Speckle diagnostics of multiple scattering media with the use of frequency-modulated laser radiation

A method for probing randomly inhomogeneous multiple scattering media with the use of frequency-modulated laser radiation is considered. The method is based on analysis of the dependence of the blinking index of time-averaged speckles formed upon scattering of the probing radiation in a medium on the frequency modulation depth of the probing radiation. In the case of a binary frequency modulation, the blinking index of the detected speckle-modulated radiation is determined by the cosine Fourier transform of the probability density of the optical path-length difference of partial components of the scattered field in the probed medium. The values of the probability density of the optical path-length difference reconstructed with the use of the proposed method from the measured blinking index of speckles for model scattering media (fluoroplastic layer and layer of TiO₂ particles on a glass substrate) are in a good agreement with the results of statistical simulation of the probing radiation transfer in multiple scattering media.     

CW laser transilluminance in turbid media with cylindrical inclusions

The present work analyzes the process of detection of small diffusive inclusions in turbid hosts. Experiments were carried out using a transillumination geometry and continuous wave laser radiation, considering cylindrical inclusions in different environments. A comparison between experimental data, theoretical approaches for the radiative transfer equation in the diffusion approximation, and numerical Monte Carlo simulations, is presented.     

Polarization aberrations of laser radiation

Polarization aberrations (PAs) of laser radiation, defined as distortions of the wave-polarization characteristics, are considered within the framework of wave theory. The parameters describing these aberrations are introduced. It is established that PAs already appear in the paraxial region. PAs of a thin lens are calculated using the method of wave-polarization matrices. It is shown that, under certain conditions, the polarization state is modified at the axis of the laser beam.     

Absorption and scattering of high-power laser radiation in low-density porous media

We have experimentally investigated the interaction of high-power neodymium laser pulses in the intensity range 10¹³–10¹⁴ W/cm² with flat low-density (0.5–10 mg/cm³) agar-agar targets under conditions of interest for problems of inertial nuclear fusion. Optical and x-ray methods with high temporal and spatial resolution were used to examine the dependence of absorption and scattering of the incident beam on the initial mean density and thickness of the irradiated samples. We show that when a porous target is irradiated, a bulk absorption layer of high-temperature plasma is produced inside the target whose dimensions are determined by the initial density of the material. The time dependence and spectral composition of the harmonics 2ω ₀ and 3ω ₀/2 observed in the plasma-scattered radiation are measured. A theoretical model is developed that describes the interaction of high-power laser pulses with a porous medium. Predictions of the model, based on the hypothesis of two stages of homogenization of the target material—a fast stage (0.1–0.3 ns) and a slow stage (1–3 ns), are in good agreement with the experimental data. Zh. éksp. Teor. Fiz. 115, 805–818 (March 1999)     

Determination of optical properties of slices of turbid media by diffuse CW laser light scattering profilometry

In this work we present a method for determining the optical parameters of turbid media, namely its absorption coefficient (μ_a) and its reduced scattering coefficient . It is based on the measurement of CW transmittance profiles and analysis of the experimental data by a theoretical model based on the diffusion approximation (DA) of the radiative transfer equation (RTE). The method developed has been investigated with solid polymer probes but it could be applied for liquid materials as well. Experimental results are presented and compared to those of other authors together with a discussion about the accuracy of measurements. In addition, measurements using integrating spheres as well as Monte Carlo simulations are also presented to validate these results.     

Time-independent polarized radiation in radiators with scattering media

The necessary condition of stationarity in radiators with scattering media is deduced. It is obtained from the fact that all primary emitted radiation disappears with probability unity when radiation is time-independent. The disappearance due to the absorption by matter and to escape from radiator is taken into account. The condition of stationarity includes the characteristics of radiation polarization, the coefficients of medium absorption, and the elements of the Green's function matrix. The relations between intensity components of radiation are derived from the condition. The results may be used in investigations of various radiators with scattering media.     

Time-dependent radiation transfer with Rayleigh scattering in finite slab media

The time-dependent radiation transfer equation in a finite plane geometry with Rayleigh scattering is studied. The traveling wave transformation is used to obtain the corresponding stationary-like equation. Pomraning-Eddington approximation is then used to find the solution. Numerical results for reflectivity at the left boundary and transmissivity from the right boundary are presented at different times. The medium is assumed to have specular-reflecting boundaries with angular-dependent externally incident flux. Two different weight functions are introduced to force the boundary conditions to fulfill.     

