A FDTD Model Based Coating Thickness Evaluation Using Terahertz Time-Domain Spectroscopy in Reflection

A optimization model based on finite difference time domain(FDTD) method is proposed to evaluate accurate coating thickness using terahertz pulse in reflection geometry, which takes into account the multiple reflections caused by Fabry-Perot effect and dispersion of coatings. A deconvolution method with a double Gaussian filter is applied to improve the depth resolution. A stochastic optimization algorithm is used to fit measured signal to simulation signal and improve the robustness of root finding. The proposed method has reached accuracy down to 8μm in single-layer aviation coating samples and can be extended to the multilayer structure easily.