Diffusion of collimated, narrow beam waves in discrete random media

The three-dimensional diffusion of a narrow beam wave in discrete random media is discussed. Mismatched boundary conditions are taken into account for surfaces on which the reflection of diffuse light occurs. An analytical expression is derived for the average diffuse intensity in terms of the sum of the residual values under practical situations of interest. The spatial spreading of beam waves for nonabsorption particles only slightly increases with an increase in the mean cosine of the scattering angle. A comparison with previously reported Monte Carlo and experimental results of the beam width versus the optical depth shows the validity of the analytical solutions obtained here. The effects of a mismatched boundary are shown to increase the transmitted diffuse flux rather than the spatial spreading.     

Measurement of aerosol-particle trajectories using a structured laser beam

What is believed to be a new concept for the measurement of micrometer-sized particle trajectories in an inlet air stream is introduced. The technique uses a light source and a mask to generate a spatial pattern of light within a volume in space. Particles traverse the illumination volume and elastically scatter light to a photodetector where the signal is recorded in time. The detected scattering waveform is decoded to find the particle trajectory. A design is presented for the structured laser beam, and the accuracy of the technique in determining particle position is demonstrated. It is also demonstrated that the structured laser beam can be used to measure and then correct for the spatially dependent instrument-response function of an optical-scattering-based particle-sizing system for aerosols.     

Thin-film characterization using a scanning laser acoustic microscope with surface acoustic waves

The use of surface acoustic waves in a scanning laser acoustic microscope for the characterization of the mechanical or acoustic properties of thin films deposited on piezoelectric substrates is demonstrated. Quantitative measurements of mass loading effects of 5000-A-thick tungsten films deposited on lithium niobate substrates were obtained using 100-MHz surface acoustic waves. No information about the tungsten film could be obtained using 100-MHz compressional waves. Methods of generating surface waves on nonpiezoelectric materials so that this technique could be used on arbitrary substrates are discussed.     

Effect of the finite dimensions of the probing beam on measurements using a laser scanning refractometer

Translated from Izmeritel'naya Tekhnika, No. 8, pp. 40–42, August, 1992.     

The manipulation of carbon nanotubes on a polymer surface using a laser beam

Controlled noninvasive manipulation of porphyrin-doped single-walled carbon nanotubes (SWCNT) by laser beam is described. SWCNT/porphyrin complexes have been deposited on a polymer surface and irradiated by a scanning beam of laser light with the wavelength of 405 nm. Laser energy was absorbed by the porphyrin and converted into heat through an energy transfer within the complexes. This led to periodical deformation of the initially flat polymer surface. As a result of the surface deformation the SWCNTs or SWCNT bundle move in the direction given by the laser scanning. It was proved that SWCNTs can be moved to a desired position using the focused laser beam.     

Reducing the uncertainty in laser beam size measurement with a scanning edge method

A modification in the analysis of a conventional laser beam spot size measurement method has been developed. The new analysis significantly decreases the uncertainty in the estimation of the beam-spot size. A conventional beam scanning approach was used in the measurement, but instead of differentiating the data and fitting the result to a Gaussian function, the data were fit to an analytical approximation to the complementary error function. As a result, fitted parameters were obtained that were consistent with the standard differentiation approach, but with considerably smaller uncertainty.     

Development of an algorithm for corneal reshaping with a scanning laser beam

The corneal-ablation rate, the beam-intensity distribution, and the initial and the desired corneal topographies are used to calculate a spatial distribution map of laser pulses. The optimal values of the parameters are determined with a computer model, for a system that produces 213-nm radiation with a Gaussian beam-intensity distribution and a peak radiant exposure of 400 mJ/cm(2). The model shows that with a beam diameter of 0.5 mm, an overlap of 80%, and a 5-mm treatment zone, the roughness is less than 6% of the central ablation depth, the refractive error after correction is less than 0.1 D for corrections of myopia of 1, 3, and 6 D and less than 0.4 D for a correction of myopia of 10 D, and the number of pulses per diopter of correction is 2500 when the beam-intensity distribution is Gaussian and 580 when it is flat.     

Sinking velocities of phytoplankton measured on a stable density gradient by laser scanning.

Two particular difficulties in measuring the sinking velocities of phytoplankton cells are preventing convection within the sedimenting medium and determining the changing depth of the cells. These problems are overcome by using a density-stabilized sedimentation column scanned by a laser. For freshwater species, a suspension of phytoplankton is layered over a vertical density gradient of Percoll solution; as the cells sink down the column their relative concentration is measured by the forward scattering of light from a laser beam that repeatedly scans up and down the column. The Percoll gradient stabilizes the column, preventing vertical mixing by convection, radiation or perturbation of density by the descending cells. Measurements were made on suspensions of 15mum polystyrene microspheres with a density of 1050kgm-3; the mean velocity was 6.28mums-1, within 1.5% of that calculated by the Stokes equation, 6.36mums-1. Measurements made on the filamentous cyanobacterium Planktothrix rubescens gave mean velocities within the theoretical range of values based on the range of size, shape, orientation and density of the particles in a modified Stokes equation. Measurements on marine phytoplankton may require density gradients prepared with other substances.     

