The OMEGA EP Facility includes two high-energy, short-pulse laser beams that will be focused to high intensity in the OMEGA target chamber, providing backlighting of compressed fusion targets and investigating the fast-ignition concept. To produce 2.6-kJ output energy per beam, developments in grating compressor technology are required. Gold-coated diffraction gratings limit on-target energy because of their low damage fluence. Multilayer dielectric (MLD) gratings have shown promise as high-damage-threshold, high-efficiency diffraction gratings suitable for use in high-energy chirped-pulse amplification [ B. W. Shore et al., J. Opt. Soc. Am. A14, 1124 (1997).] Binary 100-mm-diam MLD gratings have been produced at the Laboratory for Laser Energetics (LLE) using large-aperture, holographic exposure and reactive ion-beam etching systems. A diffraction efficiency of greater than 99.5% at 1053 nm has been achieved for gratings with 1740 grooves/mm, with a 1:1 damage threshold of 5.49 J/cm2 diffracted beam fluence at 10 ps. To demonstrate the ability to scale up to larger substrates, several 100-mm substrates have been distributed over an aperture of 47 × 43 cm and successfully etched, resulting in high efficiency over the full aperture. This paper details the manufacture and development of these gratings, including the specifics of the MLD coating, holographic lithography, reactive ion etching, reactive ion-beam cleaning, and wet chemical cleaning.
Multilayer dielectric (MLD) diffraction gratings are a key component for the construction of high-peak-power, pulse-compressed laser systems. While a great deal of effort has been devoted to the design of optimal grating structures and the etching of these structures into the MLD coating, there has not been the same effort put into the optimization of the MLD coating itself. The primary characteristics of the multilayer that must be considered during design include minimization of the standing wave created in the photoresist because of the reflectivity of the coated optical surface, creation of a sufficiently high reflectivity at the use wavelength and incidence angle in a dry environment, proper balance of the individual layer materials to yield a coating with an overall neutral or slightly compressive stress, and a high laser-damage threshold for the wavelength and pulse duration of use. This work focuses on the modification of a standard MLD mirror, while considering these characteristics, to allow the fabrication of a diffraction grating with higher efficiency and laser-damage threshold than is typically achieved. Scanning electron microscopy (SEM) images of the grating structures demonstrate smoother shapes with lower roughness due to the holographic exposure. Damage testing performed at 1053 nm with a pulse width of 10 ps demonstrates the MLD coating has a sufficiently high laser-damage threshold to form the basis of reflection gratings that survive in high-fluence applications.