Waterproof Polymer Coatings for Phosphate Laser Glass-Based Actively Cooled Laser Disk Amplifiers

All high-peak-power laser systems around the world involved in fusion research employ phosphate-based, neodymium doped laser glass (e.g., LHG-8) to amplify laser energy in the near IR wavelength region. One limitation of phosphate laser glasses is that they are attacked by water or water vapor to degrade the polished laser glass surfaces, which in turn leading to a reduction in laser energy output due to light scattering. This moisture sensitivity limits cooling methods to either “passive” gas-phase cooling (e.g., nitrogen or helium) or “active” fluid based liquid coolants such as fluorocarbons or ethylene glycol-water mixtures. Next-generation laser disk amplifiers will require active cooling in order to achieve higher energy on-target with higher shot repetition rates, but neither fluorocarbons nor ethylene glycol-water mixtures represent a viable alternative (the former is a “forever” chemical, while the latter limits laser output due to its near-IR absorption). “Heavy “water (D₂O) represents a viable alternative, but only if the LHG-8 can be coated with a protective polymer layer to eliminate or mitigate erosion of the laser glass surfaces by D₂O without compromising optical transmission or laser-induced damage. In this work, five polymer systems were evaluated as potential protective coatings for phosphate laser glass by dip-coating polished LHG-8 samples in solutions of the polymer candidates in various solvents and different % solids concentrations to produce highly transmissive coatings free of intrinsic defects ranging from tens of nanometers to tens of microns in thickness. Three of these polymer coating materials exhibited films with high surface uniformity, excellent optical transmission, and large hydrophobicity (as evidenced by contact angle measurements) on LHG-8 laser glass. These samples are currently under evaluation in a flowing coolant test bed to determine their long-term resistance to both erosion and coating defect generation induced by water.

This material is based upon work supported by the Department of Energy [National Nuclear Security Administration] University of Rochester “National Inertial Confinement Fusion Program” under Award Number(s) DE-NA0004144 and the U.S. National Science Foundation under Cooperative Agreement No. (PHY-2329970).