Fast scanning method for one-dimensional surface profile measurement by detecting angular deflection of a laser beam

A fast scanning method for one-dimensional surface profile measurement is proposed. The profile is measured by integration of a slope distribution of the surface obtained from angular deflection of a scanning laser beam. A scanning optical system that consists principally of a spherical concave mirror and a rotating scanner mirror has reasonably low cost and is insensitive to mechanical vibration because of its high-speed scanning, of the order of milliseconds. A surface profile of a polygonal mirror along a 5-mm width was measured with the scanning method and with an interferometer. The root-mean-square difference between the two measured results is 0.98 nm.     

Refraction of a narrow probe beam using layered light diodes

Translated from Izmeritel'naya Tekhnika, No. 6, pp. 14–17, June, 1991.     

Propagation through the ocean's surface at shallow angles with a laser beam

Propagation through the ocean's surface at shallow angles with a laser has been successfully demonstrated experimentally in the ocean. A 6-mJ multipulse laser at a depth of 1.5 m in water with an attenuation coefficient of approximately 3.3 m(-1) was easily detected 600 m away on a sunlit day at an angle of 1 degree to the ocean's surface. The measured efficiency was 3.6 x 10(-2) (J/J)/sr (joules received per receiver steradian per joule transmitted). Statistical ray trace analysis and physical tank modeling produced equivalent efficiencies.     

Measurement of ion beam profiles in a superconducting linac with a laser wire

A laser wire ion beam profile monitor system has been developed at the Spallation Neutron Source accelerator complex. The laser wire system uses a single laser source to measure the horizontal and vertical profiles of a pulsed hydrogen ion (H(-)) beam along a 230 m long superconducting linac, which accelerates H(-) from 200 MeV to 1 GeV. In this paper, we describe the laser optics requirement for the system, the performance of the profile measurement, and the effects of laser parameters on the measurement reliability. The result provides a practical guideline for the development of a large-scale, operational, laser-based diagnostics in accelerator facilities.     

The effect of laser surface nitriding with a spinning laser beam on the wear resistance of commercial purity titanium

Laser nitriding of commercial purity titanium using various concentrations of helium and nitrogen has been carried out. The surface appearance and microstructure of a treated layer were found to be dependent on the beam power density, interaction time, velocity and concentration of nitrogen. X-ray diffraction analyses have led to the conclusion that the dendrite layer in the resolidified zone of the nitrided specimens consisted mainly of TiN. The surface roughness of specimens after various laser treatments was investigated by SEM and a surface profilemeter. Using optical microscopy, the dendrite TiN and needle-like structure in the melt zone, and the large grain structure in the heat affected zone, were investigated. The surface wear resistance of nitriding CPTi was significantly improved compared to the untreated or laser glazed material, and the wear data were found to correlate with scanning electron microscopy observations. Two layers, having different microstructures, thickness and abrasive wear resistance were identified. Further, 100% overlapping considerably improved the wear resistance of the nitrided specimens.     

Calculation of a steady-state thermal regime in electron-beam autocrucible melting in the case of circular scanning with a beam

We suggest a mathematical model of a steady-state thermal regime in electron-beam autocrucible melting. We carry out a numerical investigation of the dependence of the thermal parameters of melting on the radius of circular scanning by the beam, the focal spot radius, and on the mean rate of mixing of the melt. The results of calculations are compared with experimental data. Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 70, No. 3, pp. 454–461, May–June, 1997.     

Power collection of a laser diode elliptical beam by a decentered circular aperture

The elliptical beam of a laser diode is collected by a circular aperture decentered with respect to the beam. The fractional optical power collected is calculated and measured as a function of the decentered distance, beam size, and aperture size. The calculation results agree well with the measurement results. An application example of the results is described.     

Exact spread function for a pulsed collimated beam in a medium with small-angle scattering

A solution has been obtained for the spatial and temporal distribution function for a pulsed fully collimated beam propagating through a homogeneous medium with Gaussian small-angle scattering. The solution was obtained first by separation of the general problem into two plane problems, which results in a partial differential equation in three variables. A Fourier transform on two projected variables (one angular and one spatial) and a Laplace transform on the projected temporal variable yielded a set of nonlinear differential equations, which were solved. A recursion relation for the moments of the distribution function was also obtained, and the software MATHEMATICA was used to evaluate these moments to high orders. The contractions on certain variables are also presented; they correspond to the solutions of less-general problems contained in the main problem. A change in the definition of the time-delay produces a remarkable change in the structure of the equations. These solutions should be quite useful for lidar studies in atmospheric and oceanic optics, x-ray and radio-wave scattering in the atmosphere and interstellar medium, and in medical physics.     

Thermal field on the surface of a material heated by a laser pulse in a narrow ring of illumination

Based on a solution of the problem of the temperature effect of instantaneous heat sources uniformly distributed in a thin plane ring on the boundary of a semünfinite body and different approximations of that solution, we constructed a dynamic model of a thermal field on the surface of a material heated by a laser pulse in a narrow ring of illumination. Scientific Center for Opticophysical Investigations Moscow. Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 69, No. 1, pp. 55–64, January–February, 1996.