Beam Shaping Technique Research Articles

Beam-shaping technique for improving the beam quality of a high-power laser-diode stack

We report a beam-shaping technique that reconfigures the beams to improve the beam quality and enhance the power density for a ten-array high-power laser-diode stack by using two optical rectangular cubes and two stripe-mirror plates. The reshaped beam has threefold improvement in beam quality, and its power density is effectively enhanced. On the basis of this technique, we focus the beam of the high-power laser-diode stack to effectively end pump a high-power fiber laser.

Laser beam homogenization: Modeling and comparison with experimental results

Beam shaping and homogenization techniques are essential to optimize a large number of laser-material processing applications and laser-material interaction studies. In this paper, we present an innovative approach for modeling laser beam homogenization by means of the integration method. The numerical results are compared with experimental data, and the influence of the measurement technique is discussed. The enhancement of the homogenization capability using an asymmetric divider is also presented and discussed.

Generation of femtosecond optical vortices using a single refractive optical element

It is shown experimentally for small spectral bandwidth that spiral phase plates (SPPs) can generate optical vortices in femtosecond beams without the need for additional dispersion compensation elements. An autocorrelation measurement performed on the obtained optical vortex (OV) beam showed that the SPP did not significantly change the pulse duration. Hence the single element SPP, already an established beam shaping technique in continuous wave and high power regimes, is presented as an efficient and practical means to obtain OVs that are free from spatial chirp in femtosecond beams.

Femtosecond pulse shaping using a liquid–crystal display: Applications to depth profiling analysis

We report on a beam shaping technique for femtosecond laser pulses based on a liquid-crystal display. The system is capable of modifying femtosecond Gaussian beams to a flattop beam. A pattern projected onto liquid-crystal display modifies the incoming Gaussian beam intensity so that flattop intensity profile is obtained. The process is monitored online using a charge-coupled device camera so that the intensity distribution of each pulse is known. An experimental example of the depth profile of a Cr layer on a Si substrate obtained using such a modified beam is presented.

Micrograting-array beam-shaping technique for asymmetrical laser beams

A beam-shaping technique is presented for asymmetrical laser beams with different beam waists and divergences in both vertical and horizontal directions. We utilize a pair of two-dimensional micrograting arrays to equalize the beam parameter products of an asymmetrical beam in orthogonal directions by deflecting the appointed parts of the beam on the longer side of the beam and by recombining the parts on the shorter side. When combined with divergent transformation by means of collimating optical components, the beam-shaping system can produce a symmetrical beam in orthogonal directions with optimized beam waists and divergences. A beam-equalization system is designed and demonstrated for a typical asymmetrical beam of a laser diode bar. In the experiment an emission beam with dimensions of 1 microm x 10 mm and half-divergences of 148 mrad x 576 mrad in the far field is transformed into an almost-square distribution with dimensions of -12 mm x 12 mm and half-divergences of -2 mrad x 2 mrad, which confirm the effectiveness of the proposed technique.

Quasi-spherical focal spot in two-photon scanning microscopy by three-ring apodization

We present a beam-shaping technique for two-photon excitation (TPE) fluorescence microscopy. We show that by inserting a properly designed three-ring pupil filter in the illumination beam of the microscope, the effective optical sectioning capacity of such a system improves so that the point spread function gets a quasi-spherical shape. Such an improvement, which allows the acquisition of 3D images with isotropic quality, is obtained at the expense of only a small increase of the overall energy in the axial sidelobes. The performance of this technique is illustrated with a scanning TPE microscopy experiment in which the image of small beads is obtained. We demonstrate an effective narrowing of 12.5% in the axial extent of the point spread function, while keeping the 82% of the spot-fluorescence efficiency.

SABCMS, a new approach to higher lateral resolution of laser probe measurement system

In order to achieve a high lateral resolution required for ultraprecision measurement of microstructural workpieces, shaped annular beam confocal measurement system (SABCMS), a new approach is proposed for ultraprecision measurement of 3D microstructures. SABCMS uses a binary optical element based on the diffraction principle to shape a Gauss laser beam into an annular beam, and optimizes the normalized inner radius of annular beam aperture ɛ to decrease the main lobe of airy spot, to increase the lateral resolution and measurement range. In comparison with an annular beam shaping technique used in a conventional annular lens confocal measurement system, the annular beam shaping technique used in SABCMS reduces the energy loss, and eliminates the effect of the inner ring edges diffraction of annular lens on the measurement system and improves the defocused property. Experimental comparison and analysis indicate that, as ɛ increases, both the lateral resolution and measurement range of SABCMS increase, and the spatial resolution improves, too, while the axial resolution of SABCMS reduces; the lateral resolution of SABCMS is better than 0.2 μm when wavelength of incidence laser beam λ = 632.8 nm, numerical aperture of measuring lens NA = 0.85, and ɛ = 0.5. It is therefore concluded that SABCMS is a new approach to further improvement of lateral resolution in laser probe measurement systems.

