Filamentation Sealing Using Dual-Laser Treatment

Allison Browar | 23-FS-032

Project Overview

Each year, the Lawrence Livermore National Laboratory optics recycle loop refurbishes several hundred glass components that are damaged by exposure to high-fluence laser energy in the National Ignition Facility (NIF). However, optics with filament damage, a type of elongated laser-induced damage through the glass thickness, cannot currently be refinished when the filaments are close to the surface and the optics are removed from service. In this work we developed a closed-loop temperature-controlled system consisting of a high power, large area, carbon dioxide (CO2) laser beam to maintain temperature nearing the softening point for fused silica (1585 degrees C) while a smaller area beam is used to increase the temperature (to 2400 degrees C) to fill in the cavity with molten material. The large area beam then lowers temperature strategically to reduce residual stress.

The system was designed, built, and calibrated for laser heating fused silica. Filaments were made in the lab on fused silica samples using a focused pulsed ultraviolet (UV) laser to achieve repeatable filaments extending from the surface. Different approaches were used to find the best way to melt material into the lab made filaments. A physics-based model was used to predict the effect of the dual-CO2 beam on temperature profile within the fused silica material. Using the simulations as a guide, we found the best approach was to increase temperature of the surface of the filament to the materials softening point and use the smaller CO2 laser to raise the temperature just below evaporation to achieve reflow temperatures deep into the thickness of the optic. Our goal was to create a seal over the breached filament at least 48 micrometers deep and results from our experiments have achieved at least 100 micrometers deep reflow, exceeding our goal by over 2-fold. This work has given us the opportunity to create a capability at the lab which is not found elsewhere and to solve refinishing problems that were not previously achievable.

Mission Impact

As a Core Competency in Lasers and Optical Science and Technology, damage repair is imperative for the economic viability of high energy lasers as it gives a means for recycling optics that would otherwise be unusable. In this work we have developed a new capability to use closed loop temperature control for heating and annealing fused silica optics as well as reflowing material into lab made filamented sites. Use of this new apparatus has provided a means for rapid manufacturing and repairing glass surfaces that had been previously unexplored. Additionally, this gives Lawrence Livermore National Laboratory the capability to utilize this method for future applications with new collaborators. The dual laser system provides an easily scalable system for large optics and usage for wider applications in Accelerated Materials and Manufacturing.

Publications, Presentations, and Patents

Allison E.M. Browar, James Vickers, Eric Strang, and Eyal Feigenbaum, "Filament Damage Reflow in Fused Silica Optics" (Presentation, SPIE Laser Damage Symposium, Dublin, CA, 2023).