Evaluation of a diffractive, microlens-array beam shaper for use in acceleration of laser-driven flyers

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 nun 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.