Optimum control of the laser beam intensity profile with a deformable mirror

A new deformable mirror control system is developed. This system consists of a deformable mirror, a CCD camera, an image processor, a computer, and actuator drive power supplies. A genetic algorithm is adopted as a control algorithm to obtain an optimum surface profile of the deformable mirror. A circular cross-sectional Gaussian beam was transformed into a beam with a rectangular contour using this system. Although the transfer function of this system is complicated and unknown, this system can be used to obtain the optimum beam profile within the achievable limits of a deformable mirror.     

The interaction between a pulsed laser beam and a steel surface

The interaction between a pulsed laser beam and the surface of a carbon steel has been investigated. The variables of the irradiation were the power intensity of the laser beam and the number of pulses. At the area of impact of the incoming laser beam concentric ripples were observed. The number of ripples and their displacement was found to be dependent on both the above parameters. No transformation products, such as martensite or bainite, or any evidence of a heat affected zone have been observed at the immediate vicinity of the impact area.     

Computer-generated microcooled reflection holograms in silicon for material processing with a CO(2) laser

We report on the design, fabrication, and testing of multilevel computer-generated reflection holograms in Si for CO(2) laser material processing for laser intensities of <2 kW/cm(2). The holograms are designed with an iterative method based on scalar diffraction theory. In this case the reconstructed intensity distribution is independent of the incident high-power laser mode. For achieving high diffraction efficiencies, multilevel staircase surface topologies are fabricated by multimask and reactive ion-etching technology on the front side of a polished Si wafer. For efficient hologram cooling, a gratinglike structure of microchannels is chemically etched on the back side of the Si wafer. Absorption and deformation measurements have been carried out on both a microcooled flat mirror and a reflection hologram. The maximum deformation amounts to 200 nm and is 10 times smaller than comparable conventional uncoated Cu mirrors. A diffraction efficiency of 88% is achieved with an eight-level reflection hologram and a 30-mm-diameter CO(2) laser beam with a power of 5 kW.     

Beam homogenizer for hollow-fiber delivery system of excimer laser light

A beam homogenizer for a hollow-fiber-based, UV laser delivery system is proposed. A rectangular glass waveguide with an inner aluminum coating that has a 1-mm square cross section is attached at the output end of the circular-core hollow fiber with a 1-mm inner diameter. The rectangular waveguide generates a number of higher-order modes and results in a uniform top-hat profile. The configuration of the waveguide is designed by a ray-tracing technique so that both the low transmission loss and the high uniformity in the output beam are obtained. The fabricated waveguide shows a low loss of 0.4 dB, and the intensity variation coefficient is 7%. The output beam from the rectangular waveguide is expanded by a lens to the size larger than 10-mm square. It is also shown that the profile does not change with the bending condition.     

Parabolic mirror optics for collimation of a crescent blue laser beam radiated from channel waveguide Čerenkov second-harmonic generation

Parabolic mirror optics to collimate the crescent-shaped blue laser beam that radiates from ?erenkov second-harmonic generation (SHG) in a channel waveguide configuration is proposed. Mirror collimation optics has a large tolerance to the variations of SHG element parameters, such as the laser source wavelength, as well as to mirror displacement. The anisotropy of a nonlinear crystal in which the waveguide is fabricated has been taken into account. The optimum mirror alignment to obtain a collimated blue laser beam is evaluated in terms of Marechal's criterion. The minimum wave-front aberration with beam intensity weighted is 0.054 λSH. The convergence of the collimated beam is less than 1.6 mrad, and, by using an objective lens, the collimated beam can be focused to a diameter of less than 1 μm, which is 1.27 times the diffraction-limited focusing point.     

Cavity with a deformable mirror for tailoring the shape of the eigenmode

We demonstrate an optical cavity that supports an eigenmode with a flattop spatial profile--a profile that has been proposed for the cavities in the Advanced Laser Interferometer Gravitational Wave Observatory, the second-generation laser interferometric gravitational wave observatory--because it provides better averaging of the spatially dependent displacement noise on the surface of the mirror than a Gaussian beam. We describe the deformable mirror that we fabricated to tailor the shape of the eigenmode of the cavity and show that this cavity is a factor of 2 more sensitive to misalignments than a comparable cavity with spherical mirrors supporting an eigenmode with a Gaussian profile.     

Mechanism of fine ripple formation on surfaces of (semi)transparent materials via a half-wavelength cavity feedback

The mechanism of the fine ripples, perpendicular to laser polarization, on the surface of (semi)transparent materials with period smaller than the vacuum wavelength, λ, of the incident radiation is proposed and experimentally validated. The sphere-to-plane transformation of nanoplasma bubbles responsible for the in-bulk ripples accounts for the fine ripples on the surface of dielectrics and semiconductors. The mechanism is demonstrated for 4H:SiC and sapphire surfaces using 800 nm/150 fs and 1030 nm/300 fs laser pulses. The ripples are pinned to the smallest possible standing wave cavity inside material of refractive index n. This defines the corresponding period, Λ = (λ/n)/2, of a light standing wave with intensity, E(2), at the maxima of which surface ablation occurs. The mechanism accounts for the fine ripples at the breakdown conditions. Comparison with ripples recorded on different materials and via other mechanisms using femtosecond pulses is presented and application potential is discussed.     

