Numerical study of thermal stress damage in dielectric film by long-pulse laser

Abstract

In this paper, we study and establish the theoretical model of thermal stress damage of dielectric film in film/substrate systems caused by long-pulse laser, based on which transient distributions of temperature field and thermal stress field are simulated using the finite element method(FEM) and then analyze the mechanism of the damage. In accordance with the basic equation of heat conduction equation and thermal stress equation, the physical model of dielectric film and the substrate transient temperature field and thermal stress field is developed with the assumption that the dielectric film and the substrate are isotropic and their thermal parameters do not change with temperature.On the foundation of theoretical analysis, transient of temperature field in dielectric film in film/substrate systems under long-pulse laser irradiation is simulated and calculated. The numerical results indicate that great temperature gradient exists in dielectric film and substrate in radial direction but smaller one exists in axial direction. When the laser power density is increasingly larger, the temperature gradient in radial direction is larger and the temperature in the center of the film is higher. Transient thermal stress field in dielectric film in film/substrate systems under long-pulse laser irradiation is simulated. Numerical results show that, the damage to the dielectric film caused by long-pulse laser is mainly due to thermal stress damage process which circumferential stress acts a primary role in. Such damage is a final result of the substrate being under thermal stress. For film/substrate system, the damage under long-pulse laser radiation starts from the substrate. When the laser power density is increasingly larger, the dielectric film is more vulnerable to damage and damage zone is greater.The result of this paper provides theoretical foundation not only for research of theories of dielectric film and substrate thermal stress damage and its numerical simulation under laser radiation but also for long-pulse laser technology and widening its application scope.Key words: long-pulse laser, dielectric film, temperature field, thermal stress field THEORETICAL MODEL Geometric modelWhen the surface of the optical dielectric film is irradiated by millisecond laser, its substrate and the film itself absorb the energy of laser which will be converted into heat form. This process is macroscopically reflected by temperature rises in the optical dielectric film and substrate and the partial expansion of the volume. Assuming that the optical dielectric film and the substrate is isotropic continuous medium and the absorption of the laser energy is volume absorption, we established the geometric model of optical dielectric thin film and the substrate under laser irradiation as shown in Figure 1. During the calculation, cylindrical coordinate system is applied because of laser spatial distribution which is Gaussian [1] .

The characterization of laser-induced thermal damage mechanism of mid-infrared optical coatings with surface contaminants

“…In order to better analyze the damage mechanism, we measured the critical ablation value of midinfrared optical coatings. Together with the absorptivity distribution of surface, a theoretical model based on the heat conduction theory was established [12] The calculated thermal damage mechanism of the optical coating agrees very well with the experimental data. As far as we know, this is the first comprehensive study on thermal damage mechanism of mid-infrared optical coatings with surface contaminants induced by CW laser.…”

“…There have been several models using thermal conduction theory to anlysis temperature field of coatings [3,12]. The camparison between the theory and experiment is still lacking.…”

“…In addition, on the irradiated surface of film, the heat flux q represents the annular heat flux induced by the CW laser with the influence of contaminants on the film surface. The heat flux can be expressed as follows [12].…”