Distribution Of Laser Beam Research Articles

Adaptive shaping of a focused intense laser beam into a doughnut mode

We present a method to shape the intensity distribution of an intense laser beam in the focal region, by means of a bimorph adaptive mirror. We emphasize the advantages in terms of flexibility with respect of previous methods making use of specially designed retarding phase plates; however, the nature of the mirror imposes constraints on the accessible wavefronts, whose consequences are analyzed. We present an experimental demonstration of the feasibility of the scheme on a high intensity femtosecond laser system.

Ideal Microlens Design for Flatting the Equiphase Distribution of a Gaussian Laser Beam

An ideal microlens profile for flatting the equiphase distribution of a Gaussian laser beam is obtained analytically by a new design tool based on the combination of Gaussian beam theory and ray optics. A comparative study with similar microlens design by ray tracing technique is performed by using a wide-angle beam propagation method with (2, 2) Pade/spl acute/ order expansion. The results show that a substantial improvement of lens profile design accuracy can be achieved by employing this new design tool.

Coherent addition of spatially incoherent light beams

We report on efficient coherent addition of spatially incoherent multimode laser beam distributions. Such addition is demonstrated within a multi-channel laser resonator configuration, obtaining more than 90% combining efficiency while preserving the good beam quality. We explain the rather surprising physical phenomenon of coherently adding spatially incoherent light by self-phase-locking of each of the modal components within the multimode beams. Our approach could lead to significantly higher output powers concomitantly with good beam qualities than were hitherto possible in laser systems.

Femto-second X-ray generation by the tera-watt laser slice method in the SPring-8 storage ring

Feasibility study of the generation of ultra-short pulse X-rays in the SPring-8 storage ring with a laser slice method is discussed. The electron beam energy is modulated by applying a tera-watt laser with a length of 100 fs in parallel to the electron beam running through a wiggler. Pulsed X-rays, emitted from a bending magnet, are separated from the X-rays due to the beam bunch with a slit. Parameters of the system are optimized based on a formalism with respect to the laser beam interaction in a wiggler, beam dynamics and laser beam optics. Numerical calculations of the interaction are carried out taking into account a Gaussian distribution of electron and laser beams. The expected flux is 8.1×10 4 photons/s/mrad/0.1% bandwidth at a bunch beam current of 10 mA with a pulse length of 240 fs in full width. If the lattice functions are modified such that an undulator is used as a radiator in a negative dispersion section, we can expect the pulsed X-rays with a flux of 2.9×10 7 photons/s/0.1% bw and a pulse length 180 fs. Because of a high beam energy of 8 GeV, a very high laser field is required, which induces limitations in the laser beam optics. Discussions are extended to the New SUBARU storage ring of 1.5 GeV.

Study of an Optical Device Used to Homogenize a Laser Beam. Application to Emissivity Measurements on Semitransparent Materials at High Temperature

On an experimental apparatus designed to measure the directional spectral emissivity of semitransparent materials at high temperature (2000°C), a specific optical device (kaleidoscope) is mounted in order to homogenize the energy distribution of a CO2 laser beam, which is used as a heating source. An objective of this work, for operating in the best conditions, is to obtain a uniform temperature of the tested sample. This study, based on Fourier optics, focuses on a square aperture kaleidoscope. A model was developed to simulate the energy distribution of the laser beam, at any transverse plane on the optical path. The final objective is to simulate the energy distribution on the sample surface to optimize the homogenization device taking into account the surface temperature gradient induced by the local energy distribution. To validate this model, quantitative comparisons of theoretical simulations and experimental thermal spots are performed.

Modelling and Analysis of UV Laser Micromachining of Copper

UV laser micromachining of metallic materials has been used in microelectronic and other industries. Knowledge and data about the process vary with feature size, material, laser wavelength and pulse duration. This paper reports on experimental and numerical investigation of micromachining of copper using a frequency tripled Nd:YAG laser with 50 ns pulse duration. An axisymmetric model is developed which allows consideration of laser beam distribution and its coupling with the target material. This is important for the process where the removal extent is in the same order of the removal depth. The model uses an enthalpy method to track the solid/liquid interface. Stefan and kinetic boundary conditions are applied at the liquid–vapour interface, and property discontinuity across the Knudsen layer is considered. Relevant experimental results are also presented and compared with the model predicted results. The range of thermal vaporisation dominated machining of copper using nanosecond time scale lasers was studied, and optimum laser intensity for micromachining of copper was suggested.

