A novel scheme of homologous differential transmissivity and reflectivity measurer

To improve the performance of differential transmissivity & reflectivity measurer (DFTRM) for coated mirrors with extremely low transmission or reflection loss, a novel scheme based on Fresnel formula and optical balance is demonstrated. Benefit from the characteristic of Fresnel formula, i.e., the specular reflectivity of the reference optical surface varies slowly with the incident angle near Brewster’s angle, so the precision measurement of transmissivity or reflectivity is then converted to the precision measurement of the incident angle in our two-optical-path scheme. An experimental system is set up to verify the feasibility of precision measurement, and the preliminary measurement results for high-reflectivity coated mirrors with low transmission loss prove its ability to measure transmissivity and distinguish the transmissivity difference of 10 parts per million (ppm) magnitude. Various potential error sources, including the responsivity of photodiodes, the scale factor, the polarization of the incident laser, the refractive index of the reference medium and the spatial relationship of components, are discussed qualitatively or quantitatively to provide guidance for the subsequent optimization of transmissivity & reflectivity measurer at ppm level in the future.