摘要:

A method is presented to calculate the force exerted on a conducting sphere by another conducting sphere in a parallel electric field. The method allows any size ratio, distance, electric charge, and field angle. It uses a number of electric charge images and dipole images, depending on the required accuracy and the distance between the spheres. ©1995 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.360516

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

Laser welding is a unique way of joining materials with less thermal distortion and minimum metallurgical damage to the workpiece. The molten pool formed during welding determines the shape of the final welded region. At high laser intensities, the molten material vaporizes and a key hole is formed during the welding process. This vapor and the shape of the molten pool affect the absorption of laser at the liquid surface. The forces generated at the liquid‐vapor interface due to surface tension gradient induce thermocapillary convection in the weld pool. This paper presents a mathematical model by considering these surface forces and the energy balance at the liquid‐vapor and solid‐liquid interfaces. The model is used to predict the surface velocity and temperature distributions, weld pool shape, key‐hole depth and diameter. The velocity field is found to be large in the radial and azimuthal directions before the key hole is formed, and it changes to a radially and axially dominant field after the formation of the key hole. The results of this model are also compared with experimental data. ©1995 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.360686

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

Transverse beam profiles on traveling‐wave optical parametric generation in KTiOPO4crystals pumped by short optical pulses are investigated. The transverse beam profile deformation and splitting due to the back‐conversion in the traveling‐wave parametric propagation process is clarified for the first time from experimental observations and calculations. The transverse beam profile becomes ring shaped when there is no walk‐off and deforms by expanding to the walk‐off direction when walk‐off exists. The deformation of the profile can be reduced by using a short crystal or decreasing the pump intensity due to the subsequent reduction in back‐conversion. However, in these cases, the conversion efficiency becomes very low (3%–4%). We can improve the conversion efficiency while keeping good transverse beam profiles by using the two‐crystal beam overlapping technique. The optimum conditions for this technique are clarified through calculations and it is shown that conversion efficiency of more than 18% can be achieved when they are used. ©1995 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.360517

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

The properties of an oxyfluoride glass ceramic that possesses high transparency after ceramming are described. Approximately 25 vol % of this material is comprised of cubic, fluoride nanocrystals and the remainder is a predominantly oxide glass. When doped with Pr+3, the fluorescence lifetime at 1300 nm is longer than in a fluorozirconate glass, suggesting that a significant fraction of the rare‐earth dopant is preferentially partitioned into the fluoride crystal phase. This material has the added advantage of being compatible with ambient air processing. ©1995 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.360518

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

A finite difference method is used to analyze the behavior of photorefractive InP:Fe at high fringe contrast under externally applied field. The evolution with increasing fringe contrastmof the electron‐hole resonance, which occurs in the presence of a continuous field, is studied. Asmincreases, the field tends to concentrate in a small zone and reaches very large local values. On the other hand, the resonance loses height and widens. For illuminations closed to the resonance, the two‐wave mixing gain presents the usual aspect of a decreasing function ofm. However, for other illuminations, it can be larger at a small pump‐to‐probe ratio than at a large one. When an ac field technique is used, nonlinearities drastically reduce the two‐wave mixing gain, even at small fringe contrast. Asmincreases, the space‐charge field tends to take a square shape. Even for rather smallmvalues (0.1), the gain presents a maximum near the dc field resonant illumination, which is not predicted by the linear theory. Finally, when drift is the dominant process for the grating formation, the applied field limits the amplitude of the space‐charge field, independently of an optimized dopant concentration which allows high gain in the smallmapproximation. ©1995 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.360519

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

The thermally induced microbending losses in initially curved double‐coated optical fibers at low temperature are investigated. The deflections in an initially curved fiber increase gradually with an increase in the thermally induced compressive axial force. The increase of deflection in an optical fiber results in an increase of microbending loss. In order to minimize such a microbending loss, the Young’s modulus and Poisson’s ratio of the primary coating should be increased. This means that a best selection of the Poisson’s ratio of the primary coating is 0.5. On the other hand, the effective thermal expansion coefficient of the primary coating, and the thickness, Young’s modulus, and effective thermal expansion coefficient of the secondary coating should be decreased. However, there exists an optimum value for the thickness of the primary coating. The thermally induced lateral pressure on the glass fiber can also produce microbending loss. Both effects of the axial force and lateral pressure on the microbending losses must be considered, so there exist optimum values for the Young’s moduli and effective thermal expansion coefficients of the primary coatings. An easy rule for the selection of the polymeric materials and their thicknesses to minimize the low‐temperature microbending losses are also proposed. ©1995 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.360520