Enhancement of emitting power density with a beam-shaping technique for a high-power laser-diode array stack

We present a beam-shaping technique to enhance emitting power density for a high-power laser-diode array stack. By use of a stripe mirror and a high-reflection mirror, the laser source height of 42 mm is folded to 21 mm and the emitting power density is increased to 365 W/cm 2 from a raw emitting power density of 194 W/cm 2 . Moreover, the beam parameter product (BPP) that is used to evaluate the beam quality is effectively improved to 168-mm mrad from a raw BPP of 336-mm mrad in the fast axis for the collimated high-power laser-diode array stack.

Optical-sectioning improvement in two-color excitation scanning microscopy.

We present a new beam-shaping technique for two-color excitation fluorescence microscopy. We show that by simply inserting a properly designed shaded-ring filter in the illumination beam of smaller wavelength, it is possible to improve the effective optical sectioning capacity of such microscopes by 23%. Such an improvement is obtained at the expense of only a very small increasing of the overall energy in the point-spread-function sidelobes. The performance of this technique is illustrated by a numerical imaging simulation.

Superresolution laser beam shaping.

The superresolution technique is well known for its ability to compress the central diffractive spot that is smaller than the Airy diffractive spot. In this paper, we extend the superresolution technique for different laser beam shaping. A complete set of superresolution diffractive elements is developed for the flat-top beam shaping, the single-circle beam shaping, and the novel circular Dammann grating. Five phase plates, corresponding to each of its applications, have been made by use of micro-optics technology. Experiments that are presented are in good agreement with the theoretical results. The superresolution technique presented in this paper should be highly interesting for the wide applications of laser beam shaping.

Laser Beam Shaping

Laser beam shaping is the process of redistributing the irradiance and phase of a beam of optical radiation. This process of controlling optical beams is an enabling technology, which is used in a number of sectors of scientific, engineering, and industrial R&D. The shape of a laser beam generally refers to its irradiance profile, while the phase of a beam generally affects its propagation characteristics. Earlier work of Frieden and Kreuzer during the 1960s articulated well the goals of some contemporary laser beam shaping applications. Namely, geometrical optics was used to determine the configuration of a twoelement optical system that would transform an input plane wave with a Gaussian irradiance profile into an output plane wave with uniform irradiance. For some contemporary applications, physical optics must be used for the optical design of the laser beam shapers. A general presentation of the theory and techniques of laser beam shaping is given in a book edited by Dickey and Holswade. Laser beam shaping has become an important component of many laser-based applications, such as materials processing, medical applications, lithography, optical data storage, laser printing, isotope separation, optical data processing, and laboratory research. The first conference on laser beam shaping was held in 2000 at the Annual Meeting of the SPIE. This Laser Beam Shaping conference included a discussion of the fundamental limits of any beam shaping technique and the foundation for successful beam shaping design, including geometric techniques and optimization-based techniques. Applications included high-power laser fiber injection, UV and deep-UV homogenizers, micromachining in the electronics industry, and beam shaping techniques for laser printing. The Laser Beam Shaping II conference continued to bring together both workers in the field and potential users of the technology. Theory and design was again addressed, but the papers were primarily in the area of de-

Spatial mode control of a diode-pumped Nd:YAG laser by an intracavity liquid-crystal light valve

We present a new beam-shaping technique with an intracavity optically addressed liquid-crystal spatial light modulator. The Nd:YAG resonator is able to deliver beams with various spatial profiles such as flat-topped super-Gaussian and square-shaped beams.

Beam shaping with diffuse light by use of a single reflection

A technique for diffuse beam shaping is presented. The beam shaping is achieved by a single reflection on an element, which consists of many displaced parallel planar reflecting facets. The reflecting facets approximate a designed curved surface. We demonstrate the validity of the method for the conversion of a diffuse Gaussian beam into a uniform one in one of the spatial dimensions.

