Liquid crystal beam shaping devices employing patterned photoalignment layers for high-peak-power laser applications

Abstract

Optical elements with spatially varying transmission are required for laser engineering and imaging systems. Although refractive systems can transform the Gaussian energy distribution of a laser beam into a "top-hat" distribution, they lack the versatility to produce the more-complex and precise intensity distributions required for high-peak-power laser systems such as the OMEGA and OMEGA EP Laser Systems at the University of Rochester's Laboratory for Laser Energetics and the National Ignition Facility at Lawrence Livermore National Laboratory. Previously, distributions of opaque metal pixels on a transparent glass substrate have been used for such beam-shaping efforts, but laser-damage thresholds of the order of 200-700 mJ/cm² in the nanosecond regime limit their applicability. By applying photolithographic patterning of coumarin-based photoalignment layers using polarized UV light to generate spatially varying molecular orientation in a nematic liquid crystal (LC) device, we have developed and demonstrated highresolution beam-shaper devices for such high-peak-power laser applications in the near-IR region. Operating at 1054 nm, these devices demonstrated a contrast ratio ranging from 280:1 to 540:1, a pixel size of 10 m with an interpixel resolution of 1.7 μm, and laser-damage resistance ranging from 20 to 40 J/cm² at 1054 nm (1-ns pulse width) using a 10-μm layer of commercially available nematic LC materials. Coupled with the ability to generate an almost infinite variety of binary and gray-scale apodization and beam-shaping profiles by the photoalignment process, the high laser-damage threshold of these devices makes them attractive and useful tools for a multitude of laser applications.