Designing Damage-Resistant Multilayer Dielectric Gratings for Petawatt-Class Lasers

Project Overview

We successfully developed high efficiency dielectric gratings for chirped pulse amplification (CPA) pulse compression with a focus on improving damage thresholds for high peak power. Specifically, we focused on developing a first-of-kind design that operates at TM polarization. Unpublished modeling within our group shows that the electric field enhancement in the solid material for gratings operating at TM can be significantly less than for TE. Subsequently, the laser damage threshold of TM gratings should be higher by potentially a factor of 2X.

Prior to this LDRD effort, no work has been conducted at optimizing the incident polarization on the grating to minimize the electric-field enhancements to increase the laser damage performance. Consequently, all MLD CPA pulse compressions gratings are designed for TE polarization. Preliminary modeling indicates that the electric-field enhancements are reduced by 2X-3X for TM versus TE polarization. Furthermore, LDT testing of polarization insensitive MLD gratings (measured >95% diffraction efficiency for both TM and TE polarization) developed for spectral beam combining (SBC) for DoD programs, shows up to a 2X higher LDT for TM versus TE polarization.

Our LDRD effort concentrated on performing a modeling study to explore the feasibility of a manufacture-able design for a high efficiency dielectric grating for CPA pulse compression for use with TM polarization. We successfully developed a design would allow for the fabrication of small-scale apertures, less than 6-inch diameter, grating optics. The project successfully simultaneously fabricated six 2-inch diameter TM grating optics and six 2-inch TE grating optics for direct comparison. Recommended follow-on work would be to perform laser damage threshold measurements and to explore scaling optics sizes towards meter-scale.

Mission Impact

The technique of chirped pulse amplification (CPA) has enabled the generation of Petawatt-class laser system throughout the world. These laser systems today use meter-scale aperture, multi-layer dielectric (MLD) gratings to compress the final amplified chirped pulse. The realization of High-Energy Petawatt (HEPW) class laser systems, targeting laser pulses of greater >1000 J energy and pulse duration of pulse duration of <500 femtosecond time regime, has been hindered by the limitation of the laser damage threshold (LDT) of the MLD gratings. If successful, our novel TM grating optics could potentially allow 2X higher energy outputs.