The calibration of dosimetry instruments used in interventional radiology.

No general agreement about the definition of the patient dose exists. As regards the radiation health risk, the doses to specific organs, Hi, are the ultimate measures for a patient dose. Values of the calibration measured, Hi, are provided only by calculational means. Out of the whole process of patient dose determination, the instruments to measure X ray spectra, FSD, field dimensions and Ka can be calibrated, X ray quality is derived from the total filtration and kV value. The actual dynamic and X ray quality ranges shall be considered when air kerma and DAP meters are calibrated. A DAP meter measurement averages the uniform radiation field specific for the X ray tube assembly used and for the beam shaping technique performed. Therefore, a DAP meter calibrated on site is preferable for patient dosimetry in interventional radiology.

Amplitude matching strategy for wave-optical design of monofunctional systems

Abstract The design of optical systems capable of transforming one given input field into an output field which maximizes one prescribed merit function is discussed in the functional embodiment of the system, that is by dealing with transmission functions instead of structured matter. Customary beam shaping techniques are identified as a special case of a more general strategy which is called amplitude matching. This strategy makes use of the field quantities amplitude and phase and therefore it is a wave-optical design approach. The flexibility of the technique is demonstrated by various examples. Parameters like conversion efficiency, signal-to-noise ratio, distances between transmission functions, and the number of transmission functions necessary for the implementation of wave transformations are discussed.

Amplitude matching strategy for wave-optical design of monofunctional systems

The design of optical systems capable of transforming one given input field into an output field which maximizes one prescribed merit function is discussed in the functional embodiment of the system, that is by dealing with transmission functions instead of structured matter. Customary beam shaping techniques are identified as a special case of a more general strategy which is called amplitude matching. This strategy makes use of the field quantities amplitude and phase and therefore it is a wave-optical design approach. The flexibility of the technique is demonstrated by various examples. Parameters like conversion efficiency, signal-to-noise ratio, distances between transmission functions, and the number of transmission functions necessary for the implementation of wave transformations are discussed.

Beam reformatting of one- and two-dimensional arrays of CO/sub 2/ waveguide lasers

The conversion of a linear to rectangular array beam is demonstrated for multi-element RF excited CO/sub 2/ waveguide lasers using the two-mirror beam shaping technique introduced by Clarkson and Hanna. High efficiency conversion of a one-dimensional array into a rectangular array format has been achieved with the reformatting mirrors in the near-field and with no auxiliary optical elements. The brightness of a 2-kW 64-element two dimensional array has been enhanced by a factor of 2 using a modified version of the reformatting technique to improve the fill factor of the array beam.

Beam-Shaping Techniques and Focusing Performance for a Diode Bar Used as a Pumping Source.

The end-pumping scheme in diode-pumped solid-state lasers has promised the achivement of high opticalconversion efficiency. However, in the case of using high-power diode bars as a pumping sourse, it is difficultto focuse the small-diamater beams required for an end-pumping laser due to the poor beam quality of thediode bar. This paper reviews beam-shaping techniques to improve the quality of the diode-bar and to achievea suitably focused spot size for end pumping.

Highly efficient diode-stack, end-pumped Nd:YAG slab laser with symmetrized beam quality

An efficient high-power cw Nd:YAG slab laser, partially end pumped by diode-laser stacks, and a novel beam-shaping technique are reported. The optical efficiency amounted to 44 %, and the slope efficiency amounted to 55 %. Introducing an intracavity Brewster plate to polarize the laser beam, we obtained an optical efficiency of 35 % and a slope efficiency of 41 %. The output beam was rectangular and the beam quality asymmetric in two orthogonal directions. To equalize the beam quality, we introduced a step-mirror beam-shaping technique. The beam-shaping technique and the results obtained are discussed.

Nd:YAG slab laser end-pumped by laser-diode stacks and its beam shaping

An efficient high power cw Nd:YAG slab laser, partially end-pumped by diode laser stacks, and a novel beam shaping technique are reported in this paper. The optical efficiency amounted to 44% and the slope efficiency to 55%. Introducing an intracavity Brewster plate to polarize the laser beam, an optical efficiency of 35% and a slope efficiency of 41% was obtained. The output beam was rectangular and the beam quality asymmetric in two orthogonal directions. To equalize the beam quality, a step mirror beam shaping technique was introduced. The beam shaping technique and the results obtained with it are discussed in this paper.