Sub surface damage measurements based on short coherent interferometry

Abstract

During grinding step in manufacturing process of glass lenses it is important to control such parameters as shape and sub-surface damage (SSD) with high accuracy which essentially influences the duration and costs of the subsequent polishing process. Typically used methods suffer from limited resolution and are time consuming. That is why the nondestructive measurement of SSD is a challenge for the metrology of grinded surfaces. In order to detect these damages, the scanning short-coherence interferometer, a method very similar to optical coherence tomography, is setup and tested at Aalen University. The lens under test is mounted on a rotation stage which can be translated in lateral direction. The sensor beam of the interferometer is focused onto the sample and can be moved along the axial direction. Lateral positioning accuracy is 2 µ m and lateral resolution is 4 µ m. The system is able to measure SSD at several positions on a lens within 10 min inside the optical workshop.