Liana V. Kuznetsova

Random media characterization using the analysis of diffusing light data on the basis of an effective medium model

The transport properties of dense random media such as rutile powder layers and polyball suspensions are analyzed in visible and near infrared on the basis of experimental data on coherent backscattering, diffuse transmittance, and low-coherence interferometry. The developed technique of retrieval of the transport parameters of examined scattering media allows the evaluation of the transport mean free path l* and the effective refractive index n(ef) of the medium without a priori knowledge of the optical properties of the scattering particles. It is found that with decreasing wavelength lambda(0) the value of localization parameter 2pin(ef)l*/lambda(0) of the studied rutile samples abruptly drops and approaches approximately 2.6 at 473 nm. This peculiarity is caused by the very large scattering efficiency of scatterers in the vicinity of the first Mie resonance.

Monitoring of tissue thermal modification with a bundle-based full-field speckle analyzer

Speckle-contrast monitoring of laser-mediated tissue modification is examined for the specific case of delivery of speckle-modulated light from the tissue to detector (CCD camera) with a fiber-optic element (bundle). The influence of the transfer properties of a bundle-based optical system on the decorrelation rate of detected dynamic speckles is analyzed. Compared with the widely used method on the base of speckle-contrast analysis in the image plane, the considered technique is characterized by a more pronounced correlation between variations of the contrast of time-averaged speckle patterns and changes in the temperature of the modified tissue. The possibility of characterization of the modification kinetics (in particular, by the evaluation of the characteristic activation energy) using the developed speckle technique is demonstrated.

Polarization videoreflectometry of multiple scattering anisotropic media in application to fibrous tissue diagnostics

Diffuse reflectance of polarized light is studied in application to characterization of multiple scattering anisotropic media (such as, e.g., the fibrous tissues) with the use of focused probe laser beam. The theoretical model for description of the transport properties (the values of transport scattering coefficients and effective refractive index for different polarization states of probe light) of closely packed systems of partially disordered dielectric cylinders is considered on the base of coherent potential approximation. The influence of structural and optical characteristics of fibrous systems (such as the volume fraction of cylindrical scatters, their average diameter, the degree of disorder, refractive indices of scatters and surrounding medium) on diagnostical parameters determined with the polarization videoreflectometry can be analyzed in the framework of the considered theoretical model. The obtained theoretical results satisfactorily agree with the experimental data on polarization videoreflectometry and spectral-polarization measurements in the transmittance mode of in-vitro samples of demineralized bone, muscular tissue and phantom samples (partially oriented polymer films).

Multiple-beam interferometry of turbid media with quasi-monochromatic light

Method for inhomogeneous media probing ny means of the contrast measurements for multiply scattered speckles is considered. Value of the speckle contrast is obtained on the basis of the discrete scatter model as the integral transform of probability density function of differences of effective optical paths characterizing the scattering system. Pathlength distributions were simulated for different detection conditions and optical properties of scattering medium by using Monte-Carlo technique. Comparison of the discussed approach with traditional method of diagnostics and visualization of turbid media such as direct measurements of the scattered light intensity is made.

Coherent backscattering diagnostics of tissue-like media and tissues

The transport properties of highly scattering media including biotissues with expressed fibrillar structure are studied by measuring coherent backscattering. Fitting of the experimental data with the coated coherent potential approximation led to evaluation of the transport parameters without a-priori knowledge of the optical properties of scattering particles for strongly scattering dense media. The peculiarities of light diffusion in multiple scattering media characterized by macroscopic anisotropy of scattering properties (such as, the collagen-containing biological tissue with partially oriented fibrillar structure) are introduced. The experimentally obtained ratio of the reduced scattering coefficients in the directions of parallel and perpendicular to the fibers is equal to 0.37 which is in a good agreement with the same value obtained with laser videoreflectometry method.

Estimation of the transport properties of strongly scattering dense random media by measuring coherent backscattering

The transport properties of densely packed layers of titanium dioxide (rutile) powder are studied in visible and near infrared region by measuring coherent backscattering (CBS). Fitting of the experimental data with the coated coherent potential approximation led to evaluation of the transport parameters (such as transport mean free path and effective refractive index) of studied samples without a-priori knowledge of the optical properties of scattering particles.

Study of the anisotropy of light propagation in biological tissues

Application of polarized probe light and polarization discrimination of backscattered light give the additional possibilities for characterization and imaging of various diseases localized in superficial layers of tissues because of the high sensitivity of polarization state of multiply scattered light, which is detected in the backscattering mode, to alterations in optical parameters of the probed tissue. In this work we compare two methods of analysis of multiple scattering anisotropic media (demineralized bone), such as laser videoreflectometry and method of coherent backscattering. The agreement between results obtained with these two techniques is satisfactory.

Dynamic laser speckles as applied to study of the laser-mediated structural changes of the collagenous biotissues

The potential for monitoring of the laser-mediated thermal modification of tissue by means dynamic laser speckles methods is examined. The specific case of delivering of speckle-modulated light backscattered by cartilage with the use of a fiber-optic bundle probe is considered. The proposed technique is experimentally compared with the conventional laser speckle contrast analysis. Both techniques similarly display the basic features of laser-mediated alterations in tissue but the bundle-based technique provides the monitoring of tissue modification with more details and more pronounced correlation between the speckle contrast and the tissue temperature. The possibility of evaluation of the fundamental physical-chemical parameters of tissue modification (in particular, the activation energy) from the temperature-dependent dynamics of the speckle contrast is demonstrated.

Study of relaxation of thermally treated collagenous tissues with the cumulant analysis of speckle intensity fluctuations

The possibilities of characterization of the relaxation processes in thermally treated collagenous tissues with the use of the cumulant analysis of speckle intensity fluctuations are discussed. This method deals with the correlation analysis of spatial-temporal fluctuations of laser light multiply scattered by thermally modified tissue under the condition of polarization discrimination of detected speckle-modulated optical signals. Experimental results, which were obtained with ex-vivo cartilage samples, are presented.

Anomalous diffusion of light in TiO2-powder layers near the absorption edge

The transfer of visible and near-IR radiation in layers of polydisperse TiO2 (rutile) particles has been studied by measuring diffusion transmission and coherent backscattering in order to determine the optical parameters of the samples under investigation. An approach based on the coherent potential approximation is applied. It has been shown that, in the short-wavelength region of the visible range, anomalous diffusion of light occurs due to the effect of interference at mesoscopic scales on the transport characteristics of the scattering medium.