Dielectric Grating Technology for High Power Ultrashort Laser Source

High-power lasers operating at high operation rates based on gold-coated grating compressors are limited by the absorption of the gold coating [1], [2] that leads to heating and deformation of gratings under vacuum. An alternative to decrease absorption is the use of all-dielectric gratings that consists in dielectric ridges on a reflective dielectric multilayer stack. Such approach enables broadband acceptance for the compression step of the chirp pulse amplification [3] of high power femtosecond pulses. Dielectric gratings have been reported but can have a limited spectral bandwidth and requires an out-of-the plane layout for the associated pulse compressor design, resulting into a more complex and higher volume architecture as compared to in-plane layout. In this paper, we propose all-dielectric compressors for high power ultrashort laser source, enabling both high broadband efficiency (> 90% efficiency on spectral bandwidth> 100nm centered at 800nm) and high laser induced damage threshold (LIDT). Two architectures have been proposed with grating periods of 610nm and 560nm, enabling operation at 36° and 55° incidence angle in TE polarization. As an example, the theoretical efficiency of the -1 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">st</sup> diffraction order of the designed 560nm-period grating, given in Fig. 1a, shows a bandwidth larger than 100nm for 90% efficiency computed by RCWA (Rigorous Coupled Wave Analysis). Several components with different ridge profiles and reflective stacks are simulated in terms of efficiency and electric field intensity distribution, and manufactured by nanoimprint lithography and high-density plasma etching on a multilayer dielectric reflective stack. A picture of the 15mm x 15mm gratings manufactured on a 2 inch wafer is given in Fig. 1b. On Fig. 1c, the Scanning Electron Microscope (SEM) image of a FIB (Focused Ion Beam) cut of the grating shows the multilayer dielectric stack and the ridges of the gratings.