Telescopic refractive index gradient diagnostic of an ion diode anode plasma

We present a general analysis of possible variants of refractive index gradient (RING) diagnostics with a laser beam probe. Using a differential bicell photodiode as a detector, the sensitivity, dynamic range, and geometric restrictions of RING deflectometry have been found for lensless, one lens, and “three-telescope” optical schemes. The three-telescope method is found to be the most flexible and easily aligned. If the refracted/deflected laser beam cross section in the back focal plane of the output lens is also recorded using a fast framing camera, measurements of the beam deflection and its spatial frequency spectrum after passing through the refractive medium can be obtained simultaneously. A general relation is presented between the Fourier transform of a Gaussian beam by the output lens and the spatial frequency spectrum of the inhomogeneities. From these considerations, we present the specific design of a RING diagnostic for study of anode plasma evolution in a magnetically insulated ion diode on the Cornell Beam Research Accelerator (COBRA) (800 kV, 80 ns pulse). The maximum density gradient was found to be located about 0.4 mm from the anode surface at the peak of the diode voltage pulse. The electron density at this position was about 2×1015 cm−3. The transverse spatial frequency distribution of the probe laser beam after passing through the anode plasma was recorded using the optical Fourier transform technique. This experiment demonstrates that the combined integrated deflection. (RING) and Fourier transform optical techniques can give a great deal of information about this thin (∼1 mm) anode plasma layer.

Rotating shutters: a mechanical way of flattening Gaussian beam profiles in time average

A mechanical method of flattening the Gaussian intensity distribution of laser beams in time average is presented. Specially shaped rotating shutters are the key feature of this method, which has been applied to achieve homogeneous submicrometer patterning of macroscopically large samples by laser interference exposure. This method represents a simple yet useful alternative to applying beam broadening or degaussing plates (apodizing filters).

Bimodal temporal distribution of photons in ultrashort laser pulse passed through a turbid medium

The criteria were determined for simultaneous observation of scattered and unscattered (ballistic) peaks in temporal distribution of a narrow pulsed laser beam passed through a strongly scattering medium. Conditions were found for such an observation on the basis of non-stationary two-flux model for radiation transport. Calculation results permitted the definition of a compromise between the parameters of the initial laser pulse and the properties of a scattering medium. This provided the separate observation of various types of photons. The theoretical calculations were confirmed by experiments on recording pulse profiles of passed radiation of fs-laser in a water–milk solution.

Thermal analysis of the volume absorber in pulsed excimer laser calorimeters

Thermal modeling and analysis of the volume absorber in the pulsed excimer laser calorimeters are very important. In this work, Gaussian distributions are used to model the temporal and spatial distributions of the laser beam and an exponential decay function is used to model the internal absorption of the laser power. A finite-element method is employed to simulate the space- and time-dependence of temperature in the volume absorber. A three-dimensional model and an axial-symmetric model are built and used to study the heating effects of single pulse and multiple pulses on the present design, respectively. Furthermore, a new design is proposed, in which the absorber is not optically thick. A one-dimensional model and an axial-symmetric model, in which the reflection at the interface and the absorption on the copper surface are considered, are used to study the heating effects of single pulse and multiple pulses on the new design. The comparison of the present design to the new design shows that the energy loss and nonequivalence in the new design are smaller than those in the present design. Hence, the new design can increase the accuracy and dynamic range of calorimeters. This work will help the future design of optical calorimeters for measuring the pulse energy of excimer lasers in the deep ultraviolet.

Calculation of mode contents of high-power CO 2 laser beam according to the changes of transverse intensity distribution

A method for calculation the mode contents of high-power CO 2 blending-mode laser beam is presented. The basic idea is based on the changing rule of transverse intensity distribution of blending-mode laser beam. Based on two arbitrary measured transverse intensity distributions, the beam's mode contents and phase shifts of blending-mode laser beams are calculated by means of the least squares method and iterative process. Moreover, through the calculation results, any transverse intensity distribution for blending-mode laser beam is simulated and beam quality is also calculated readily.

Design of Computer-Generated Hologram With Ring Focus for Nonmechanical Corneal Trephination With Er:YAG Laser in Penetrating Keratoplasty

