Wavefront Profile Research Articles

Acoustic emission source location in bidirectionally reinforced composites, Part 2: Wavefront profile and elastic modulus correlation : Ibitolu, E.O.; Summerscales, J. Royal naval engineering college, Plymouth (United Kingdom), RNEC-RP-87033, 12 pp. (Sep. 1987)

Acoustic emission source location in bidirectionally reinforced composites, Part 2: Wavefront profile and elastic modulus correlation : Ibitolu, E.O.; Summerscales, J. Royal naval engineering college, Plymouth (United Kingdom), RNEC-RP-87033, 12 pp. (Sep. 1987)

Wavefront and amplitude profile for astigmatic beams in semiconductor lasers: analytical and graphical treatment

The problem of the propagation of astigmatic beams, in particular those produced by semiconductor lasers, is treated in analytic and graphical forms. The authors use hyperbolic and ellipsoidal curves to describe the propagation of the Gaussian width and the wavefront surface in both perpendicular planes defined by the astigmatic beam. To find all the parameters they need only the divergence of the beam.

Propagation of ultrashort optical pulses and computation of pulse evolution in free space by numerical simulations.

Pulse propagation in a vacuum is described as being the superposition of elementary pulses given by the time derivative of the initial pulse and weighted by local inclination factors attached to the pulse wave front. Optical pulses with any wave-front profile and propagating in near-field and far-field regions are considered. The analytical expressions thus obtained demonstrate the possibility to compute the pulse structure as a function of time and its evolution through numerical simulations.

Compensation of nonlinear distortions of light beams in the case of restricted deformation of the control mirror

An analysis is made of the influence of constraints relating to the profile of a flexible mirror on the quality of compensation of nonlinear distortions of light beams which are scanned and have a specific transverse distribution. The effects of propagation across thick and thin nonlinear layers are considered. Studies are made of the problems of control of the focusing and tilt of the wavefront of the scanning beam and of the influence of restrictions on the aberration modes when adaptive systems are tuned using the peak intensity or the beam width. It is shown numerically and analytically that it is desirable to use beams with a specific transverse distribution when restrictions are imposed on the wavefront profile.

Spatial and temporal properties of a tunable picosecond dye-laser oscillator–amplifier system

In this paper, we describe a high-power picosecond dye-laser system providing tunable 1.5-mJ pulses with a uniform wave front and Gaussian spatial profile. With the aid of a computer-controlled data acquisition system, we have analyzed for the first time the temporal coherence properties of such a system.

Shock Compression and Shock-Induced Radiation Measurements on Calcium Fluoride

Results are presented from a study of a compression wavefront profile for a single crystal of calcium fluoride (CaF2) carried out using a particle velocity gauge. The Hugoniot elastic limit and two polymorphic phase transitions are observed at 1.8 GPa, 5.2 GPa, and 12 GPa, respectively. Shock-induced thermal radiation measurements are also carried out in the shockpressure range from 30 GPa to 70 GPa. A simultaneous spectrometric multichannel analyzer system is developed to observe the shock-induced radiation emitted from a CaF2 crystal. The resultant color temperatures are high, with a small value of emissivity. The pressure dependence of the color temperature behaves anomalously owing to the phase transition, whereas the brightness increases monotonically with increasing pressure.

