偏振相移与相位共轭结合的散射光聚焦技术研究

散射介质对光的随机散射作用是制约其光学聚焦和成像的重要因素，光学相位共轭技术能够通过对散射光场共轭还原实现透过散射介质的光学聚焦和成像，其中散射光场相位的获取是共轭还原的核心。阐述了偏振相移的基本原理，将偏振相移与相位共轭技术相结合，设计了新的基于偏振相移的数字光学相位共轭系统。采用633 nm的HeNe激光器作为系统光源，毛玻璃作为散射介质开展散射光聚焦实验研究。实验结果表明:该装置能够成功实现透过散射介质的光学聚焦，其中聚焦点与背景光强的比值可达约400倍。     

Factors affecting the spectrum of an electromagnetic light wave on scattering from a semisoft boundary medium

The spectrum of an electromagnetic light wave on scattering from a semisoft boundary medium is discussed within the accuracy of the first-order Born approximation. It is shown that spectral shifts and spectral switches are affected both by the polarization of the incident light wave and by the characters of the scat-tering medium. Moreover, numerical results show that the direction at which the spectral switch occurs is governed by the characters of the scattering medium, whereas the magnitude of the spectral switch is affected by the polarization of the incident light wave.     

Polarization visualization of scattering media with CW laser radiation

The method of polarization visualization of a multiply scattering medium containing macroinhomogeneities based on analysis of polarization spatial distribution of a scattered linearly polarized light is discussed. The treatment is based on statistical properties of the effective optical path distribution of scattered field components. The influence of media scattering properties and the geometry of the experiment on the inhomogeneity image contrast obtained with use of polarization degree and of normalized scattered intensity of radiation as visualization parameters are discussed, as well as spatial resolution achieved in these both cases. Using the results of theoretical analysis and of the experimental model, the relationship between the shapes of spatial distributions of polarization degree and the intensity of the scattered light is considered as a function of the position of the visualized object (an absorbing half-plane immersed in a plane layer of the scattering medium). The opportunities for enhancing the quality of the images formed in this way are also discussed.     

Interference effects in backscattering of light by a layer of a discrete random medium

Multiple backscattering of light by a layer of a discrete random medium is considered. A brief derivation of equations for describing the coherent and incoherent components of scattered light is presented. These equations are solved numerically in the approximation of doubled scattering of light by a semi-infinite medium of spherical scatterers having a size comparable with the wavelength in order to study the effect of the properties of particles on the angular dependence of interference effects. Calculations show that the half-width of the interference peak decreases upon an increase in lateral scattering by particles and that the degree of polarization has a complex angular dependence on the properties of the particles. For an optically thin layer of the medium, the relations defining the interference peak half-width and the scattering angle upon extreme linear polarization as functions of the effective refractive index are given.     

透射光偏振度与散射次数关系的研究

偏振态与散射次数的关系对于提取深入组织内部的后向散射光有很重要的价值.本文分析了透射光的偏振度与光在介质中的散射次数的关系,给出了偏振光完全失去偏振态所需的散射次数,并用直接散射的实验方法进行了验证,得出当散射次数为14次时偏振光将失去其偏振特性的结论.     

双折射光纤受激拉曼散射偏光特性的实验研究

对椭圆芯光纤受激拉曼散射偏光特性进行了系统的实验研究。实验巾观察到8级斯托克斯线和2级反斯托克斯线,对不同偏振态的抽运光激励下各级斯托克斯线的偏振特性、拉曼频移等参量进行了分析,并给出了经验公式。其结果与实验数据符合良好。实验表明,双折射光纤受激拉曼谱的各级斯托克斯线的偏振状态不但与拉曼介质有关还与抽运光的偏振态有关,入射抽运光偏振态对低阶次的斯托克斯线拉曼频移的影响较小,而对高级斯托克斯线拉曼频移影响较大。     

Ultimate degree of residual polarization of incoherently backscattered light for multiple scattering of linearly polarized light

By invoking ideas about the distribution of the optical paths of partial components of the scattered field, we obtain an expression for estimating the degree of residual polarization of light that is incoherently backscattered from a disordered multiply scattering semi-infinite medium illuminated by linearly polarized light. In the backscattering regime, the depolarization length of the linearly polarized light in the disordered medium becomes smaller with the passage from the isotropic to anisotropic scattering. Experiments with model media featuring substantially anisotropic scattering (the anisotropy parameter of 0.90 ≤ g ≤ 0.95) demonstrated that for backscattering of linearly polarized light, the depolarization length is close to the transport length of the scattering medium.     

