The Coherent Addition of Gratings for Pulse Compression in High-energy Laser Systems

LLE Review, Volume 96 207 Introduction Short-pulse, high-energy, and high-irradiance laser systems provide many new opportunities for studies in light–matter interaction and inertial confinement fusion, including x-ray radiography and fast ignition. Research in high-intensity, high-energy backlighting for high-energy-density physics experiments under ignition conditions and integrated fast-ignition experiments with cryogenic targets depend on the development of short-pulse, high-irradiance lasers.1 Highpower, solid-state lasers, using the chirped-pulse-amplification (CPA) scheme, incorporate pulse compressors containing holographic gratings.2 The most-promising grating technology is a holographically formed grating combined with a multilayer dielectric (MLD) coating to form a highly efficient grating used in reflection.3–5 The four primary grating requirements, namely high diffraction efficiency, high wavefront quality, large aperture, and high damage threshold, make it a highly constrained optical system. The aperture size and damage threshold of reflection gratings determine the short-pulse energy capability of petawatt laser systems. The critical compressor component is the last grating, which experiences the shortest pulse and therefore the highest power. The highest reported damage threshold for commercially available MLD gratings is 0.6 J/cm2 at 275-fs pulse width.6 Assuming a square root of time scaling, these gratings would have a damage threshold of ~1.2 J/cm2 for a 1-ps pulse width. For a grating with 1740 l/mm and a Littrow angle of 66.5∞ for l = 1054 nm, this surface fluence corresponds to a beam fluence, measured in a plane normal to beam propagation, of approximately 3 J/cm2. These gratings are currently available in approximately 50-cm lengths. Assuming a 1.8 safety factor for diffraction modulation, the above provides an energy of less than 1 kJ. Gratings with larger apertures can further extend the shortpulse energy capability of petawatt laser systems; however, since the difficult fabrication process for MLD reflection gratings may limit the ultimate size of an individual grating to less than 1 m, alternative approaches are critical to scaling toward multiple-kilojoule, short-pulse laser systems. The Coherent Addition of Gratings for Pulse Compression in High-Energy Laser Systems