Dynamic Plasticity

Recent advances in the understanding of the dynamic plastic response of crystalline solids are discussed. At the level of individual dislocations progress is being made on measurements of dislocation mobility at high stress levels and on elastodynamics solutions for transient dislocation motions. More progress is required on the understanding of changes in mobile dislocation density during dynamic plastic deformation. Widespread use of the Kolsky (or split-Hopkinson) bar has resulted in a reasonably clear picture of the dependence of flow stress on plastic strain rate for polycrystalline metals deformed at strain rates up to 103s−1. Influences of strain-rate history, temperature, and pressure require further investigation. At strain rates of approximately 103s−1’ to 105s−1 there is increasing evidence of a marked increase in flow stress with increasing strain rate. Pressure-shear plate impact experiments appear to be attractive for studying plastic response in this high strain-rate regime. Differences, if any, between stress-path effects at quasi-static strain rates and at strain rates of 103s−1 and higher remain poorly understood. Larger-than-predicted precursor decay observed in plate impact experiments on single crystals remains unresolved and of continuing fundamental interest; however, the importance of precursor decay measurements in determining dynamic plastic response appears to be diminishing because surface effects and limitations on the resolution of wave-front profiles represent serious constraints on the inferences that can be drawn from precursor decay measurements. Modeling of dynamic plastic response of polycrystals in terms of the response of slip systems is in an early stage of development. Kinematics of finite deformation by crystalline slip and consistent averaging techniques for modeling polycrystalline response are understood reasonably well. Increased emphasis on the understanding of the dynamic plastic response of single crystals and on the influence of microstructure appear desirable for sustained progress. Physically based models of wide applicability are required.

Performance of a laser beam wave-front sensor

Laser beam wave-front measurement is important for laser beam quality evaluation and beam control. In this paper we report on a laser beam wave-front sensor consisting of a phase detector and a scanner. With the use of this sensor, the wave-front phase profile across a hydrogen fluoride (HF) laser beam has been obtained. The major advantages of this technique are that it is fast, sensitive, and compact. Other features are electric scanning, self-alignment, noise cancellation, and no drift. Wave-front sensor operational principles, features, and experimental results are presented. The sensitivity of the sensor was 5°. This sensitivity corresponds to the change of beam quality parameter, the Strehl ratio, by 1%. Hence it is a sensitive beam quality measurement technique.

A critical analysis of ionising wave propagation mechanisms in breakdown

The formation and propagation of ionising waves in impulsive overvolted gas discharges for parallel plane electrode geometry is studied. The spatio-temporal evolution of the electronic and ionic densities together with the space-charge distorted field was thoroughly examined for different initial electron distributions. The study elucidates the important role of the electrons present ahead of both the anode and cathode ionising waves and shows particularly how the wavefront profile evolves during its propagation. In all cases, provided that the discharge gap is sufficiently large, the wavefront is transformed into an electron shock wave defined by a shock zone of extremely high electronic density and field gradients. This limits the domain of validity of the gas discharge models that neglect both the thermal diffusion of charge carriers and gas photoionisation of photodetachment.

Electrophysiological studies in four patients with atrial flutter with 1:1 atrioventricular conduction

Electrophysiological studies of atrioventricular conduction during rapid atrial overdrive pacing and during programmed premature atrial stimulation are reported in four patients with an unusually rapid 1:1 ventricular response to atrial flutter (ventricular rates 240 to 310 per minute). Second-degree AV block developed during atrial overdrive pacing at rates well below those during which 1:1 AV conduction was sustained during spontaneous atrial flutter. Although none of the four patients showed evidence of pre-excitation on the standard 12-lead electrocardiogram, evidence suggesting a partial AV nodal bypass was demonstrated at electrophysiological study in one case. It is postulated that the profile of the atrial wavefront presented to the normal AV node by atrial flutter differs from that during high right atrial pacing and may account for the lower ventricular rates achieved during high right atrial overdrive pacing than during spontaneous atrial flutter in the remaining three cases.

Fluctuations in radiation scattered into the Fresnel region by a random-phase screen in uniform motion

The scattering of radiation by a rigid deep random-phase screen in uniform linear motion is studied and formulae are derived for the spatial and temporal correlation functions of the scattered field and intensity in the Fraunhofer and Fresnel regions. A joint Gaussian distribution is used to represent the phase screen and the illuminating beam is assumed to have a curved wavefront and Gaussian intensity profile. It is shown that the coherence properties of the scattered radiation depend on an apparent area of illumination which is a function both of the actual width of the illuminating beam and of the slope distribution of the scattered wavefront. The dependence of the second intensity moment on distance from the screen is discussed and compared with that predicted by previous authors.