This contribution focuses on the study and comparison of different design methods for designing phase-only diffractive optical elements (PDOEs) for different possible applications in laser beam shaping. Both binary and multilevel phase-only diffractive optical elements are considered, theoretically studied, and compared, using diffusive types of input objects. Especially, new results and approaches, concerning the iterative Fourier transform algorithm, are analyzed, implemented, and compared (the scale factor and the algorithm convergence characterization through the spectrum representation). The two important output design parameters, i.e. the signal-to-noise ratio and the diffraction efficiency characteristics are presented by means of evaluating their dependencies on, e.g. the number of iterations, the number of phase levels, and the signal-window position). Next, we concentrate on the design of Fresnel domain PDOEs, generalizing the classical IFTA scheme (both with a single and double focus) types of PDOEs. The application of the appropriate scale factors for each particular case is presented, and typical design results for both cases are shown. For practical realization, e-beam lithography technology is used, showing a good qualitative agreement with theoretical designs.

A novel hybrid phase-retrieval non-iteration algorithm combining random searching algorithm and simulated annealing algorithm has been proposed for designing the diffractive optical elements (DOE'S) used for producing a uniform focal spot required with top head, steep edge, low side lobe or concentrating high power performance in the main lobe. The continuous phase distribution of diffractive optical elements with a good geometrical structure has been obtained by the new hybrid algorithm. Starting the optimal process with a continuous phase profile and taking the special phase function, this new algorithm can construct fully continuous phase screens for tailoring far-field intensity profiles (beam shaping). The continuous phase distribution using the new algorithm will be fabricated easily , make the elements have high diffractive efficiency, heighten the threshold value resisting the high power laser and get the far-field intensity profile with high uniformity at the main lobe, very small side lobe and steep edge. The computer simulation has shown that the algorithm is robust and maintains the continuous phase distribution throughout the optimal process. The DOE'S with continuous phase have been designed with more than 99% energy efficiency, less than 1% side slobe non-uniformity, and, with more than 90% energy efficiency, less than 2% top non-uniformity, respectively.

Geometrical methods have been used to design two-mirror laser beam shaping systems with rectangular symmetry and no central obscuration. These systems are able to transform an input laser beam with an elliptical cross section and a two-axis Gaussian irradiance profile into a rectangularly symmetric output beam with uniform irradiance. The optical design software ZEMAX has been used for modeling and performance analysis of these systems.

Previous efforts have shown that optimization-based techniques, particularly genetic algorithms, can be used to effectively design beam shaping systems for several classes of problems. These techniques have used external programs to perform the optimization, which allow a great deal of optimization flexibility. However, the external program must be understood and tuned for beam shaping applications. For many problems, the flexibility of an external program is not required. This work shows that the internal optimization routines in a typical lens design program can perform effective optimization of several classes of beam shaping systems. In addition, previous beam shaping merit functions have used a large number of rays, typically 200, to perform the optimization. Several pieces of data must be recorded for each ray. This results in a fairly lengthy merit function evaluation by the lens design program. This work shows that by setting up certain problems using up-front geometrical considerations, reasonable optimizations can be obtained with as few as 20-40 rays being traced. Example problems are presented, including the merit functions used. Guidance is provided for the designer interested in picking appropriate parameters and optimization routines for some typical problems.

Methods of shaping laser beams within the laser resonator are studied. The simplest form of shaping is the spatial mode generation inherent in the laser cavity due to the geometry of the resonator in conjunction with gain to compensate for roundtrip losses, such as diffraction and output coupling. Typically the fundamental mode or multimode behavior is exhibited from such configurations. Passive mode shaping can be accomplished by introducing static amplitude or phase masks. An example of an amplitude mask is provided in order to generate a higher-order spatial mode. Active mode shaping can be accomplished by altering the optical pump distribution. This case is studied experimentally with a diode-end-pumped Nd:YVO₄ laser and compared to modeling of expected Hermite-Gaussian mode generation. Active mode shaping allows the preferred mode distribution to be altered in real time. Additional shaping can be done following the resonator to modify a Hermite-Gaussian mode into a pseudo-Laguerre-Gaussian mode. This work also shows that using the coherent propagation method of Gaussian beam decomposition is capable of modeling and describing intracavity beam shaping.

A method for converting a gaussian laser beam into a propagating Airy pattern is described using a diffractive optic. This propagating Airy pattern is focused by a lens to obtain a flat top intensity at the focal plane. The technique is based on the idea of Fourier transform pairs and produces a small spot diameter with a useable depth of field. Experimental results will be presented for round and square focused uniform intensity profiles. A focused uniform intensity profile can prove useful for many laser applications.

A method for designing and fabricating aspherical transmissive beam shaping elements for the visible and near-IR is described. Both rotationally symmetric and non-symmetric aspheres can be fabricated as Near-Index-Match^TM optics. The aspherical surface is machined or mastered using a conventional NC machine. A sandwich of two transmissive materials is formed, with the refractive phase the product of the surface relief and the index difference. Both far-field (Dickey, Romero, and Holswade) beam shapers and near-field (Rhodes and Shealy) beam shapers can be realized, as well as two-element systems with phase correction of the shaped beam. Design methodologies and results are presented.

