-stopping after interaction with photons can be observed in substances. The dependence of the photon-wavelength variation after interaction with electrons on substance parameters and electron propagation velocity is obtained.     

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     

The Study of the Diffuse Equation About a Three-Layered Matched Medium

Near-IR radiation is often utilized to detect the properties in tissues, up to now, a semi-infinite medium photon migration model and a two-layered turbid medium model are applied widely. But the solution is approximate. In this paper, According to the diffusion equation, employing the extrapolated boundary condition, we analyze the diffusion of photons of a three-layered matched medium, set up the accurate solution of the diffuse equation. In order to validate our solution, we apply the Monte-Carlo simulation of the time domain and the steady-state, we find that the solution of a three-layered matched medium diffusion equation not only accord with the Monte-Carlo simulation. The solution can still solve the problems of a two-layered turbid medium model and a semi-infinite medium photon migration model     

Superfluidity of coherent light in self-focusing nonlinear waveguides

We establish the superfluidity theory of coherent light in waveguides made of nonlinear polar crystals.It is found that the pairing state of photons in a nonlinear polar crystal is the photonic superfluid state.The photon-photon interaction potential is an attractive effective interaction by exchange of virtual optical phonons.In the traveling-wave pairing state of photons,the photon number is conserved,which is similar to the Bose-Einstein condensation(BEC) state of photons.In analogy to the BCS-BEC crossover theory of superconductivity,we derive a set of coupled order parameter and number equations,which determine the solution of the traveling-wave superfluid state of photons.This solution gives the critical velocity of light in a self-focusing nonlinear waveguide.The most important property of the photonic superfluid state is that the system of photon pairs evolves without scattering attenuations.     

Quantum interference effects of γ radiation under crossing-anticrossing of nuclear levels in an RF field

The quantum interference effects occurring in resonant scattering of a γ photon in the multilevel γ-optical medium ⁵⁷Fe, affected by an external RF field, have been theoretically investigated. It is shown that a significant reconstruction of the hyperfine structure of nuclear states leads to the interference of partial forwardscattering amplitudes of a γ photon and, therefore, to a significant change in the nuclear refractive index of the γ-optical medium, depending on the amplitude, frequency, and phase of the external RF field. The conditions are found under which the effect of electromagnetically induced transparency can be observed in the technique of in-phase detection of γ photons.     

Spatially distributed two-photon excitation fluorescence in scattering media: Experiments and time-resolved Monte Carlo simulations

We report on measurements and Monte Carlo analysis of the spatial distribution of light in two-photon excitation fluorescence (TPEF) microscopy operating through scattering media. The axial profile of TPEF produced by a high numerical aperture microscope objective focusing through a tissue-like optical phantom at depths more than five photon mean-scattering lengths is studied. The measured profile is quantitatively interpreted with time-resolved Monte Carlo simulations that take into account the spatio-temporal distribution of the photons within the turbid medium under a femtosecond excitation regime. It is shown that considering only the ballistic photons leads to an under-estimation of the actual out-of-focus distribution of the fluorescence, whereas it is over-estimated when the ballistic and scattered photons are treated alike. Comparison of the in-focus signal to the overall out-of-focus background clearly pointed out that the signal-to-background ratio of TPEF microscopy images benefits from a shortening of the excitation pulses. Moreover, in homogeneously labeled specimen, the background is shown to overcome the signal at imaging depths of about five to six photon mean-scattering paths.     

Acousto-optic laser optical feedback imaging

We present a photon noise and diffraction-limited imaging method combining an imaging laser and ultrasonic waves. The laser optical feedback imaging (LOFI) technique is an ultrasensitive imaging method for imaging objects through or embedded within a scattering medium. However, LOFI performances are dramatically limited by parasitic optical feedback occurring in the experimental setup. In this Letter, we have tagged the ballistic photons by an acousto-optic effect in order to filter the parasitic feedback effect and to reach the theoretical and ultimate sensitivity of the LOFI technique. We present the principle and the experimental setup of the acousto-optic laser optical feedback imaging technique, and we demonstrate the suppression of the parasitic feedback.     

Experiment at SPring-8

The GeV photon beam at SPring-8 is produced by backward-Compton scattering of laser photons from 8 GeV electrons. The maximum energy of the photon will be above 3 GeV, and the beam intensity will be 10⁷ photons/sec. Polarization of the photon beam will be 100 % at the maximum energy with fully polarized laser photons. We report the outline of the quark nuclear physics project with this high-quality high-intensity beam.