Table of contents

Methods of optical tomography of biological tissues are considered, which include pulse-modulation and frequency-modulation tomography, diffusion tomography with the use of cw radiation sources, optical coherent tomography, speckle-correlation tomography of nonstationary media, and optoacoustic tomography. The method for controlling the optical properties of tissues is studied from the point of view of increasing a probing depth in optical coherent tomography. The modern state and prospects of the development of optical tomography are discussed.

The spatial distribution of laser radiation intensity in turbid condensed media is studied analytically, numerically, and experimentally by the optoacoustic method. Based on optoacoustic measurements and Monte Carlo numerical simulations, the relation is obtained between the optical characteristics of a scattering medium and the position of the maximum of the spatial distribution of radiation intensity in the medium. It is shown that, when the anisotropy factor exceeds 0.8, this dependence has a universal type in the ranges of absorption and scattering coefficients typical for biological tissues. The method is proposed for measuring the extinction and absorption coefficients in light scattering media from the temporal shape of an optoacoustic pulse detected in relative units. An approximate method for solving a radiation transfer equation is verified, and the regions of application of the P₃ and P₅ approximation are established.

The possibility of increasing the human-skin probing depth by the method of reflection confocal microscopy (RCM) upon decreasing the amplitude of spatial fluctuations of the refractive index of the upper skin layers is considered. A change in the probing depth is estimated by analysing the spatial distribution of the probability density of the effective optical paths of detected photons calculated by the Monte Carlo method. The results of the numerical simulation are interpreted within the framework of the possible application of RCM to the study of the human skin exposed to an immersion liquid compatible to it. A diffusion of the immersion agent into the skin depth involves the equalising of the refractive indices of the structural elements of near-surface skin layers, which in turn causes a decrease in the scattering intensity and a certain increase in the transparency of the upper tissue layers. It is shown that a decrease in the light scattering in the near-surface skin layers leads to a significant increase in the probing depth obtained with the RCM technique.

The scattering phase functions of light are obtained for a layer of the erythrocyte suspension by the Monte Carlo method. At the erythrocyte concentration corresponding to a whole blood, these functions substantially differ from the phase function of a single erythrocyte. Contributions from the low-order and multiple scattering to the light intensity measured at different angles are compared. It is shown that scattering of light from a suspension layer of thickness of about 100 μm to the forward half-plane is mainly determined by the low-order scattering (by snake photons), whereas scattering to the back half-plane is mainly determined by multiple scattering. The possibility of using the diffuse approximation for the theoretical description of scattering is analysed.

A distorting influence of light refraction at the boundaries of scattering media on the results of tomographic reconstruction of images of radially symmetric objects is investigated. The methods for the correction of such refraction-caused distortions are described. The results of the image reconstruction for two model cylindrical objects are presented.

The method of autodyne detection of backscattered radiation is used to study the passage of CO₂ laser radiation through interfaces of model media and biological tissues in vitro during their laser perforation. It is shown that a stepwise change in the weighted mean frequency of the autodyne-signal power spectrum is a criterion for this passage in real time.

The formation of macromolecular protein clusters is studied by the Rayleigh — Debye method of scattering of laser radiation in solutions containing salts of toxic heavy elements, such as lead, copper, and cadmium, at low concentrations. It is found that the mass of clusters is maximal in the region of the isoelectric point of a protein and increases with increasing the ionic strength of the solution. It is shown that clusters decompose upon heating solutions up to the temperature about of 40 °C.

A compact adaptive system capable of imaging a human-eye retina with a spatial resolution as high as 6 μm and a field of view of 15° is developed. It is shown that a modal bimorph corrector with nonlocalised response functions provides the efficient suppression of dynamic aberrations of a human eye. The residual root-mean-square error in correction of aberrations of a real eye with nonparalysed accommodation lies in the range of 0.1 — 0.15 μm.

A new approach is proposed for correcting the eye refraction by controlled variation of the mechanical properties of the sclera and cornea upon nondestructive laser heating. Experimental ex vivo studies of rabbit and pig eyes show that laser-induced local denaturation of the sclera changes the refraction of the cornea by 3 diopters on the average, and the subsequent nondestructive irradiation of the cornea increases its plasticity, which leads to a further increase in its radius of curvature and a decrease in refraction down to 7 diopters.

The structural changes in adipose and fibrous tissues caused by 2- and 3-W IR laser irradiation are studied by the methods of IR and Raman spectroscopy and differential scanning calorimetry. It is shown that heating of fibrous tissue samples to 50 °C and adipose tissue samples to 75 °C by IR laser radiation changes the supramolecular structure of their proteins and triacylglycerides, respectively, without the intramolecular bond breaking. Heating of fibrous tissue to 70 °C and adipose tissue to 90 — 110 °C leads to a partial reversible denaturation of proteins and to oxidation of fats.

The effect of 675-nm semiconductor laser radiation of various power densities on the degree of oxygenation and the relative concentration of hemoglobin in blood in the microcirculatory vessels in the skin of a hand is studied in vivo by the spectroscopy of backward diffuse reflection in the visible range. No noticeable variations in these quantities were observed at relatively low power densities (up to 0.5 W cm^-2) of the radiation power density. At higher power densities, an increase in the degree of oxygenation and the relative concentration of hemoglobin was observed, which is apparently caused exclusively by heating.

The intracavity summing of the frequencies of lasing (λ = 1.084 μm) and pump radiation from a 0.81-μm diode laser is performed in a periodically poled active nonlinear Nd:Mg:LiNbO₃ crystal.

It is proposed to use radiation from a femtosecond laser for absolute length measurements. The possibility of creation of a unified frequency and length standard is shown.

The process of melt formation upon irradiation of a metal by a train of nanosecond laser pulses is studied theoretically and experimentally. A method is developed for determining the parameters of a train at which the amount of melt is minimal, and the material is removed in the vapour state upon laser irradiation.

An H₂ — F₂ amplifier initiated by radiation from a pulsed hydrogen fluoride laser is studied theoretically. Numerical calculations are made by taking into account the inhomogeneity of the initial HF concentration appearing upon preparation of the laser mixture in experiments. The mean theoretical output energy for the H₂:F₂:O₂: He = 100:600:30:100-Torr mixture is about 15 JL^-1 for an average initial pressure of hydrogen fluoride equal to 0.5 Torr along the amplifier axis. The results of calculations show that, if a sufficiently homogeneous medium is obtained in experiments, lasing becomes possible according to the kinetic scheme of initiation proposed for the H₂ — F₂ laser.

The effect of the intensity modulation on the operation of an adaptive system with a shearing interferometer in an optical feedback loop is studied numerically. It is shown that intensity fluctuations at the system input deteriorate the quality of compensation of phase fluctuations. The dependences of the residual compensation error on the statistical characteristics of the radiation phase and intensity fluctuations at the system input are obtained.