We discuss the development and applications of a new approach to Diamond-Like Carbon (DLC) coating that provides the durability of traditional DLC coatings, with the addition of significantly more transmission at visible wavelengths and greater transmission in the IR. We developed a deposition system design that incorporates multiple coating technologies, allowing for multiple material design approaches. This has enabled the manufacture of DLC coatings with improved extended spectral properties, suitable for applications in which the coating must withstand airborne particulate impacts, corrosive fluids, environmental extremes, and abrasive physical handling, while offering better than typical transmission in the visible or infrared wavelength regions, or both.
Multilayer dielectric (MLD) diffraction gratings are an essential component for the OMEGA EP short-pulse, highenergy
laser system. The MLD gratings must have both high-optical-diffraction efficiency and high laser-damage
threshold to be suitable for use within the OMEGA EP Laser System. Considerable effort has been directed toward
optimizing the process parameters required to fabricate gratings that can withstand the 2.6-kJ output energy delivered by
each beam.
In this paper, we discuss a number of conventional semiconductor chemical cleaning processes that have been
investigated for grating cleaning, and present evidence of their effectiveness in the critical cleaning of MLD gratings
fabricated at LLE. Diffraction efficiency and damage-threshold data were correlated with both scanning electron
microscopy (SEM) and time-of-flight secondary ion-mass spectrometry (ToF-SIMS) to determine the best combination
of cleaning process and chemistry. We found that using these cleaning processes we were able to exceed both the LLE
diffraction efficiency (specification >97%) and laser-damage specifications (specification >2.7 J/cm2).
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.
Thin-film polarizers are essential components of large laser systems such as OMEGA EP and the NIF because of the need to switch the beam out of the primary laser cavity (in conjunction with a plasma-electrode Pockels cell) as well as providing a well-defined linear polarization for frequency conversion and protecting the system from back-reflected light. The design and fabrication of polarizers for pulse-compressed laser systems is especially challenging because of the spectral bandwidth necessary for chirped-pulse amplification.
The design requirements for a polarizer on the OMEGA EP Laser System include a Tp greater than 98% over a spectral range of 1053±4 nm while maintaining a contrast ratio (Tp/Ts) of greater than 200:1 (500:1 goal) over the same range. An allowance must be made for the uniformity of the film deposition such that the specifications are met over the aperture of the component while allowing for some tolerance of angular misalignment. Production results for hafnia/silica designs will be shown, illustrating high transmission and contrast over an extended wavelength/angular range suitable for the 8 nm spectral bandwidth of OMEGA EP. Difficulties in production will also be illustrated, as well as the methods being implemented to overcome these challenges. A key challenge continues to be the fabrication of such a coating suitable for use on fused-silica substrates in a dry environment. Laser-damage thresholds for 1-ns and 10-ps pulse widths will be discussed.
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