Large range closed-loop scans with a focal-distance-modulated confocal sensor

Abstract The article presents large range closed-loop scans with a high-speed focal-distance-modulated fibre-coupled confocal sensor integrated into a nano measuring machine (NMM-1). Combined with an appropriate signal processing this axial beam modulating sensor yields a linear characteristic curve, which enables areal closed-loop scans. Such scans are conducted under thermally stable conditions on a concave mirror with a nominal focal length of 19 mm. Prior to the scans the mirror is used to determine and adjust the probe direction of the sensor in order to align the probe direction with the z-axis of the NMM-1. Free-form scans without specification of the geometry of the concave mirror and scans with the specification of the spherical geometry are compared regarding the achievable scan velocity and the influence of the scan velocity on the achievable measurement precision of profile scans. The measurement precision for these scans over a double-digit millimeter-range is in the low double-digit nanometer-range. Finally, the ultimate areal measurement precision is calculated by the standard deviation of each single measurement point of repeated areal scans of the concave mirror.