The Effect of Surface Processing Methods on the Laser Induced Damage Threshold of Fused Silica

Abstract

High peak power laser systems, such as National Ignition Facility (NIF), Laser Mega Joule (LMJ), and High Power laser Energy Research facility (HiPER), include a large amount of optics. Fused silica glass is one of the most common optical materials which is used in these high peak power laser systems owing to its excellent optical properties, especially for the 355nm ultraviolet laser. However, it is generally found that fused silica optics damage under irradiation with a high peak power laser beam, and the laser induced damage (LID) becomes the limit to increasing the laser power. Theoretically, the laser induced damage threshold (LIDT) of fused silica substrates is high, while it drops significantly due to the poor surface quality created in the manufacturing process.
This project aims to find a series of fused silica optical surface processing techniques which are able to improve the surface quality and increase its LIDT when irradiated using high peak power lasers. This work consists of the following contents：
1. According to the mechanisms of LID, the effects of surface structural defects and contaminants on the LID are analysed and some simulation work is done.
2. By means of the Magnetorheological Finishing (MRF) method, surface structural defects, i.e. surface and sub-surface damage, are removed because the MRF method is a non-fracture polishing process.
3. Parameter optimisation in the MRF process is done by the Taguchi designing method. This optimisation mainly focuses on the surface roughness because it is also another factor that limits the LIDT.
4. Two post polishing treatments, Ion Beam Etching (IBE) and a HF-based etching process (buffered oxide etch (BOE)), are used in this work to remove contaminants left by the former polishing steps (conventional polishing and MRF processes).
5. A series of LIDT tests are done to verify the validity of the above work. Results show that the MRF process, BOE etching and IBE treatment are all useful in improving the LIDT of fused silica optics.
The main contribution to knowledge of this work is that this work provides a series of processing techniques to increase the LIDT of fused silica optics. These techniques involve the MRF procedure, IBE method and BOE etching in sequence after the conventional grinding and lapping processes. These processing techniques are validated by several groups of LIDT tests.