Purpose: To calculate a beam-shaping optical element for homogeneous intensity distribution within a focal ring to be used in nonmechanical trephination with the Er:YAG laser in penetrating keratoplasty instead of a spot guiding device. Methods: The phase distribution behind a holographic optical element (HOE) kΨ( u) can be described by the addition of the hologram phase Φ H ( u) to the beam phase Φ E( u ): k Ψ( u ) = Φ H( u ) + Φ E( u ), k = 2 π/ λ , where u denotes the coordinates inside the hologram aperture, k an integer, and λ the laser wavelength. To avoid discontinuous wavefronts leading to speckle noise, a smooth phase function is necessary. After transforming the hologram aperture coordinates into the focal plane x in a focal distance f, Ψ can be retrieved from the slope equation: ▿ Ψ( u ) = x( u ) − u /f . Results: Creating a ring focus can be reduced to an essentially one-dimensional problem by separation of variables due to the symmetry condition. We calculated a computer-generated eight-level phase-only HOE with 4096 × 4096 pixels from a Gaussian-distributed 2.94 Er:YAG laser spot with a beam diameter of 10 mm and a focal distance of 100 mm. Thereby, a ring focus with an inner/outer radius of 7/8 mm can be created. To avoid Poisson's spot, the symmetry of the problem was broken by circular modulation of the phase leading to a spiral-like structure. The calculated efficiency of the HOE relating the energy within the ring to the total energy was 91%. Conclusion: With an HOE it is possible to redistribute the energy along the desired focal ring. The HOE design can be adapted to the intensity distribution of the impinging laser beam with its characteristic aperture shape. A circular homogeneous corneal trephination depth is possible, because the energy fluctuation from pulse to pulse does not locally affect the ablation process. A ring focus for the Er:YAG laser has the potential to render superfluous a manual beam control via micromanipulator and to allow a more rapid and more regular corneal trephination along aperture masks.

ＣＯ２レーザ加工における照射部温度と吸収率

The temperature of workpiece irradiated with CO2 laser is measured using a new type of infrared radiation pyrometer with an optical fiber. In the pyrometer, a chalcogenide optical fiber and a two-color detector which consists of InSb and InAs detector are used. The power intensity of the laser beam during the exposure time and the two dimensional picture of the intensity distribution of the laser beam are measured experimentally. The measuring system for the intensity distribution of laser beam consists of MCT cell and an optical fiber. The temperature distribution on the workpiece irradiated with laser is calculated numerically using FEM for unsteady state. The change of physical properties of materials is considered. Comparing the experimental results with the calculated ones, the absorptivity of the work material for CO2 laser is obtained.

Depth-resolved analysis by laser-induced breakdown spectrometry at reduced pressure

The 581 nm output from a dye laser in a —uence range between 2.86 and 11.47 J cmo2 was used to ablate pure Zn and Fe foils. The average ablation rate (AAR, lm per shot) was calculated for diUerent experimental variables (buUer gas, pressure, laser —uence and focal conditions). Deposition of previously ablated material in the ablation crater results in large variation of the observed AAR values. This eUect was observed in air and argon buUer gases at atmospheric pressure. The situation is largely alleviated at reduced pressure due to free expansion of the ablated material. Under these circumstances the capability of laser-induced plasmas to resolve interfacial structures is improved. The eUect on depth-resolved studies was checked with a commercial Zn-coated steel sample. Due to the Gaussian-like energy distribution of the incident laser beam, the material is ablated to produce a conical crater. This fact ensures that the Zn signal remains for a longer time because the ablated region spreads over the edge gradually. At low pressure the emission peaks are better de—ned and the background becomes —at. However, these conditions produce also the lowest net intensities and some peaks are not detected. An Ar atmosphere produces more intense spectral lines at both pressure levels. Best analytical results were obtained at reduced pressure, with a slight improvement in depth resolution in the presence of Ar. 1998 John Wiley & Sons, Ltd.

Delivery of 10-MW Nd:YAG laser pulses by large-core optical fibers: dependence of the laser-intensity profile on beam propagation

A large-core multimode optical fiber of a few meters length is studied as a 10-MW beam delivery system for a 15-ns pulsed Nd:YAG laser. A laser-to-fiber vacuum coupler is used to inhibit air breakdown and reduce the probability of dielectric breakdown on the fiber front surface. Laser-induced damage inside the fiber core is observed behind the fiber front surface. An explanation based on a high power density is illustrated by a ray trace. Damaged spots and measurements of fiber output energies are reported for two laser beam distributions: a flat-hat type and a near-Gaussian type. Experiments have been performed to deliver a 100-pulse mean energy between 100 and 230 mJ without catastrophic damage.

Reduction of small-scale intensity modulation in a line focus produced by a cylindrical-lens array using polarization control and wedge angles

Based on the Huygens-Kirchhoff propagation law the intensity distribution of a coherent laser beam, line focused by a cylindrical-lens array and a spherical lens is investigated. With the aim to reduce the depth of the small-scale intensity modulation along the line focus, the effect of alternatingly orthogonal linear polarization and controlled misalignment of the individual line foci and the influence of the divergence of the laser beam is demonstrated. Numerical calculations show a considerable reduction of the small-scale modulation depth of the intensity in the line focus. The physical-optics analysis of the cylindrical-lens array is compared with experimental results.

