Uniform Laser Beam Research Articles

Formation of the uniform irradiation of a target in high-power laser facilities

The methods are developed for obtaining highly uniform laser beams for direct irradiation of targets in high-power laser facilities being designed. The methods are based on the optimisation of the mutual arrangement of the beams irradiating a target, the use of stationary phase plates for the formation of the specified radiation intensity distribution on the target, and the employment of the spatiotemporal beam smoothing to suppress small-scale inhomogeneities. The requirements on different systems of the facility are considered which provide the necessary accuracy of the beam steering on a target and the admissible difference in the intensities of the beams. A method is proposed for the spatiotemporal beam smoothing using the nonstationary properties of a laser plasma. The possibilities of this method are studied experimentally.

Holographic Recording and Control of Diffraction Efficiency Using Photoinduced Surface Deformation on Azo-Polymer Films

Surface relief holograms were fabricated on azo-polymer films by the irradiation of interference laser fringes. The side-chain azo-polymer, poly-orange tom-1 isophoronediisocyanate, was used in this study. Recording characteristics of surface relief structures were investigated; they needed no post-treatment, and could be erased by heating or irradiating a uniform laser beam. The diffraction efficiency of the recorded hologram was markedly increased by corona charging. It was also controlled by irradiation of the laser beam (488 nm) with corona charging.

Laser pulse modulation instabilities in plasma channels

In this paper the modulational instability associated with propagation of intense laser pulses in a partially stripped, preformed plasma channel is analyzed. In general, modulation instabilities are caused by the interplay between (anomalous) group velocity dispersion and self-phase modulation. The analysis is based on a systematic approach that includes finite-perturbation-length effects, nonlinearities, group velocity dispersion, and transverse effects. To properly include the radial variation of both the laser field and plasma channel, the source-dependent expansion method for analyzing the wave equation is employed. Matched equilibria for a laser beam propagating in a plasma channel are obtained and analyzed. Modulation of a uniform (matched) laser beam equilibrium in a plasma channel leads to a coupled pair of differential equations for the perturbed spot size and laser field amplitude. A general dispersion relation is derived and solved. Surface plots of the spatial growth rate as a function of laser beam power and the modulation wave number are presented.

Excitation of electrostatic waves at the beat frequency of two laser beams in a semiconductor plasma

A theoretical investigation has been made of the parametric excitation of electrostatic waves at the beat frequency of two high-power, collinear, uniform laser beams in a highly collisional, magneto-active, compensated semiconductor plasma. The gyro-kinetic approach has been employed to find the linear and nonlinear response of electrons and holes in this semiconductor plasma. It is found that for typical plasma parameters in compensated Ge, two collinear laser beams of equal power density, 2.37 MW cm-2, can excite beat waves of power density 1 kW cm-2. The effects of electron-phonon collision frequency (v), transversely applied external magnetic field (B0), plasma density (n0), and variation of the effective mass of electrons and holes on this parametric excitation process in a semiconductor plasma are discussed.

Corneal ablation profilometry and steep central islands.

Photorefractive keratectomy with large diameter ablations using a uniform laser beam has produced central undercorrections, or "steep central islands" in patients, as seen with videokeratography. Using a custom optical profilometer to measure corneal ablation profiles and a VISX excimer laser system, we measured the effect of ablation algorithms, diameter, depth, and dioptric correction on enucleated porcine eyes and living rabbit eyes. Our profilometer was verified using a 43.00 diopter (D) spherical surface and a 35.00 and 43.00 D bicurve test surface as a model for the ablated cornea. The profilometer measured the test surfaces to within 3 microns of predicted values. Photorefractive keratectomies showed over-ablation peripherally and under-ablation centrally which increased with ablation diameter and dioptric correction. Fixed diameter ablations 2 to 6 mm in diameter and 10 to 80 microns deep showed stromal ablation rates vary spatially but not with ablation depth. These spatially variant ablation profiles were used to re-engineer the ablation algorithm and to produce photorefractive keratectomies with improved sphericity. Steep central islands are caused by the spatial variance of tissue ablated with a uniform laser beam irradiance. This aberration can be corrected by modifying the laser ablation algorithm to correct for the spatial variance of stromal ablation.

