Optical Components. Strong Laser And Plasma Surface Clean Processing Research

With the development of laser inertial confinement for fusion(ICF),it is important to build large scale high-power solid laser driver. The effect of impurities is a key factor to keep the laser driver in normal operation. Experiments shown that the surface of optical elements is polluted by silicones grease(s7501#) in the vacuum environment. As a result, laser beam quality is decline, and laser damage threshold of optical elements is also descended. Therefore, it is necessary to study the cleaning technique of optical elements.Firstly, this paper provides a review of the normal laser-induced damage mechanism of optical elements, and points out that the effects of impurities on the surface of optical elements is a key factor to keep the laser driver in normal driver .Then both of the laser-induce cleaning mechanism and the plasma-induce cleaning mechanism are described. Secondly, two aspects related to cleaning methods are discussed. One of the cleaning effect of silicone greases on the surface of fused silica and K9 glass induced by 351nm short pulse is described. For the K9 glass, the cleaning effects of front surface are better than back surface .However, for the fused silica, the cleaning effects of back surface are better than front surface. Base on the theoretical of light pressure, the pressure of back surface are stronger than front surface for the K9 glass and fused silica, but according to the measured results of the laser photo-thermal transmission technique, the thermal absorption of the K9 glass is greater than fused silica irradiated by 351nm laser .Therefore, the effects of heat expansion of front surface of K9 glass is stronger than the front surface of fused silica, and corresponding effects of cleaning are achieved. The another important effect is cleaning silica-oil-and-grease on the surface of fused silica the by CO2 laser cleaning method combining plasma cleaning method. The fused silica surface, which polluted by evaporative silicones grease, are cleaned by CO2 laser and plasma. The sample is characterized via the optical microscopy, the surface contact angle, the optical transmittance and the laser damage threshold measurements. The experimental results show that the substrate could be well cleaned by low energy CO2 laser combined with vacuum plasma. After cleaning, the contact angle of quartz substrates decreases from 63°to 4°, the transmittance increases from 92.3% to 93.0%, and the damage threshold increases from 3.77 J/cm2 to 5.09 J/cm2 when irradiated by 351nm, 6ns single pulse laser.