Implications of the Gaussian intensity distribution of laser beams on the performance of the phase Doppler technique. Sizing uncertainties

The influence of the Gaussian intensity distribution of the incident laser beams on sizing measurements by phase-Doppler velocimetry has been investigated using geometrical optics approximation, considering the interference between contributions from reflection at the external surface of a droplet, refraction through the droplet and those refracted after one and two reflections at the inner surface of the droplet. The results have considered the use of forward-(scattering angles between 30 and 75°), back-(scattering angles between 140 and 180°) and side-(scattering angles between 80 and 130°) scattered light to size transparent non-absorbing spherical droplets in the range of relative refractive indices between 1.33 and 1.4 and normal, or oblique trajectories relative to the fringes of the velocimeter. The results have demonstrated the influence of phase and amplitude validations and the logic of the signal processor on sizing uncertainties. The Gaussian intensity profile of the laser beams caused variations of phase difference and associated amplitude of the signal with droplet trajectory and, for the optical arrangement of the current work, the resulting sizing uncertainties after amplitude and phase validations were between 5 and 10% for forward-scattered light collected at 30° and +9%/−100% and +20%/−16% for back-scattered light collected at 150° for droplet-to-probe diameter ratios of 1.67 and 0.24, respectively. For side-scattered light, the phase validation logic is not valid and the sizing uncertainties after amplitude validation were +19%/−26% and ±5% for droplet-to-probe diameter ratios of 1.67 and 0.24, respectively, and scattering angle of 104°. Although these sizing errors correspond to optical arrangements, which may not be available in commercially available phase Doppler systems, experimental results from Willman et al. ( Proc. 7th Int. Symp. on Applications of Laser Technologies to Fluid Mechanics, 1994) using commercial instruments indicate similar errors. Sizing with forward-scattered light is preferable overall, but for flows with limited optical access, sizing with side-scattered light is preferable to back-scattered light due to lower sizing errors. Droplet refractive index influences the response curves and for refractive indices of 1.33 and 1.4, back-scattered light should be collected between scattering angles of 145 and 160° and of 155 and 160°, respectively, and that for side-scattered light at around 120 and 104°, respectively, and for both arrangements the collection optics are required to be within a small range of angles around the bisector plane of the beams.

レーザ照射処理におけるカライドスコープによるパワー密度分布の変換 円管・テーパ管のカライドスコープを用いた穴あけ加工

Kaleidoscope is a device to convert power density distribution of laser beam. Cylindrical pipe kaleidoscope and taper pipe kaleidoscope are applied to laser drilling process in the present research. The shapes and dimensions of the kaleidoscopes are designed based on the theoretical analysis of the power density distribution of the laser beam converted by the kaleidoscopes. Laser drilling experiments of acrylic resin plates are carried out by applying both the kaleidoscopes and the conventional lens. The experimental results are discussed from the viewpoints of the depth and the cylindricity of the acrylic burn patterns. The following remarks are concluded based on the experiments. The shapes and dimensions of the burn patterns are not affected so much by the distances between the kaleidoscopes and the work plates in the case of drilling process by use of the kaleidoscopes. The taper pipe kaleidoscope is most suitable for the drilling process from the viewpoint of the cylindricity of the burn patterns.

Beam propagation in an injection-controlled copper vapor laser system

Far-field intensity distribution as well as its temporal evolution of the output from a copper vapor laser oscillator at different operating conditions are measured in detail. Experimental observation has been performed about the influence of delay time between the oscillator and amplifier on the far-field intensity distribution of the output from the amplifier. Beam propagation in an injection-controlled copper vapor laser system has been investigated on account of thermal lens effect. The application of a single-pass unfolded amplifier model has been successfully demonstrated to describe beam propagation in the power oscillator by the temporal measurement. It has been found that the thermal lens effect greatly influences the spatial distribution of laser beam from an injection-controlled system.

Investigation into absorption of the incident laser beam during Nd:YAG laser processing of metals

Laser materials processing is highly affected by the existence of surface plasma. The absorption of surface plasma during drilling alters the power intensity distribution of the incident laser beam across the irradiated spot. The present study is carried out to measure the electron number density and temperature using a Langmuir probe while a mathematical formulation is conducted for the absorption coefficients due to electron-ion, electron-neutral atom collisions, inverse Bremsstrahlung, and photoionization processes. Consequently, a computer program is developed to compute the relevant absorption coefficients as well as the overall absorption coefficient. The laser power intensity distribution before and after the plasma absorption is computed at a plane 2.6 mm above the workpiece surface. It is found that 13% of the reduction occurs in the incident laser output power intensity at this plane in the plasma.