Research on surface plasma characteristics of CaF2 and MgF2 windows induced by laser

Fluoride optical windows with low refractive index and good transparency have been widely used in photoelectric systems, laser systems and optical protective windows. The temperature and the electron number density of surface plasma of fluoride windows induced by 355 nm laser was calculated. The Saha-Boltzmann plot method was used for the calculation of the temperature of plasma. The approximate linear relationship between the electron number density and the Stark broadening of independent spectral lines was used for the calculation of the electron number density. The polynomial function was used to fit the background radiation intensity to solve the peak intensity and the signal-to-background radiation ratio of the emission spectrum. The effect of different laser fluence on the temperature of plasma and electron number density was analyzed. The temperature of plasma of CaF₂ and MgF₂ reached about 5178 K and 7230 K and the electron number density of CaF₂ and MgF₂ reached 6.052 x 10¹⁷ cm^-3 and 5.127 x 10¹⁶ cm^-3 at a laser fluence of 98.2 J/cm² . The signal-to-background radiation intensity ratio of the calcium atom emission spectrum of plasma of CaF₂ induced by 355 nm laser was between 0.2 and 0.5, while the calcium ion emission spectrum after one ionization showed a ratio of approximately 1.19.