The basic instrument used for optical data storage is a scanning laser microscope. Each device contains a beam from a laser diode that is collimated, shaped and focused with an objective lens to produce a microscopic spot on the recording media. The reflected light is collected by the objective lens and directed to data and servo detectors with a beam splitter. Data density on the storage medium is primarily defined by the size and shape of the focused laser beam used to scan the data. Several interesting techniques have been used to shape the focused spot in a way that decreases the primary feature of the spot, thus increasing density. For example, both amplitude and phase filters have been used to decrease the central lobe, at the expense of increased sidelobe levels. Effects of the sidelobes can be minimized with special electronic circuits. The configuration of the readout optics also can influence density. That is, optical filters can be placed in the collection pupil to improve the system transfer function. When combined with electronic shaping circuits, the optical filters significantly improve device performance. This paper reviews the techniques used for beam shaping in optical data storage with an explanation of each technique and its success or failure.

For the next generation of DVD we have designed an 'asymmetric Gaussian to flat-top' converter which uses two refractive optical elements. The design method is based on conservation of energy. This approach leads to an analytically solvable mapping equation which was integrated numerically. The properties of the designed element were analyzed by numerical nonparaxial wave-propagation as well as by experimental characterization. In detail, we have studied the tolerance to phase noise, the tolerance to deviations from the specified form and the tolerance to positioning errors. We observed, that the flat-top-intensity shows a very height sensitivity to phase noise. The sensitivity is increased by rotationally symmetric distributions of phase noise e.g. produced by diamond turning. The influence of these deviations on the intensity distribution in the focal plane of the high NA-focusing lens are investigated. First experimental results of a shaping element fabricated by direct write in resist are presented.

A high-power vertical cavity surface emitting laser (VCSEL) is described that can emit over 40 mW in pulsed operation at 780 nm. The mode structure of the device is unusual, in that only the HE³¹ mode lazes over a broad environment of mechanical and thermal conditions. The stable mode structure suggests that this laser may be used in an optical system like those used in optical data storage, which demand diffraction-limited performance. However, the laser mode cannot be used directly in a classical optical system due to its ring amplitude and periodic phase properties. In this paper, we discuss potential beam-shaping techniques that can be used to overcome these limitations. For example, a simple phase mask can be used to eliminate the periodic phase properties. Optical systems for focusing the shaped laser light are also discussed.

A system of two plano-aspheric lenses which transforms a collimated, radially symmetric Gaussian beam to a flattop is described. To minimize diffraction of the output beam, the lenses were designed to accept essentially all (99.7%) of the input beam, and the output intensity distribution was chosen to have a controlled roll-off, given by the Fermi-Dirac function. Both aspheric surfaces were convex, simplifying fabrication by the technique of magnetorheological figuring. The optics were made of fused silica and, with suitable antireflection coatings, a single prescription can be used at any wavelength from 250 to 1550 nm. Measurements of the output intensity distribution were made by directly illuminating a CCD sensor with the flattop beam, and these results were compared quantitatively to the theoretical design. At the 8 mm diameter output aperture, 78% of the total beam power is contained in a region with 5% rms intensity variation, representing a fourfold improvement in power utilization over the Gaussian input. The beam propagates approximately 0.5 m without significant change in the intensity profile. Both the intensity uniformity and the propagation range are believed to be limited by the accuracy of the aspheric surfaces. It was verified that expanding the linear dimensions of the beam by the factor m increases by m² the range over which the beam retains its shape as it propagates. The optics have been successfully used in a holographic data storage test stand and for deep-UV interferometric lithography.

Laser micromachining has been a part of the manufacturing process for semiconductors and microelectronics devices for several decades. More recent applications such as the drilling of microvia holes in high-density electronic packages have recently entered broad industrial use for high-volume production. In such applications, process stability and throughput are key drivers of commercial success. Particularly in the UV, where solid-state laser power is growing rapidly but is still limited to less than 10 watts, innovations that permit the available laser power to be applied at the work surface more efficiently are of interest. Within the last two years, the use of beam shapers to create round laser spots with near-uniform irradiance at the work surface has been demonstrated. Shaping the irradiance profile has been shown to both increase process speed and improve the quality of the drilled holes, which range in diameter between 20 and 150 micrometers . This paper gives an historical overview of laser via drilling, presents the Gaussian-to-flattop beam shaping optics used in the microvia laser drills, and discusses the process results obtained.

