Calculation for the optical parameters of the Sn-doped SiO2 thin films by fitting the entire transmitted spectrum

The preparation of the Sn-doped SiO2 optical thin films with different Sn concentration at 70%, 90%, 100% has been conducted using sol–gel method. Several times of dipping are adopted to increase the thickness of the optical films. After that the transmission spectrum of the optical thin films is measured at a normal incidence using spectrophotometer. For double coated optical thin film, the relations among the transmittance T, incidence wavelength λ, thickness d and extinction coefficient k are present. According to the external point theory, the initial values of n and d are calculated. And two models are used for the expression of n, Cauchy and polynomial. The comparison of these two kinds of expressions is presented after curve fitting. Finally three curves for refractive index n are fitted, which shows that the refractive index becomes larger with the increase of SnO2. Also, it turns out that the refractive index decreases when the wavelength increases. And the curve of the extinction coefficient falls when the wavelength becomes larger.