Scintillation index of optical spherical wave propagating through biological tissue

Effects of the tissue turbulence on the propagation of an optical spherical wave are analysed. For this purpose, scintillation index of an optical spherical wave which is propagating in a soft tissue is formulated and evaluated in weakly turbulent soft tissue. Scintillation index of the optical spherical wave is examined against the changes in the tissue parameters which are the tissue length between the optical spherical wave source and the detector, random variations in the refractive index of the tissue and the outer scale of the tissue turbulence. According to our graphical outputs, it is observed that increase in the random variations of the refractive index of the tissue results in an increase in the scintillation index at a certain realization of the turbulence spectrum. On the other hand, larger outer scales and longer tissue lengths yield larger scintillations. The variation of the scintillation index of the optical spherical wave versus the wavelength is also investigated. It is found that at small tissue lengths, wavelength has almost no effect on the scintillations; however, when the tissue length reaches a certain value, shorter wavelengths give rise to larger intensity fluctuations.     

Trivariate characteristics of intensity fluctuations for heavily saturated optical systems

Trivariate cumulants of intensity fluctuations have been computed starting from a trivariate intensity probability distribution function, which rests on the assumption that the variation of intensity has a maximum entropy distribution with the constraint that the total intensity is constant. The assumption holds for optical systems such as a thin, long, mirrorless gas laser amplifier where under heavy gain saturation the total output approaches a constant intensity, although intensity of any mode fluctuates rapidly over the average intensity. The relations between trivariate cumulants and central moments that were needed for the computation of trivariate cumulants were derived. The results of the computation show that the cumulants have characteristic values that depend on the number of interacting modes in the system. The cumulant values approach zero when the number of modes is infinite, as expected. The results will be useful for comparison with the experimental triavariate statistics of heavily saturated optical systems such as the output from a thin, long, bidirectional gas laser amplifier.     

Generalized modified atmospheric spectral model for optical wave propagating through non-Kolmogorov turbulence

A new generalized modified atmospheric spectral model is derived theoretically for wave propagating through non-Kolmogorov turbulence, which has been reported recently by increasing experimental evidence and theoretical investigation. The generalized, modified atmospheric spectrum considers finite turbulence inner and outer scales and has a spectral power law value in the range of 3 to 5 instead of the standard power law value of 11/3. When the inner scale and outer scale are set to zero and infinity, respectively, this spectral model is reduced to the classical non-Kolmogorov spectrum.     

Computation of quasi-discrete Hankel transforms of integer order for propagating optical wave fields

The method originally proposed by Yu et al. [Opt. Lett. 23, 409 (1998)] for evaluating the zero-order Hankel transform is generalized to high-order Hankel transforms. Since the method preserves the discrete form of the Parseval theorem, it is particularly suitable for field propagation. A general algorithm for propagating an input field through axially symmetric systems using the generalized method is given. The advantages and the disadvantages of the method with respect to other typical methods are discussed.     

Partially coherent dark hollow beams propagating through real ABCD optical systems in a turbulent atmosphere

A closed-form analytical expression is derived for a partially coherent dark hollow beam (DHB) propagating through an arbitrary real ABCD optical system in a turbulent atmosphere. The average intensity of the beam in the output plane is investigated in the presence of, respectively, a thin lens image system and a two-lens system along the optical path. For a special thin lens image system, the partially coherent DHB and the fully coherent DHB have the same evolution properties, and the comparative analysis is made between the propagation of the focused DHB and the collimated DHB for direct propagation in turbulence to show the effect of the thin lens on the average intensity. As for the two-lens system, the effects of the lens systems, the structure constant in the turbulent medium and the parameters of the incident beam on the average intensity are evaluated and illustrated. The result shows that different lens systems and propagation parameters can evidently affect the evolution properties of the beam.     