Thermal efficiency of pulsed laser radiation increased by methods of multibeam acousto-optic diffraction

Bragg multibeam acousto-optic diffraction of pulsed laser radiation (with 150-ns pulse duration) has been studied under conditions of significant overlap between the neighboring beams. It is established that the initial beam with a Gaussian angular distribution of intensity can be transformed into diffracted radiation with a nearly rectangular profile of intensity provided that the laser pulse duration is much shorter than the period of interference beats in the overlapping regions. Using this circumstance, it is possible to increase the efficiency of high-power lasers in material processing (laser cutting, welding, engraving, etc.) with a threshold character of the radiation effect. This possibility was experimentally verified for a fiber laser operating at a 1.07 μm wavelength.     

Kinoform-only Gaussian-to-rectangle beam shaper for a semiconductor laser

A kinoform that shapes the divergent beam from a semiconductor laser without using any other optical components was designed and fabricated. The kinoform-only concept means that the kinoform must perform both the actual beam shaping as well as focusing the divergent laser beam, correcting for the astigmatism of the laser, and correcting for the spherical aberration of the laser exit window. A rectangular beam of dimensions 1000 μm × 300 μm is formed 42 mm behind the kinoform. Of the total output from the laser, some 50% is incident upon the kinoform, of which ~50% will appear in the rectangular beam. The intensity uniformity error within the rectangle increases from the design value of 8% to 38% because of sensitivity to fabrication errors. The kinoform-only design for beam-shaping applications requires high manufacturing accuracy but is attractive because a system using such a component is easily mounted and aligned and, with the use of kinoform-replication techniques, can be mass produced at low cost.     

Fabrication of small complex-shaped optics by plasma chemical vaporization machining with a microelectrode

We have developed plasma chemical vaporization machining by using a microelectrode for the fabrication of small complex-shaped optical surfaces. In this method, a 0.5 mm diameter pipe microelectrode, from which processing gas is drawn in, generates a small localized plasma that is scanned over a workpiece under numerical computer control to shape a desired surface. A 12 mm x 12 mm nonaxisymmetric mirror with a maximum depth of approximately 3 microm was successfully fabricated with a peak-to-valley shape accuracy of 0.04 microm in an area excluding the edges of the mirror. The average surface roughness was 0.58 nm, which is smooth enough for optical use.     

Intracavity Beam Behavior in Hybrid Resonator Planar-Waveguide CO(2) Lasers

We describe a combined computer simulation and experimental investigation of the intracavity spatial beam profile characteristics of a planar-waveguide rf-excited CO(2) laser that incorporates a hybrid waveguide confocal unstable negative-branch resonator. The study includes results for the intracavity lateral beam intensity profile and output power of the laser as a function of resonator mirror misalignment. In addition, the behavior of the unstable resonator, observed experimentally and predicted by the simulation, in generating localized high intensity hot-spots when it is subjected to relatively large misalignment angles is reported.     

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.     

Diffractive optical elements for intra-cavity beam-shaping of laser modes

Abstract

Diffractive optical elements (DOE) are applied as intra-cavity mode selection devices for customizing the fundamental mode of laser resonators for high power laser systems. Using a phase-conjugating mode selecting element (MSE) in a laser oscillator, we are able to produce a good approximation to a super-Gaussian mode with a near flat intensity profile. This offers higher energy extraction from any following laser amplifiers compared to an unmodified Gaussian TEM₀₀ mode. Two different designs for operation in a 1 m cavity length Nd:YAG master oscillator are presented. Both designs are surface relief phase elements fabricated in fused silica using photolithography with reactive-ion etching to produce 16 level elements for use in transmission. One element is designed to replace the cavity end mirror, while the other stands off an arbitrary distance from the end mirror. A novel iterated design for these transmissive elements is introduced. Numerical results and experimental measurements are presented and discussed.     

Taper array in silica glass for beam splitting

We proposed taper array in silica glass for beam splitting which was fabricated by water-assisted femtosecond laser direct writing technology and the subsequent heat treatment. We divided the array into many fabricating cells which were executed automatically in sequences as specified by the program that contained the information for the three-dimensional stage movements. Each cell could fabricated a rectangular cylinder. The size and distribution of the rectangular cylinder could be controlled by adjusting the position of the fabricating cells. Then the heat treatment should be used to reshape the rectangular cylinders into taper array. The experimental results show that the taper periodic microstructures in silica glass are uniform and smooth, and the tapers can divide the incident light into beam array. The results demonstrated that the combination of the water-assisted femtosecond laser direct writing technology and the heat treatment is accessible and practical for the high quality micro-optical elements. These micro-optical elements will have potential applications in fluorescence detection and beam splitter.     

