Reflectivity mapping of large-aperture mirrors with cavity ringdown technique.

A high-precision reflectivity measurement device based on an optical-feedback cavity ringdown technique is developed for mapping reflectivity distributions of large-aperture highly reflective (HR) mirrors with a diameter up to 300 mm. Reflectivity maps are obtained by two-dimensionally raster scanning the large-aperture mirrors and measuring the reflectivity as a function of position. By employing a laser source with a beam diameter of approximately 0.4 mm, reflectivity maps with sub-millimeter spatial resolution are achieved. The reflectivity non-uniformity of HR mirrors is investigated by statistically analyzing the experimental reflectivity distributions. The measurement repeatability of the device is also experimentally investigated, with a standard deviation of approximately 0.0001% for reflectivity higher than 99.99%.