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

It has been observed experimentally that bubbly layers generated by breaking waves form nearly exponential waveguides near ocean surfaces and that noise propagation in such waveguides occurs in distinct frequency bands [D. M. Farmer and S. Vagle, J. Acoust. Soc. Am.86, 1897 (1989)]. It was conjectured that the well‐defined frequency bands are due to the normal mode propagation of sound. Motivated by the experimental observations, a theory is presented to investigate the normal mode propagation in exponential waveguides, by analogy with the optical waveguides formed by diffusion [Z. Ye, Appl. Phys. Lett.65, 3173 (1994)]. The theoretical results compare favorably with the experimental data. In addition, we show that a waveguide with an exponential sound speed profile has great mathematical convenience in that it has simple characteristic equations which can be solved in a closed form. The number of normal modes, which is shown to correspond to the number of discrete acoustic bands, can be determined by waveguide parameters through a simple equation. The results are useful for solving inverse problems associated with exponential waveguides. The present exponential sound speed profile is also compared to the inverse square profile that has been studied extensively in the past. ©1995 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.360521

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

A fast method is presented for accurately calculating just the steady‐state temperature in thin‐film multilayers heated by continual laser irradiation. The key features are a simple coordinate transformation and a special lattice with which the steady‐state portion of the heat conduction solution reported earlier [J. Appl. Phys.75, 4382 (1994)] may be quickly and efficiently extracted. The earlier solution, expressed in terms of Fourier‐transformed Green’s functions, was designed for temperature problems as typically found in optical disk recording situations. In particular, the laser irradiation was assumed to be modulated arbitrarily in time and finite in duration. As the pulse length increases, though, both execution time and memory requirements increase. Thus when long laser pulses are used to model continual laser irradiation, the numerical performance may degrade to unacceptable levels. For the case of periodically modulated laser irradiation (including constant laser irradiation as a special case) which is important for magneto‐optical disk recording, the time and memory requirements can be reduced substantially by defining a new coordinate and lattice system in the space spanned by the time and scan directions. In frequency space, the new lattice filters out the frequencies which contribute to the transient part of the solution and retains only the frequencies which contribute to the steady‐state part. The temperature formulas themselves remain unchanged; they are merely evaluated over a specially defined lattice which yields just the steady‐state temperature more quickly. This paper describes the new coordinate and lattice systems, discusses some numerical issues, and compares the results of the steady‐state theory to the results of the earlier original theory which includes the transients. ©1995 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.360522

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

The three‐dimensional equation of heat conduction is solved to obtain the time‐dependent (harmonic) temperature field in an opaque, thermally anisotropic layer on a thermally thick and isotropic substrate when the system is heated by a modulated Gaussian laser beam. The influence of the anisotropy on the amplitude and phase response of the temperature variation as a function of the position on the surface and depth in the layer is studied. The limiting case of one‐dimensional heat diffusion in highly anisotropic media is discussed. For one example the influence of thermal anisotropy on the frequency dependence of the surface temperature distribution is studied. ©1995 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.360523

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

X‐ray emission from pinch plasma devices with pinch currents ranging from 200 to 400 kA operated with pure high‐Zgases are investigated with temporal spatial and spectral resolution. If operated using elementsZ<18 (e.g., nitrogen or neon)K‐shell emission is observed from column‐like volumes several 100 &mgr;m in diameter and several mm in length (bulk or column mode). ForZ≳18 (e.g., krypton or xenon) emission withh&ngr;≳1 keV is only observed from micropinches. For argon (Z=18) both modes of operation can be observed. The occurrence of a specific mode depends on the initial gas pressure. In this paper the transition regime between column and micropinches is investigated with particular regard to argon. A criterion is proposed to decide whether column mode or micropinch mode is expected. ©1995 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.360524

出版商:AIP    

年代:1995

数据来源: AIP