均一化レーザ光によるフォトリフラクティブ媒質からの位相共役波の発生と光画像伝送への応用

均一化レーザ光によるフォトリフラクティブ媒質からの位相共役波の発生と光画像伝送への応用

可変反射体を用いた波動の強度分布整形

In some applications of wave like laser or micro wave, it is necessary to reshape the beam intensity profile. For example, a rectangular and uniform intensity laser beam is very suitable for the laser photo chemistry. In this paper, we propose a beam-forming system which consists of deformable reflectors whose surfaces are specially deformed like deformable mirrors. We simulate the beam forming property using the Fresnel equation which is solved by Fourier transformation. A Gaussian beam is transformed to a rectangular uniform beam. The surface profile of the phase distortion compensation reflector is also calculated and it is simple enough to be compensated by a reflector with simple surface profile.

Amplification of fibre Bragg grating reflectivityby post-writingexposure with a 193 nm ArF laser

The reflectivity of weak in-fibre gratings (a few percent reflectivity) written by an ArF laser illuminated phase mask can be dramatically enhanced (reaching >or=95%) by subsequent exposure using a uniform ArF laser beam. This new effect offers the possibility of post-writing control of grating properties.

レーザによる薄板の部分硬化技術の開発

Selective laser-hardening of steel sheet has been developed. By this method, only selected region of steel sheet can be hardened. So, forming process can be followed by hardening process. It is very effective, because steel sheet can be formed while it is ductile and after that the only region required to be hardened can be heat treated. It is necessary to develop two core technologies ; (1) uniformly heating of selected region of steel sheet and (2) realizing the optimized heat cycle for laser-hardening. As for (3), the special integration mirror for shaping a uniform laser beam of 25 mm × 25 mm square has been developed based on the ray-tracing method. Moreover, the combined optical system, which enlarges the uniformly heated area 9 times as large as the case of a uniform heat source, has been designed based on the heat conduction model for laser static heating. As for (2), the optimum heat cycle including soaking for hardening has been determined. And, in order to realize this heat cycle, a control system consisted of a non-contact temperature detector, a PID (proportional integral differential) controller and a laser power regulator, has been developed. Finally, by this selective laser-hardening method using the special integration mirror and the heat cycle control system, hardness of test piece of steel sheet has been increased from 130 to 200 in Vickers hardness.

Formation of a laser beam with a uniform spatial distribution

The feasibility of obtaining a uniform laser beam intensity distribution in a system formed by a hard aperture and a spatial filter was studied. Various levels of limitation of a Gaussian beam by the aperture were analyzed and methods of transfer of an image of its intensity profile in a narrow spectrum of spatial frequencies were examined. Conditions for the formation of a near-uniform spatial distribution in a laser beam were determined. These can be applied to a linear medium and to a laser amplifier.

Self-focusing and ion wave generation in laser-produced plasmas

Two-dimensional hydrodynamic simulations of laser light self-focusing in a hydrogen plasma are presented. The simulation code includes a model for laser beam propagation which accounts for inverse bremsstrahlung absorption, refraction, diffraction, and ponderomotive forces. A Gaussian hot spot, superimposed upon a collimated, spatially uniform laser beam, is used to initiate self-focusing. Intense filaments provide a driving source for ion waves near the axis of the laser beam. The radially propagating ion waves cause spatiotemporal modulations of the flux where it is initially uniform, as well as the more usual focusing that occurs along the axis. Some of the factors affecting the generation of the ion waves are considered. In particular, the effect of changing the amplitude and width of the imposed nonuniformity is investigated. The intensity thresholds for thermal and ponderomotively driven self-focusing have also been determined by artificially turning the ponderomotive force on and off.

Laser-target interaction with induced spatial incoherence.