Polarization of radiation of distributed-feedback dye lasers with one- and two-component gain media

Polarization of the radiation generated by a distributed-feedback laser on one-component solutions and binary mixtures of coumarin dyes is investigated. It is established that, with a marked excess of the generation threshold, the degree of polarization P of the radiation generated by one-component solutions is practically independent of the wavelength. For binary mixtures, the spectral behavior of P depends on the donorlacceptor concentration ratio. Here, with the anisotropy of the distribution of excited molecules being partially disrupted in the process of energy transfer (from the donor to the acceptor), the degree of polarization decreases appreciably when radiation frequency is tuned from the short-wavelength region to the long-wavelength one. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 67, No. 3, pp. 340–343, May–June, 2000.     

Theory of laser action in scattering gain media

A laser model based on feedback produced by scattering has been developed to explain the narrow linewidth emission and input-output behavior observed in scattering gain media. The model is based on the transient two-level laser equations and includes the detailed spectral properties of the dye gain system. Monte Carlo methods were employed to calculate the threshold gain required for modeling the input-output and linewidth emission characteristics.     

Role of light-induced scattering in the optical limitation of laser radiation on the basis of fullerene-containing media

A theoretical and experimental study of the contribution of light-induced scattering to nonlinear optical limitation is made. It is shown that light-induced scattering makes a considerable contribution to the optical limitation of radiation by fullerene-containing solutions. Radiation is scattered by small-scale (1–10 μm) inhomogeneities of density, and the corresponding increments are rather large. Because of this, scattering can be initiated by small-scale inhomogeneities of the input beam. Numerical simulation of nonlinear scattering is made, and the angular distribution and limitation of radiation in optical limiters are calculated. The calculation results are compared with the experiment.     

Polarization structure of biospeckles and the depolarization of laser radiation

The formation of a fine polarization structure of biospeckles of derma is studied. In the case of scattered laser radiation, the degree of polarization is shown to be an integrated characteristic of randomly distributed types and forms of polarization of an ensemble of biospeckles. A method for phase compensation of the degree of depolarization is proposed, which makes possible a severalfold increase of the measurement sensitivity.     

Certain features of radiation transport in spatially bounded media with anisotropic scattering

The problem of radiation transport (RT) in media with a nonsymmetric scattering index is examined under multiple scattering conditions. The governing influence on the nature of the RT is exerted by the integral index parameters that take account of the forward-backward radiation. Nonsymmetry of the index /, equal to three produces a relative energy redistribution through the side faces of the medium within 20% limits, where the maximum redistribution is observed for small transverse dimensions _y=_z=5. For large transverse dimensions the effect of index nonsymmetry does not appear. It is shown that the degree of anisotropy is less in spatially bounded media as compared with an unbounded layer of equal optical thickness.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 12, pp. 69–72, December, 1989. Original article submitted January 11, 1988.     

Interaction of laser radiation with an axially symmetric polarization with condensed media

The interaction of laser radiation with an axially symmetric polarization with condensed media is studied theoretically. The interaction results in the formation of ordered micro- and nanoscale structures that can be described in terms of the universal polariton model of laser-induced media damage. It is shown that the radially polarized radiation induces a periodic ring resonance grating that focuses the excited surface plasmon-polaritons, thus increasing the total light intensity in the center of the irradiated zone.     

Principles of fluorescence laser tomography of strongly scattering media

The propagation of laser radiation and fluorescence excited by it in a strongly scattering medium are analyzed in the diffusion equation approximation. The possibility of applying efficient algorithms of the tomographic reconstruction with the use of mean paths of photons in biotissues to fluorescence laser tomography is substantiated.     

Spectral composition of radiation of supersonic CW chemical lasers: A DF laser

We experimentally studied the spectral parameters of the radiation generated by supersonic continuous-wave DF lasers with nozzle blocks of two different configurations (nozzle-nozzle and nozzle-radial-expansion-injector mixing schemes). We demonstrate the effects of the excess secondary-fuel (deuterium) ratio, the orientation of the optical cavity axis relative to the nozzle section, and gas-dynamical reactant mixing conditions on the spectral composition of the radiation. We show that the DF-laser spectrum can be controlled to a certain extent by varying these parameters.