Arbitrary Space-Variant Polarization Control for Large Aperture, High-Energy Lasers

Project Overview

Thermally-induced stress birefringence often severely limits the performance of high-energy laser systems. In this work, we explore the feasibility of a depolarization mitigation scheme that is scalable to larger apertures and higher fluences than any existing solution. In the proposed polarization correction scheme, magnetorheological finishing (MRF) tools are used to carve space-variant thickness profiles in two or more quartz waveplates to achieve arbitrary space-variant polarization control. To test the feasibility of this method for polarization compensation, a small-aperture (11 mm) custom retarder was designed to generate a "distorted" space-variant polarization distribution resembling the output of a typical thermally driven laser amplifier head. After expanding the beam to a 2-inch diameter, thickness prescriptions for two MRF freeform crystals were designed to convert the distorted polarization distribution to a uniform linearly polarized beam, thus eliminating the energy loss incurred upon selecting a single linear polarization mode.

Although the feasibility goal of fabricating both freeform waveplates and demonstrating a 20x reduction in depolarization losses could not be met due to procurement delays, significant progress was made on the fabrication of the first waveplate. Based on polarimetry measurements taken with the in-process waveplate as well as additional simulated results and error studies, a 20x energy-loss reduction is realistically achievable over a large subaperture of the 2-inch beam. Further study is required on the mitigation of energy losses near the edge of the beam resulting from residual wavefront errors, but the proposed technology remains a promising solution to vastly improve the performance of high-energy laser systems and enable new operating regimes.

Mission Impact

This work supports several of LLNL's core competency areas, including laser and optical science and technology, high-energy-density science, and additive manufacturing. Our results enable a new class of experiments and laser-operating regimes that will advance Livermore's inertial fusion science research in support of the NNSA mission. Example programs which might presently benefit from this capability span the HEDS community, NIF&PS PAD, including DoD (SSL high-average-power projects), APT, engineering (additive manufacturing), PLS (characterizing/mitigating laser-plasma interactions), and global security (imaging polarimetry for remote sensing applications).

Publications, Presentations, and Patents

Di Nicola, J.-M. et al. 2021. "Polarization Manipulation of Free-Space Electromagnetic Radiation Fields." US patent application 2021/0239893 A1, filed September 1, 2020, published August 5, 2021.

Vella, A. et al. 2023. "Freeform Crystals for Arbitrary Space-Variant Polarization Control in Large Aperture, High Energy Lasers." Presentation, Photonics West 2023: High Power Lasers For Fusion Research VII, San Francisco, CA, February 2023.