With the aim of reducing the heat-affected zone to improve edge quality, we present results of drilling microholes using reshaped pulsed Gaussian laser beams. A diode-pumped, high repetition rate, nanosecond pulse duration 3rd harmonic Nd:YAG laser was reshaped such that the intensity gradient in the outer region of the focussed laser beam profile is increased. Compared to focussed Gaussian laser beams, such hard-edged intensity distributions produce smaller heat-affected zones. As a result there is less associated collateral damage, debris, remelt produced by the near-ablation threshold fluences. Specially designed spherically-aberrating Galilean telescopes are used to reshape the primary Gaussian laser beam into a quasi-tophat distribution at the mask plane. Gaussian illumination propagation simulations using Monte-Carlo ray tracing calculations compare well with measurements of reshaped distributions made with a beam profiler. Drilling trials in polymers and silicon nitride demonstrated improved edge quality, reduced debris and wall roughness and a significant reduction in the energy density required for drilling microholes of high aspect ratio.

CO₂ laser material processing is now being used increasingly in the area of electronics, specifically, for drilling micro holes (min. (phi) 50 micrometers ) in printed circuit. This has become possible through the use of galvanometer mirrors and Aspheric ZnSe f-theta lenses. ZnSe is a good infrared material because of wide transmission. We have succeeded in producing aspheric lenses by using a Single Point Diamond Turning (SPDT) lathe. Distribution of laser beam intensity mainly based on Gaussian distribution is non- uniform. Therefore, the rise great demand for uniform intensity distribution in the fields of heat processing. To obtain higher uniformity, attempts have to be made to convert non-uniform Gaussian distribution into top-hat shaped uniform intensity distribution for smoothly bending laser beams. In this study we propose it is possible to allow the surfaces of a lens to perform more than one function (the center portion functions as a concave lens and the rim portion functions as a convex lens). The Aspheric ZnSe beam shaper consists of two aspheric lenses. First one converts Gaussian profile to uniform irradiation, Second one performs phase matching (compensation). We design this optical component with our special method based on the optical ray tracing. The most important optical property, wave front distortion were measured by infrared interferometer. And we also show the intensity distribution through after beam shaper with high power CO₂ laser (>100 W).

A common method of shaping laser beams into patterns for product marking and machining involves the use of masks. These masks result in inefficient use of the available laser energy, causing increased fabrication time and cost. Simple patterns, such as rings and tophat profiles can be easily formed with multi-aperture beam integrators, thus utilizing nearly 100% of the laser energy. This paper discusses multi-aperture beam integrators for forming a continuously variable ring pattern on the target plane. Both on-axis and off-axis systems are discussed.

A promising new tool in shock wave physics is the generation of shock waves in test materials through the impact of small, laser-accelerated discs ('flyers'). In order to achieve the necessary one-dimensional condition of uniaxial strain in the shock-loaded material, it is vital that flyers maintain a nearly planar geometry during the acceleration and impact processes. The geometry of the flyer is significantly influenced by the spatial intensity profile of the driving laser beam. With the goal of achieving a nearly uniform drive intensity for this application, we have evaluated a diffractive, microlens-array beam shaper for use with a high-energy, Nd:Glass laser driver. Based on the near-field spatial profile of this multimode laser, a 30-mm-diameter array containing multiple hexagonal diffractive lenslets was designed and fabricated. In combination with a primary integrator lens of 76.2-mm focal length, this optical element was intended to produce a uniform intensity distribution over a 2-mm-diameter spot at the focal plane of the primary lens. Beam profiling studies were performed to determine the performance of this optical assembly. At the focal plane of the primary lens, the beam shaping optics generated a reasonably uniform profile over a large portion of the focused beam area. However, a small amount of undiffracted light resulted in a high-intensity, on-axis spike. A beam profile approaching the desired 'top hat' geometry could be obtained by moving the flyer launch plane a few mm inside or outside of the focal plane. The planarity of flyers generated using this optical assembly was evaluated using a line-imaging, optically recording velocity interferometer system (ORVIS). Results of these measurements demonstrate the deleterious effect of the on-axis spike on flyer planarity. Acceptable conditions for useful flyer impact experiments can be obtained by operating at a position that provides a near-top-hat profile.

Diffractive optical element (DOE) can be used for beam smoothing, owing to its high light efficiency, design flexibility and so on. The phase design of the DOE can be considered as an optimization problem and optimized by many kinds of algorithms. In this paper, the hybrid algorithm, merging Hill-climbing with Simulated Annealing, is adopted to design the phase of the DOE. The designed phase is the sum of many kinds of sine function with different period, amplitude and initial phase, which the continuous phase DOE is ensured. The continuous phase DOE is fabricated by ion etching, with a rotating hollowed-out mask. Experiments of beam smoothing are carried out with a pulse working Nd:YAG laser, and spots with a rather good beam smoothing performance are obtained.

Diffractive optical element (DOE) can be used for beam transform to obtain a certain required focused spot. The phase design of the DOE can be considered as an optimization problem and can be optimized by many kinds of algorithms. In optimization, the intensities of the sampling points are optimized to be consistent with the required beam transform, but the other points are not consistent with such demand. In this paper, fine design of the DOE is completed with a new sampling interval on the focal plane. The simulated results show that the intensity of any point on the focal plane is fully filled with the required beam transform, besides the used sampling points in the optimization.