A critical component for the OMEGA EP short-pulse petawatt laser system is the grating compressor chamber (GCC).
This large (12,375 ft3) vacuum chamber contains critical optics where laser-pulse compression is performed at the output
of the system on two 40-cm-sq-aperture, IR (1054-nm) laser beams. Critical to this compression, within the GCC, are
four sets of tiled multilayer-dielectric- (MLD) diffraction gratings that provide the capability for producing 2.6-kJ output
IR energy per beam at 10 ps. The primary requirements for these large-aperture (43-cm × 47-cm) gratings are diffraction
efficiencies greater than 95%, peak-to-valley wavefront quality of less than λ/10 waves, and laser-induced-damage
thresholds greater than 2.7 J/cm2 at 10-ps measured beam normal. Degradation of the grating laser-damage threshold due
to adsorption of contaminants from the manufacturing process must be prevented to maintain system performance.
In this paper we discuss an optimized cleaning process to achieve the OMEGA EP requirements. The fabrication of
MLD gratings involves processes that utilize a wide variety of both organic materials (photoresist processes) and
inorganic materials (metals and metal oxides) that can affect the final cleaning process. A number of these materials
have significant optical absorbance; therefore, incomplete cleaning of these residues may result in the MLD gratings
experiencing laser damage.
By design, point-diffraction interferometers are much less sensitive to environmental disturbances than dual-path interferometers, but, until very recently, have not been capable of phase shifting. The liquid crystal point-diffraction interferometer (LCPDI) utilizes a dye-doped, liquid crystal (LC) electro-optical device that functions as both the point-diffraction source and the phase-shifting element, yielding a phase-shifting diagnostic device that is significantly more compact and robust while using fewer optical elements than conventional dual-path interferometers. These attributes make the LCPDI of special interest for diagnostic applications in the scientific, commercial, military, and industrial sectors, where vibration insensitivity, power requirements, size, weight, and cost are critical issues. Until very recently, LCPDI devices have used a plastic microsphere embedded in the LC fluid layer as the point-diffraction source. The process for fabricating microsphere-based LCPDI devices is low-yield, labor-intensive, and very "hands-on"; great care and skill are required to produce devices with adequate interference fringe contrast for diagnostic measurements. With the goal of evolving the LCPDI beyond the level of a laboratory prototype in mind, we have developed "second-generation" LCPDI devices in which the reference-diffracting elements are an integral part of the substrates by depositing a suitable optical material (vapor-deposited thin films or photoresist) directly onto the substrate surface. These "structured" substrates eliminate many of the assembly difficulties and performance limitations of current LCPDI devices as well as open the possibility of mass-producing LCPDI devices at low cost by the same processes used to manufacture commercial LC displays.