The advent of very large laser systems with thousands of optical components has necessitated an investigation of methods for cleaning both coated and uncoated optical elements. To perform adequately in laser systems, optical surfaces must be free from both films and particulate matter. Films have undesirable absorption bands and particles scatter light. Furthermore, in high power laser systems contaminants on the surface or trapped between the layers of coatings are known to lower the damage threshold. All surfaces must therefore, be clean when the optical elements are coated and installed and must remain clean for years after installation. Accomplishing this goal of long term cleanliness requires detailed consideration of cleaning methods and all mechanisms of recontamination. Generally, the nature of contaminants are unknown and therefore several cleaning steps may be necessary. Many conventional cleaning methods have proven unsuitable because they damage the precision surfaces they are to clean. Once cleaned, optical elements must be handled in such a manner as to preclude or at least minimize recontamination by airborne particles, aerosols or direct contact transfer. When finally installed the optical housing should not shed particles from fasteners or seals and should not outgas, which could cause an undesirable film to condense on the optical element. This paper discusses many of the techniques currently being used for cleaning optics and several new methods under investigation. It also discusses clean room procedures for reducing recontamination after cleaning.
Irving F. Stowers,
Howard G. Patton,
"Cleaning Optical Surfaces", Proc. SPIE 0140, Optical Coatings: Applications and Utilization II, (26 September 1978); doi: 10.1117/12.956265; https://doi.org/10.1117/12.956265