Study On Laser Damage Of Optical Thin Film Induced By Inclusion Absorption Based On ANSYS

Optical coatings are almost the indispensable components of all the optical systems and are also one of the weakest links in the laser system.Previous studies have found that for long pulse laser inclusions absorption in the optical coatings is the main reason for the decline of laser induced damage threshold.In the analysis of the mechanism and process of damage induced by inclusions,it is very important to evaluate the temperature distribution.The rationality of temperature distribution will affect the following mechanics analysis.So the study of inclusions absorption has an important theoretical value and practical significance.The existing models have ignored the nonlinear absorption effect of inclusions.In this paper, we have made some improvements in this aspect.In accordance with classic thermal conduction equation,transient temperature field in coatings under laser radiation are analyzed in this paper.It is provided that dielectric film and substrate are isotropic,and the thermal physical properties of inclusion are variable,while the coating's are not.An optimized finite element is developed to simulate transient temperature filed in coating/substrate systems based on a well understanding of the influences of element size and integration time step on the solution stability.Based on this theoretical model,transient distributions of temperature field around kinds of inclusions with different shapes,sizes and depths are simulated by using the finite element method(FEM).Numerical results indicate:In the time of single impulse irradiation,the highest temperature rise doesn't synchronized with the peak of laser power density,but delayed more or less.For the body inclusion,every orientation of globosity,cylindrical and sphenoid inclusions will cause a decline of damage threshold.But the absorptions of inclusions with larger sizes and lighter positions will cause a higher temperature rise and result in a lower damage threshold.There is a relationship similar to a quadratic function between the highest temperature rise and the sizes of globosity inclusions and cylindrical inclusions whose axis is parallel to the direction of the coatings' thickness.For the surface inclusions,there is a size range,in which the inclusions will cause a laser damage and the range can be calculated by sophisticated simulation.The result in this paper shows that under the irradiation of YAG laser generator with the the wavelength of 1064nm,pulse-width of 12ns,power density of 2.22×10~6 W/cm~2,the globosity inclusions in the radius of about 50～120nm will cuase a damage of coatings.These conclusions may provide reference for the theory research and numerical simulation of optical coatings' thermal damage induced by the inclusions absorption under laser radiation.And they can guarantee the credibility of the following mechanical analysis.At the same time,they can also be a guide to take effective measures to control the inclusions in a certain position and size.