A combined method to reduce residual subsurface damage in optical grade polishing process

摘要

There are residual subsurface damage induced in conventional optical polishing, including hydrolyzed layer, polishing contaminant (embedded in hydrolyzed layer), and subsurface plastic scratch, which will sharply deteriorate laser induced damage threshold, imaging quality, and reliability of optical elements. Therefore, a combined method is proposed to reduce and finally remove subsurface damage from polished surface. Firstly, depth of hydrolyzed layer and contaminant distribution were detected with secondary ion mass spectrography. Then, dimension of subsurface plastic scratch was measured with atomic force microscope. Finally, a combined method, that is magnetorheological finishing and ion beam figuring, was adopted to reduce plastic scratch and contaminant in turn. The results show that magnetorheological finishing is effective in subsurface damage elimination induced in conventional polishing, and maximum depth of subsurface plastic scratch and embedded contaminant decrease from 18 to 4nm, and from 97.3 to 55.5nm, respectively. The polishing contaminant produced in MRF process is entirely removed by IBF. The method will improve the wave front precision and optical performances of optics in the mean time.