Microlenses and Microlens Arrays Formed on a Glass Plate by Use of a CO(2) Laser

Microlenses and microlens arrays were formed directly on a surface of a glass plate by use of a CO(2) laser. When the surface of a glass plate is heated locally to a working point of the glass material by use of a focused CO(2) laser beam, it tends to become a hyperboloid owing to surface tension, which results in a microlens. A profile of the microlens was measured with an ultrahigh accurate three-dimensional profilometer (Model UA3P, Matsusita Electric Industrial Company Ltd.) that utilizes a specially designed atomic force microscope. An intensity profile and a spot diameter at the focus of the microlens were measured with a microscope and a CCD system utilizing a He-Ne laser as a light source. The focused spot FWHM diameter of 1.35 mum was obtained, and the modulation transfer function was derived from the spot profile. Microlens arrays were also fabricated and characterized.     

Diffractive shaping of excimer laser beams

Abstract

We address the problem of shaping the intensity distribution of a highly directional partially coherent field, such as an excimer laser beam, by means of diffractive optics. Our theoretical analysis is based on modelling the multi-transverse-mode laser beam as a Gaussian Schell-model beam. It is shown numerically that a periodic element, which is unsuitable for the shaping of a coherent laser beam, works well with an excimer laser beam because of its partial spatial coherence. The conversion of an approximately Gaussian excimer laser beam into a flat-top beam in the Fourier plane of a lens is demonstrated with a diffractive beam shaper fabricated as a multilevel profile in SiOl by electron-beam lithography and proportional reactive-ion etching.     

Development of line-shaped optical system for green laser annealing used in the manufacture of low-temperature poly-Si thin-film transistors

We have developed a new optical system that transforms the circle profile beam generated with near-Gaussian intensity distribution by a pulse green laser (YAG2omega laser; second harmonics of a Q-switched Nd:YAG laser) into a line-profile beam. For homogenization in the longitudinal direction, we employed a waveguide plate-type homogenizer. We successfully reduced interference fringes. In the width direction, the laser beam was focused up to the limited M2 value. This transformed beam has a uniform distribution to within 5% in the longitudinal direction, and it is approximately 100 mm long and 40 microm wide.     

Two-faceted mirror for active integration of coherent high-power laser beams

A new integration method suited for spatially coherent high-power laser beams is demonstrated. The integrator system is based on a mirror with two facets, one of which can vibrate under the action of a piezoelectric translator. After reflection in the faceted mirror, the beam intensity distribution is modified to obtain greater uniformity. However, because of the coherence of the reflected beamlets, this distribution is affected by an interference pattern. The active integration consists of a periodic displacement of the moving facet that causes the interference pattern to vibrate, and its contribution to the intensity profile therefore averages out (fringe visibility within a 5% range). The combination of a faceted mirror and a simple imaging system results in an intensity profile with good uniformity over large spot sizes. Both simulated and experimental results are presented, the latter showing that a final uniformity within a 10% range can be achieved and it is limited mainly by diffraction at the edges of the facets.     

Wave-front design algorithm for shaping a quasi-far-field pattern

To design a fully continuous wave-front distribution suitable for focused beam shaping by a deformable mirror, we modify the phase-retrieval algorithm by employing a uniformly distributed phase as a starting phase screen and spatial filtering for the near-field phase retrieved during the iteration process. A special phase unwrapping algorithm is not required to obtain a continuous phase distribution from the retrieved phase since the boundary of the 2pi-phase-jumped region in the designed phase distribution is perfectly closed. From the computational result producing a uniform square beam transformation from a circular defocused beam, this algorithm has provided a fully continuous wave-front distribution with a lower spatial frequency for a deformable mirror. The transformed square beam has a normalized intensity nonuniformity of varsigma(rms) = 0.14 with respect to a desired flat-topped square beam pattern. This beam-shaping method also provides a high energy-concentration rate of more than 98%.     

Output power enhancement of a palmtop terahertz-wave parametric generator

Broadband terahertz (THz) waves were generated by optical parametric processes based on laser light scattering from the polariton mode of a nonlinear crystal. By using the parametric oscillation of a MgO-doped LiNbO3 crystal pumped by a nanosecond Q-switched Nd:YAG laser, we have realized a broadband, high-energy and compact THz-wave source. We report the development of a THz-wave parametric generator (TPG) using a small pump source with a short pulse width and a top-hat beam profile. These characteristics of the pump beam permit high-intensity pumping especially close to the output surface of the THz wave without thermal damage to the crystal surface. We also calculated the outcoupled THz wave for beams with two different intensity profiles: a top-hat beam (in this experiment) and a Gaussian beam (previously reported). The result shows the mechanism of the output energy and/or power enhancement.