Rotational Laser Scanning Microscopy for Measuring Roughness and Point Absorbers in Optical Surfaces

High-precision laser interferometric instruments require optical surfaces with low absorption and high reflectivity and with almost perfect surface contours to achieve the optimum measurement sensitivity. Problematic are surface roughness, causing scattering, and so-called point absorbers, heating up at high intensities and locally deforming otherwise flat surfaces. Here, we report on an interferometric technique for efficiently profiling optical substrate figures underneath the dielectric coating in terms of spatially resolved reflection phase and optical loss. We implement a proof-of-concept experimental setup, with which we reconstruct images of an over-coated mirror substrate profile with a sensitivity of up to (3.1 $\pm$ 1.4) fm/$\sqrt{\mathrm{Hz}}$ and a (5.29 $\pm$ 0.06) $\mathrm{\mu}$m lateral resolution.