An optical technique called induced spatial incoherence can produce the highly uniform laser beams required for direct-illumination laser fusion. Here we present experimental data on laser-plasma interactions showing that the technique, when used in combination with short laser wavelength, also improves the coupling physics.

Determination of cluster size distributions using an optical pulse particle size analyzer

We have developed an instrument that is specifically designed to measure cluster size distributions in colloidal dispersions. In this instrument, individual clusters pass single file through an optical flow channel, a portion of which is illuminated by a uniform laser beam. As each cluster passes through the illuminated region, it scatters a pulse of light, which is collected at angles of less than 3° and imaged at the surface of a photomultiplier tube. Multichannel pulse-height analysis yields a histogram of the cluster size distribution. The distinguishing feature of this instrument is its specific use of lowangle light scattering for cluster size analysis. We demonstrate that at sufficiently low angles, pulse height is proportional to the square of the number of monomeric units in a cluster—independent of cluster shape or orientation. Using polystyrene microspheres in water (relative refractive index m = 1.2), we show that this proportionality holds for sphere diameters as large as three times the wavelength of the incident light. Scattering from individual particles and from clusters of spheres is discussed in terms of Rayleigh, Rayleigh-Gans-Debye, and Mie theories.

Theoretical analysis of fluorescence photobleaching recovery experiments

We derive an exact closed formula for the fluorescence recovery curve measured in fluorescence photobleaching recovery experiments employing uniform circular laser beams. In contrast to the expression used currently, this result is very simple and free of mathematical drawbacks, thus facilitating the quantitative analysis of experimental data.

Temporal growth rate of filamentation instability in a strongly ionised magnetoplasma

Presents an investigation of the temporal growth rate of filamentation instability of a uniform laser beam propagating along and perpendicular to the static magnetic field in a strongly ionised plasma. The nonlinearity of the dielectric constant arises on account of diffusion of non-uniformly heated carriers (on account of fluctuations in the intensity of the main beam). Both the mechanism of energy loss of electrons, namely the thermal conduction and collisions with heavy particles have been taken into consideration.

Correlation of the structure and electrical properties of ion-implanted and laser-annealed silicon

Both transmission electron microscopy and four-point probe measurements have been used to compare the structure and electrical properties of As+- and P+-ion-implanted and Q-switched laser-annealed Si. Use of a uniform laser beam permitted accurate correlations to be made. It was found that the thickness of the initial amorphous surface layer controlled the radiation energy densities required for annealing onset and completion. Annealing onset variations were probably due to changes in the coupling of the radiation with the ion-damaged region. The variation in depth distribution of residual defects with changes in annealing conditions has been determined and shown often to produce a two-stage annealing characteristic. The effect of such defects on annealed-layer electrical properties has been determined. The presence of very-small-scale undulations on annealed surfaces is thought to be due to the propagation of shock waves through the annealed region.

Production of flat top beam profiles for high energy lasers

Production of uniform (flat top) high energy laser beam profiles with relatively little beam energy loss is possible through the use of a modified kaleidoscope, multiple reflection beam scrambler.

Millimeter Wave Power Transmission and Reflection in Semiconductor Image-Converting Systems (Correspondence)

Power transmitted or reflected at millimeter wave-lengths in image-converting systems using a semiconductor illuminated panel is determined. The panel assembly response is defined and evaluated for two modes of operation. In the reflection mode both the cases of uniform circular light illumination and laser beam illumination of the semiconductor material are considered. In the transmission mode of operation the panel is assumed to be uniformly illuminated apart from a circular shadow region. The effect of recombination velocity at the surface of the semiconductor panel is determined.

Millimeter Wave Attenuation through Illuminated Semiconductor Panel

Attenuation of millimeter wave energy through light-illuminated semiconductor panels is determined. The expressions for the attenuation are applied to cases of uniform circular light excitation and laser beam excitation of the panel. Families of curves are given for the attenuation as a function of ratio of distance from the center of excitation to the diffusion length of the semiconductor material. The effect of surface recombination velocities of semiconductor panels is also considered.