All-optical isolator under arbitrary linearly polarized fundamental wave in an optical superlattice

We theoretically investigate an all-optical isolator under arbitrary linearly polarized fundamental wave (FW) input in an optical superlattice (OSL). The scheme is based on simultaneously phase matching the first-order Type I (oo-e) quasi-phase-matching (QPM) second-harmonic generation (SHG) process and higher-order Type 0 (ee-e) QPM SHG process in an OSL with a defect inserted in an asymmetrical position. Simulation results show that the contrast ratio of the all-optical isolator can maintain close to 1 under arbitrary linearly polarized FW. Thus, an all-optical isolator based on an OSL that is not sensitive to the direction of linear polarization can be realized. We also show that, with the defect in a strong asymmetry position, the length of the defect can be designed flexibly to maintain a high contrast ratio. Additionally, if the length of the OSL is longer, the nonreciprocal response can be realized for low optical intensities.     

Emission of a surface acoustic wave by an internal acoustic wave propagating near the surface

It was observed that the propagation of an internal acoustic wave near a surface is accompanied by the excitation of a surface acoustic wave directed at an angle to the internal wave. Pis’ma Zh. Tekh. Fiz. 24, 57–61 (September 26, 1998)     

Wind profiling based on the optical beam intensity statistics in a turbulent atmosphere

Reconstruction of the wind profile from the statistics of intensity fluctuations of an optical beam propagating in a turbulent atmosphere is considered. The equations for the spatiotemporal correlation function and the spectrum of weak intensity fluctuations of a Gaussian beam are obtained. The algorithms of wind profile retrieval from the spatiotemporal intensity spectrum are described and the results of end-to-end computer experiments on wind profiling based on the developed algorithms are presented. It is shown that the developed algorithms allow retrieval of the wind profile from the turbulent optical beam intensity fluctuations with acceptable accuracy in many practically feasible laser measurements set up in the atmosphere.     

Fractality of speckle intensity fluctuations

Coherent-light diffraction on random phase screens with fractal properties leads to the formation of speckle patterns with peculiarities in correlation characteristics in the small-scale region. Such peculiarities are manifested in asymptotic behavior in intensity autocorrelation and structure functions in the vicinity of the zero values of their arguments. Intensity fluctuations in the far and the near diffraction zones are also characterized by values of fractal (Hausdorff-Besicovitch) dimensions D(HB), differing from the corresponding Euclidean dimension. Relationships between the exponential factors of the structure functions of boundary field phase and scattered-light intensity fluctuations as well as between values of D(HB) have been obtained as a result of speckle-formation analysis for different conditions. Their dependencies on the illumination and observation conditions obtained in experiments with fractallike scatterers (rough glass plates) are in satisfactory agreement with theoretical results.     

Beam spreading of vortex beams propagating in turbulent atmosphere

We present some results obtained by numerical modeling of the propagation of vortex beams LG(0l) through a randomly inhomogeneous medium. The vortex beams are the lower order Laguerre-Gaussian modes. Such beams, if propagated under conditions of weak turbulence, also experience distortions, like a Gaussian beam. However, the statistically averaged vortex beams (LG(0l)) conserve the central intensity dip with a nonzero intensity on the beam axis. The beam broadening of vortex beams is analyzed. The average vortex beams are found to be broadened less than the Gaussian beam while propagated through a randomly inhomogeneous medium. The higher the topological charge l is, the smaller the beam broadening is.     

Stress intensity factor histories of a steadily propagating crack under 3-D combined mode traction

Elastodynamic stress intensity factor histories of an unbounded solid containing a semi-infinite plane crack that propagates at a constant velocity under 3-D time-independent combined mode loading are considered. The fundamental solution, which is the response of point loading, is obtained. Then, stress intensity factor histories of a general loading system are written out in terms of superposition integrals. The methods used here are the Laplace transform methods in conjunction with the Wiener-Hopf technique. This revised version was published online in August 2006 with corrections to the Cover Date.     

Analogy between turbulent momentum and heat transfer under complex conditions

The analogy between turbulent momentum and heat transfer under complex conditions, i.e., under the action of several perturbing factors on the flow, is extended for a broad range of variation of the Prandtl number.Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 49, No. 3, pp. 406–413, September, 1985.     

