Cleaning Of Multilayer Dielectric Pulse Compressor Gratings And Research Of Laser Induced Damage Threshold

Rapid development of ultra-high power laser device is attributable to the chirped pulse amplification (CPA) technology, and the induced laser damage of multilayer dielectric film gratings (PCG) wich are core components in CPA system is key factor influence on the operation of high power laser systen stable and sustainable. A wide variety of PCG surface contaminants, such as organic photoresist, fluoro-hydrocarbon deposits, metals deposits, airborne organic, and particulate contamination and so on, are introduced during the process of manufacturing gratings. These residual contaminants which have great absorption coefficient from incidencing laser pulses can easily form heat-absorbing centers and further lead to physics damage to the PCG. Therefore, selection of appropriate and effective cleaning method process to remove the surface contaminants is an ideal way to improve the laser induced damage threshold of PCG.The cleaning of surface contaminants that impact on the laser damage threshold of PCG is studied systematically in this paper. Complete physical and chemical surface analysis techniques of USTC (University of Science and Technology of China) can be carried out to qualitatively and quantitatively evaluate the cleanliness of PCG. The laser facility for laser damage test with picosecond pulses which has been built by CAEP (China Academy of Engineering Physics) and a similar test device which has been setting up by SIOM (Shanghai Institute of Optics and Fine Mechanics) both can be employed to evaluate the laser damage threshold. And a series significant achievements of PCG cleaning have been obtained.(1) The system research of wet cleaning with different solutions has been made. The optimization of cleaning by SPM solution has been got in the experiment. The diluted hydrofluoric acid to remove the thin hafnium oxide film which exist the top layer of PCG has been explored. The ideal process which solves the problem of secondary contaminante from chromium-etching solution used to remove chromium mask of PCG has been obtained. The reason which lead to cleaning-damage by wet cleaning has been comprehensively analysised, and new demands relating to the pre-process of PCG manufacturing have been put forward. Safety and efficiency of different frequency of ultrasonic-assisted cleaning have been explored, and the suitable type of ultrasound for PCG cleaning has been gained. (2) The dry cleaning of various gas-plasma has been researched. The high effectiveness of removing photoresist, fluorocarbon, and some particular contaminants has been gained by using corresponding gas-plasma cleaning. The source of metal contaminants from ion beam machine and photoresist-stripping device is analysised to direct improvements of the vacuum equipment design in future. The ability of HPM solution to remove metal contaminants of PCG surface has been explored, and a new cleaning method based on the HPM solution combined with oxygen plasma for PCG has been developed.(3) Basing on evaluation of picosecond-laser threshold of different cleaning methods, cleaning process Ⅰ and Ⅱ of PCG have been confirmed. HPM solution of cleaning process Ⅱ, combined oxygen plasma cleaning, is operated at lower temperature and shorter time than the SPM solution of cleaning process I, and the damage threshold of cleaning process Ⅱ is equate to (or even better) cleaning process I.(4) Storage condition of PCG of the laboratory of different functional areals of humidity, temperature and other environmental paramters has been studied. It found that PP plastic may contaminate the PCG surface when it is degraded by the humity and sun in the air. Therefore, PCG should be stored in sealing-container of glass materials. The samples polluted with the degradation of PP plastic should be further cleaning before use.