Edge steepness and plateau uniformity of a nearly flat-top-shaped laser beam (original) (raw)
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Characterizing flat-top laser beams using standard beam parameters
Canadian Journal of Physics, 2006
We examine the correspondence between various models describing flat-top laser beam profiles using two standard parameters; namely, the M2 factor and the kurtosis parameter. Numerical expressions for M2, based on the second moment of the beam irradiance distribution in the near and far fields and for the kurtosis parameter, k, based on the fourth moment at the near field, are obtained. Plots of k in the near field versus M2 demonstrate the similarities between the different analytical models used to describe flat-top profiles. Using the Padé approximation, a relationship between k and M2, a new reference formula, is derived that predicts the values of M2 to within less than a percent for these flattened beams. This method is then extended to define numerical expressions relating the beam parameters (i.e., M2 and k) and the parameters describing the beam characteristic in each analytical model (model parameters). The results obtained using the Padé method are used to describe the out...
Diffractive shaping of excimer laser beams
Journal of Modern Optics, 2000
We address the problem of shaping the intensity distribution of a highly directional partially coherent ® eld, such as an excimer laser beam, by means of diå ractive 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¯at-top beam in the Fourier plane of a lens is demonstrated with a diå ractive beam shaper fabricated as a multilevel pro® le in SiO 2 by electron-beam lithography and proportional reactive-ion etching.
2000
Industrial, military, medical, and research and development applications of lasers frequently require a beam with a specified irradiance distribution in some plane. A common requirement is a laser profile that is uniform over some cross-section. Such applications include laser/material processing, laser material interaction studies, fiber injection systems, optical data/image processing, lithography, medical applications, and military applications. Laser beam shaping techniques can be divided in to three areas: apertured beams, field mappers, and multi-aperture beam integrators. An uncertainty relation exists for laser beam shaping that puts constraints on system design. In this paper we review the basics of laser beam shaping and present applications and limitations of various techniques.
High-energy flat-top beams for laser launching using a Gaussian mirror
Applied Optics, 2010
Converting a Gaussian to a flat-top beam is useful for many applications including laser-launched thin-foil flyer plates. A flat-top beam is needed to maintain a constant launch velocity across the flyer; otherwise, the flyer can disintegrate in flight. Here we discuss and demonstrate the use of a variable reflectivity mirror (VRM) with a Gaussian reflectivity profile with an additional hard aperture and compare it to a refractive beam shaper. An ideal VRM would generate a flat-top beam with 37% efficiency. Readily available high-power Gaussian or super-Gaussian mirrors create an approximate flat-top profile, but there is a trade-off between flatness and efficiency. We show that a super-Gaussian mirror can, in principle, convert an input Gaussian beam with 30% efficiency to a flat-top beam with 3% (maximum-tominimum) variation. With a Gaussian mirror and a high-energy pulsed Nd:YAG laser having relatively poor beam quality, we generate flat-top beams with 25% conversion efficiency having 6% variation (standard deviation σ ¼ 4:2%). The beams are used to launch 400 μm diameter, 25 μm thick Al flyer plates, whose flight was monitored by a high-speed displacement interferometer. The plates flew across a 300 μm gap at 1:3 km=s. The distribution of arrival times at the witness plate was 5 ns, as determined by the rise time of the impact emission. Compared to a total flight time of 260 ns, the velocity spread of different parts of the flyer plate was 2%.
Production of high energy, uniform focal profiles with the Nike laser
Optics Communications, 1995
Nike, a KrF laser facility at the Naval Research Laboratory, is designed to produce high intensity, ultra-uniform focal profiles for experiments relating to direct drive inertial confinement fusion. We present measurements of focal profiles through the nextto-last amplifier, a 20 X 20 cm* aperture electron beam pumped amplifier capable of producing more than 120 J of output in a 120 ns pulse. Using echelon free induced spatial incoherence beam smoothing this system has produced focal profiles with less than 2% tilt and curvature and less than 2% rrns variation from a flat top distribution,
Single-element laser beam shaper for uniform flat-top profiles
Optics Express, 2003
We investigate theoretically and experimentally the decomposition of high-order Bessel beams in terms of a new family of nondiffracting beams, referred as Hermite-Bessel beams, which are solutions of the Helmholtz equation in Cartesian coordinates. Based on this decomposition we develop a geometrical representation of first-order Bessel beams, equivalent to the Poincaré sphere for the polarization states of light and implement an unitary transformation within our geometrical representation using linear optical elements.
Laser beam-quality/aperture-shape scaling relation
Applied Optics, 1986
Many high-energy lasers (HELs) have noncircular output apertures. Some are rectangular in shape with or without a central or noncentral (up to 30%) obscuration. However, most high-energy laser propagation codes (especially those developed for systems analysis) model the aperture as either an unobscured circle or as a circle with fixed (e.g., 10%) obscuration. We present a beam-quality/aperture-shape scaling relation which can be useful when applying these codes to realistic designs for HELs. Our analysis also yields a generalized formula for angular size of the Airy disk and definitions of a characteristic aperture length and aperture quality.
About particularities of intensities distribution in a cross-section of powerful laser beams
Laser Optics 2000: High-Power Gas Lasers, 2001
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Modeling of the laser beam shape for high-power applications
Optical Engineering
Aperture losses and thermo-optic effects (TOE) inside optics as well as the effective beam width in far field should be taken into account in the analysis of the most appropriate laser beam profile for high-power applications. We have theoretically analyzed such a problem for a group of super-Gaussian beams taking first only diffraction limitations. Furthermore, we have investigated TOE on far-field parameters of such beams to determine the influence of absorption in optical elements on beam quality degradation. The best compromise gives the super-Gaussian profile of index p ¼ 5, for which beam quality does not decrease noticeably and the thermo-optic higher order aberrations are compensated. The simplified formulas were derived for beam quality metrics (parameter M 2 and Strehl ratio), which enable estimation of the influence of heat deposited in optics on degradation of beam quality. The method of dynamic compensation of such effect was proposed. © The Authors. Published by SPIE under a Creative Commons Attribution 3.0 Unported License. Distribution or reproduction of this work in whole or in part requires full attribution of the original publication, including its DOI.
Gaussian laser beam shaping: test and evaluation
Current Developments in Optical Design and Engineering VI, 1996
A method for converting single mode Gaussian beams into beams with uniform irradiance profiles is described. This technique has application to laser cutting and welding, laser ablation, semiconductor mask fabrication, and other tasks. Currently, designs for rectangular and circular flat top profiles have been investigated. Experimental results are presented for an element that converts a single mode Gaussian beam into a square, flat top spot. The design is based on a Fourier transform relation between the input and output beam functions and can be implemented as a diffiactive or reffactive element. The form of the element reduces to a common equation that is scaled for the particular geometry involved. This scale factor contains the product of the widths of the input and output beams, the focal length of the system, and the wavelength. It is a dimensionless quantity that uniquely determines the quality of the target spot, regardless of wavelength or system geometry. A designer can thus start ftom a desired target quality and lay out the required optical system to achieve that quality, in contrast to an iterative approach.