Measuring surface deformation of optical components with surface thermal lens technique

Abstract

When a modulated laser beam irradiates an optical component, the laser-induced surface deformation has both direct current (DC) and alternating current (AC) portions. Explicit surface deformation and surface thermal lens (STL) theory models are developed to describe the DC and AC portions of the surface deformation and corresponding STL signals. Experimentally, a setup combining laser calorimetry (LC) and STL technique is developed to measure the absolute absorptance and laser-induced surface deformation of optical components. The absorptance measurement is implemented by LC with excellent stability and repeatability. The surface deformation measurement is realized with STL amplitude by defining an approximately linear relationship between the AC (or DC) STL amplitude and the maximum AC (or DC) deformation. As an example, the deformation value of a BK7 substrate coated with a TiO2/SiO2 film stack of absolute absorptance 1.32×10 -3 , irradiated by a 1064nm laser with 3.8W power is determined to be 34.3 nm with the experimental STL amplitude, in good agreement with the theoretical value of 35.8 nm calculated by the explicit surface deformation model. An indirect approach is proposed to determine accurately the irradiation beam radius by fitting the experimental data of the radial AC intensity change at the detection plane to the explicit STL model. By performing a theoretical fit to the experimental frequency dependence of the STL amplitude, the thermal properties of the optical component (i.e. the thermal diffusivity) can also be determined.