Effect of dimensions on thermal deformation of high reflectance sapphire mirrors under high power laser irradiation

In order to reduce thermal deformation of high reflectance sapphire mirrors under high power laser irradiation and improve the beam quality, effect of the diameter and thickness on thermal deformation of the high reflectance mirror was studied. The heat conduction equation expressed in polar coordinate and the thermal deformation formula were used to describe the distribution of the temperature field and the displacement field of the sapphire mirror. Then, the numerical calculation model was built with the finite element analysis software and the temperature field and displacement field in different thickness and diameter was calculated. The thermal deformation with respect to the thickness and diameter was obtained. The results indicate that the main factors affecting the PV value of the sapphire mirror surface is the temperature variation and the change of the size has effect on both temperature and stiffness. Appropriate size and thickness effectively reduce the rise of mirror's temperature and keep suitable axial structural stiffness, and thus the thermal deformation of the reflective mirror is decreased. The study results are useful for the size determination and selection of high reflectance sapphire mirrors under high power laser irradiation.