The kinetics of excited atoms and optical radiation under conditions of wave mechanism of breakdown in inert gases

Based on an analysis of the results of integrated experimental and theoretical investigations, modern concepts are described of the kinetics of excited atoms and optical radiation under conditions of the wave mechanism of breakdown in inert gases. Primary attention is given to optical investigations in the gas pressure range from 1 to 1000 torr with voltage pulse amplitudes of up to 60 kV. The effect of fast electrons, formed at the ionization wave front, on the intense and polarization profiles of spontaneous spectral lines is treated. The role of the processes involving molecular ions in the formation of the optical properties of plasma.developed by wave breakdown is discussed. The possibility is studied of using such discharges for pumping recombination gas lasers.     

Numerical investigation of heat transfer under laminarization conditions of turbulent flows

Based on the mathematical boundary layer model, a numerical investigation of heat transfer is carried out for a wide range of turbulent Reynolds numbers in nozzles of experimental devices under laminarization conditions of turbulent flows.Translated from Inzhenerno-fizicheskii Zhurnal, Vol. 61, No. 2, pp. 223–229, August, 1991.     

Simulation of unsteady-state hydrodynamics under conditions of turbulent flow in pipes

Modern approaches to simulation of turbulent flows are used to analyze the effect of nonstationarity of the flow rate on the parameters of turbulence. The flow rate variation in the case of homogeneous turbulence with constant shear of velocity is treated. The dynamics of the second moments of velocity fluctuations at constant flow rate are investigated. Calculations of unsteady-state pipe flow are performed within the two-parameter model of turbulence. Qualitative comparison is made of the theoretical and experimental results.__________Translated from Teplofizika Vysokikh Temperatur, Vol. 43, No. 2, 2005, pp. 231–248.Original Russian Text Copyright © 2005 by I. V. Derevich.     

Comparison of the heate-xchange intensity under nonstationary and stationary conditions

It is established experimentally that the coefficients of heat exchange in nonstationary processes differ from the coefficients of heat exchange obtained from the classical heattransfer formulas. Experimental results are obtained in conformity with which the coefficient of heat exchange depends on the volume specific heat and geometric size of the body.Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 18, No. 2, pp. 210–215, February, 1970.     

Experimental investigation of heat transfer under conditions of laminarization of turbulent flows

The results of an experimental investigation of the temperature in model nozzles of an experimental setup under conditions of laminarization of turbulent flows are presented.Translated from Inzhenerno-fizicheskii Zhurnal, Vol. 60, No. 2, pp. 181–185, February, 1991.     

Some discussion on moving strong discontinuity under plane stress conditions

This paper deals with moving strong discontinuities in an elastic ideally plastic solid under plane stress conditions for a Huber-Mises material. Small strain formulation is employed, inertia terms are ignored (quasi-static), and the in-plane displacements are assumed to be continuous in the body during the deformation process. It is shown that stress discontinuity is not permissible anywhere in the body. The only permissible jumps are the strains and material velocities, and they must occur across a characteristic surface. A restriction for material velocity jumps is given. The results are discussed and compared with those obtained from rigid plastic theory.     

Limits on achievable intensity reduction with an optical occulter

Deep shadowing of a normally incident plane wave by an opaque circular disk is partially negated by the formation of a region of strong intensity surrounding the axis passing normally through the disk center. This local intensity enhancement, historically referred to as the Poisson Spot (also known as the Spot of Arago), has been the principal source of difficulties in applications where a significant reduction of the incident intensity is essential. In particular, the NASA Terrestrial Planet Finder's (TPF) mission requires suppression of direct starlight by at least 10 orders of magnitude over the entire visible spectral range. One technique that has been proposed for blocking the direct starlight is to use a rotationally symmetric disk with petallike segments along its boundary. We find that, even though such configurations could, indeed, theoretically provide the desired intensity reduction, they would require unreasonably small radii of curvature at the petals' tips (in the range of micrometers). When the radii of curvature are increased to 3 mm, the intensity reduction drops to a modest 5 to 6 orders of magnitude. Given that for the NASA's TPF mission the proposed occulter radius would be on the order of 25 m, even the 3 mm radius of curvature would be too small for any practical implementation. Further increases of the radius of curvature result in progressively poorer intensity suppression. As an alternative solution we propose an apodized circular disk. We show that with an optimized apodization function, intensity reductions of at least 10 orders of magnitude can be achieved over the entire visible spectral range. Numerical results are presented for parameters appropriate to the NASA TPF mission.