Imaging and multiple scattering through media containing optically active particles

This paper examines the behaviour of polarized light scattered by a medium containing small chiral spheroidal particles. We show that for single scattering the observed phenomena of optical activity may be interpreted in terms of an averaged Mueller matrix and describe how the degree of polarization is affected by such a medium. The polarization properties of multiply scattered light by chiral particles are considered through the use of Monte Carlo simulations. It is shown that the effects of chirality under multiple scattering can be interpreted as an order-preserving influence in a disordered system and that this influence can, in principle, be exploited for the purposes of imaging.     

Imaging and multiple scattering through media containing optically active particles

Abstract

This paper examines the behaviour of polarized light scattered by a medium containing small chiral spheroidal particles. We show that for single scattering the observed phenomena of optical activity may be interpreted in terms of an averaged Mueller matrix and describe how the degree of polarization is affected by such a medium. The polarization properties of multiply scattered light by chiral particles are considered through the use of Monte Carlo simulations. It is shown that the effects of chirality under multiple scattering can be interpreted as an order-preserving influence in a disordered system and that this influence can, in principle, be exploited for the purposes of imaging.     

A Monte Carlo approach for the calculation of polarized light: application to an incident narrow beam

Monte Carlo approaches to compute multiple scattering of polarized light are examined. A Backward Monte Carlo (BMC) method is developed to solve the Stokes vector of the multiple scattered light for an inhomogeneous scattering medium with boundaries. A generalized form of the BMC method in vector notation is proposed. This method can determine the scattered light with sufficient accuracy in both intensity and polarization compared to the same calculation using the doubling-adding method for a plane parallel medium.For application to a narrow incident beam and an inhomogeneous medium, a modified BMC method is developed, borrowing a concept from the Forward Monte Carlo (FMC) method for the first scattering events. Furthermore, a modification of the total scattering matrix, i.e., the combination of the derived scattering matrix with its time inverse, is discussed. This BMC method can be used successfully for model calculations of lidar and other laser measurements of polarized light.     

偏振光在多层散射介质中传输的蒙特卡罗模拟研究

基于米氏散射理论,运用斯托克斯-穆勒形式,结合子午面法和拒绝法,提出了偏振光在多层散射介质中传输的蒙特卡罗模型,并首次分析了当菲涅耳公式失效时,多层介质界面处的全反射行为.同时用该模型研究了微粒直径和介质折射率对漫反射和漫透射的影响.结果表明:无论是折射率匹配介质还是非匹配介质,当粒子浓度一致时,漫反射率都随微粒直径的增加而增加,随介质折射率的增加而减小,而漫透射率则相反.研究结果为生物组织的偏振光散射研究提供了理论验证模型.     

偏振光在多层散射介质中传输的蒙特卡罗模拟研究*

基于米氏散射理论,运用斯托克斯-穆勒形式,结合子午面法和拒绝法,提出了偏振光在多层散射介质中传输的蒙特卡罗模型,并首次分析了当菲涅耳公式失效时,多层介质界面处的全反射行为.同时用该模型研究了微粒直径和介质折射率对漫反射和漫透射的影响.结果表明：无论是折射率匹配介质还是非匹配介质,当粒子浓度一致时,漫反射率都随微粒直径的增加而增加,随介质折射率的增加而减小,而漫透射率则相反.研究结果为生物组织的偏振光散射研究提供了理论验证模型.     

Physical bounds to the entropy-depolarization relation in random light scattering

We present a theoretical study of multimode scattering of light by optically random media, using the Mueller-Stokes formalism which permits us to encode all the polarization properties of the scattering medium in a real 4 x 4 matrix. From this matrix two relevant parameters can be extracted: the depolarizing power D(M) and the polarization entropy E(M) of the scattering medium. By studying the relation between E(M) and D(M), we find that all scattering media must satisfy some universal constraints. These constraints apply to both classical and quantum scattering processes. The results obtained here may be especially relevant for quantum communication applications, where depolarization is synonymous with decoherence.     

Monte Carlo simulation of photon transport in a randomly oriented sphere-cylinder scattering medium

A Monte Carlo simulation tool for simulating photon transport in a randomly oriented sphere-cylinder medium has been developed. The simulated medium represents a paper pulp suspension where the constituents are assumed to be mono-disperse micro-spheres, representing dispersed fiber fragments, and infinitely long, straight, randomly oriented cylinders representing fibers. The diameter of the micro-spheres is considered to be about the order of the wavelength and is described by Mie scattering theory. The fiber diameter is considerably larger than the wavelength and the photon scattering is therefore determined by an analytical solution of Maxwell’s equation for scattering at an infinitely long cylinder. By employing a Stokes–Mueller formalism, the software tracks the polarization of the light while propagating through the medium. The effects of varying volume concentrations and sizes of the scattering components on reflection, transmission and polarization of the incident light are investigated. It is shown that not only the size but also the shape of the particles has a big impact on the depolarization.     

