Reseach On The Influence Of Optical Components Surface Contamination On The Damage Resistance Capability

Damage resistance is one of the important performances of the large laser facilities. The continuous improvement of the damage resistance of optics and coatings is the basis of the laser facilities operating at high fluence. However, laser induced damage is still the bottle-neck problem for the fluence improvement. While surface contamination of optical components plays an important role in laser induce damage.This work focuses on the influence of surface contamination on the damage resistance capability of large laser facility. The methods of on-line monitoring, sampling and components analysis of contaminants have been studied and proposed. The influence of various contaminants on the optical and damage resistance performance of optics and sol-gel antireflective films has been experimentally studied. The relationship between deposition mass of contaminant and laser induced damage threshold（LIDT） has also been investigated. The “red line” where optical components can safely operate for the specific contamination is obtained. The electric field modulation caused by surface contaminant is simulated. The important results are summarized as follows:1. Three methods of on-line monitoring, i.e., on-line imaging system, surface acoustic wave sensor and optical microfiber sensor, are proposed according to the different contaminant types. The results show that on-line imaging system is suitable to monitor particulate contaminant whose size is larger than a few tens of microns, the surface acoustic wave sensor can effectively monitor the airborne molecular contamination, and the advantage of the optical microfiber sensor is to monitor particulate contaminant whose size is micron level or even smaller.2. Sampling and components analysis of contaminants:（a） Wiping method is used for sampling of particulate contaminant or nonvolatile residue. The morphology of contaminant is observed by scanning electron microscope（SEM）, and the component of contaminant is analyzed by energy dispersive spectrometer（EDS）. The results show that the main components of those contaminants are organics, metal and mineral.（b） Vacuum sampling tank is used for sampling of airborne molecular contamination, and the component of contaminant is analyzed by mass spectrometer. The results show that a great variety（no less than a few dozens） of molecular organics existed in laser facility including alkane, chlorine hydride, aromatic hydrocarbons, and so on. The total content and each concentration of molecular organics decrease with the increasing laser shot number and time.3. The influence of contamination on the optical and damage resistance performance of pristine optical components（e.g. fused silica and K9 glass） is studied.（a） The influence of dust on the damage morphology and LIDT of pristine K9 glass is studied. The size of damage pit in dust increases with the increasing laser shots and energy. The LIDT of K9 glass is inverse ratio with the size of dust. The “red line” where K9 glass can safely operate is obtained for different sizes of dust.（b） The influence of vacuum oil on the performances（e.g. morphology, transmission, LIDT, photo-thermal absorption） of pristine fused silica and K9 glass are studied. The results show that oil droplets appear on the surface of pristine optics after contamination. The transmission and LIDT of optics decrease with the increasing oil mass, and the corresponding correlation and formula are obtained. The “red line” where each pristine optical component can safely operate is obtained for different pollution levels.4. The influence of contamination on the optical and damage resistance performance of SiO₂ sol-gel antireflective film（e.g. 1ω and 3ω） is studied.（a） The influence of dust on the damage morphology and LIDT of 1ω sol-gel antireflective film is studied. The damage morphology of film aggravates with the increasing laser energy. The LIDT of film is inverse ratio with the size of dust. The “red line” where 1ω SiO₂ sol-gel antireflective film can safely operate is obtained for different size of dust.（b） The influence of vacuum oil on the performances（e.g. morphology, transmission, LIDT） of 1ω and 3ω sol-gel antireflective films is studied. The results show that the oil contamination will permeate into film at extremely low concentration, and the oil spots, condensation, and particles successively appear on the film surface with the increasing oil concentration. The transmission and LIDT of film decrease with the increasing oil mass, and the corresponding correlation and formula are also obtained. The “red line” where each film can safely operate is obtained for different pollution levels.（c） The influence of rubble outgassing on the performances of 1ω and 3ω SiO₂ sol-gel antireflective film is studied. The results show that the transmission and LIDT of sol-gel antireflective film decrease with the increasing contamination time or decreasing pressure. Additionally, the resistance against contamination of the 1ω sol-gel antireflective film is stronger than that of the 3ω sol-gel antireflective film. In addition, the transmittance peak red shifts when the film suffers heavy contamination.5. The three-dimensional contaminant models of the ellipsoid, circular cone, cylinder, and film are constructed. The light field modulation induced by different contaminants is simulated by using the three-dimensional finite-difference time domain（FDTD） method and the perfectly matched layer（PML） absorption boundary conditions. The relationships between contaminants parameters, e.g., shape, location, size, relative dielectric constant, numbers and distance, and light field modulation are obtained. The peak of electric field for each case is also obtained.The results above show that some conclusions on the influence of surface contamination on damage resistance have been achieved via experimental investigation and numerical simulation. It has a meaning of guidance and reference for engineering（laser system）.