Polarization preservation in diffusive scattering from in vivo turbid biological media: effects of tissue optical absorption in the exact backscattering direction

There is considerable recent interest in using polarized light to investigate turbid biological media. Although tissue multiple scattering randomizes incident polarization states, there are circumstances when appreciable degree of polarization can be observed in diffusive scattering. In this study, we use polarization modulation and synchronous detection to examine in the exact backscattering direction the polarization properties of diffusely reflected visible light from hands of human volunteers of varying pigmentation levels. The surviving polarization fraction increases with increasing pigmentation, likely due to preferential loss of highly scattered, long-pathlength photons; this mechanism lowers the average pathlength traversed by the detected light and hence increases the measured polarization preservation. This behavior is contrasted with the overall diffuse reflectance intensity, whose magnitude decreases with increasing absorption. These experiments demonstrate the important influences of medium optical properties on the polarization characteristics of multiply scattered light, which must be further investigated to enable quantitative polarization evaluation of turbid media such as biological tissues.     

Stokes parameters of an electromagnetic light wave on scattering

Stokes parameters of an electromagnetic light wave on scattering have been discussed, which permit us to study the spectral degree of polarization of the far-zone scattered field. An example of scattering of a spatially coherent electromagnetic light wave from a quasi-homogeneous medium has been discussed to illustrate the changes of the Stokes parameters and the changes of the spectral degree of polarization of the far-zone scattered field.     

Dependence of the circular polarization of backscattered light in random media on anisotropy of scatterers

The scattering of linearly or circularly polarized light from a semi-infinite randomly inhomogeneous medium is considered. Using the Monte Carlo method, it is shown that, in the case of a wide front of incident and scattered optical radiation and irrespective of the degree of scattering anisotropy, the copolarized component of backscattered light dominates the cross-polarized component for the linear polarization and the cross-polarized component dominates the copolarized component for the circular polarization. If the beams of incident and scattered radiation are spatially separated and the size of scatterers exceeds the wavelength, the circular copolarized component dominates the cross-polarized one. A similar effect of the change in direction of the rotation of the plane of polarization in relation to the size of scatterers is revealed for pulsed radiation.     

Coherent opposition effects for semi-infinite discrete random medium in the double-scattering approximation

The rigorous equations of the theory of multiple scattering of light by a layer of disordered medium have been used in the double-scattering approximation for semi-infinite medium to determine the influence of the particle properties on the coherent opposition effects. The effects were found to be strongly dependent on the concentration of scatterers in the medium. The polarization opposition effect is more sensitive to the properties of the scatterers than the photometric opposition effect. The interference of waves could result in the negative polarization at the backscattering direction as well as in the positive polarization.     

Stokes vector determination of polarized light propagation in turbid medium

We report a two dimensional Stokes vector imaging technique for transamination measurements of the polarization state of scattering medium. Measurement of the depth resolved Stokes parameters allows determination of the degree of polarization, birefringence, retardation, optical activity and characterization of the medium. The polarized light preserved and degree of polarization very with scatterer concentration. The transmitted intensity patterns by varying a polarization state of the incident laser light (λ = 632.8 nm) and changing analyzer configuration provides a useful information about concentration, orientation, and shape of the sample under investigation. The results are important for the understanding of polarization phenomenon in turbid media, like biological tissues.     

Optical characterization of metallic aerosols

Airborne metallic particulates from industry and urban sources are highly conducting aerosols. The characterization of these pollutant particles is important for environment monitoring and protection. Because these metallic particulates are highly reflective, their effect on local weather or regional radiation budget may also need to be studied. In this work, light scattering characteristics of these metallic aerosols are studied using exact solutions on perfectly conducting spherical and cylindrical particles. It is found that for perfectly conducting spheres and cylinders, when scattering angle is larger than 90° the linear polarization degree of the scattered light is very close to zero. This light scattering characteristics of perfectly conducting particles is significantly different from that of other aerosols. When these perfectly conducting particles are immersed in an absorbing medium, this light scattering characteristics does not show significant change. Therefore, measuring the linear polarization of scattered lights at backward scattering angles can detect and distinguish metallic particulates from other aerosols. This result provides a great potential of metallic aerosol detection and monitoring for environmental protection.