Non-ablative removal of sub surface damages in ground optical glass substrates by controlled melting of thin surface layers using CO_2-laser radiation

摘要

The form generation of optical surfaces by grinding and mechanical polishing results in small sub surface damages in the form of micro cracks which conventionally have to be removed by further removal of the damaged surface layers. In order to reduce process time and material cost non-ablative methods for removal of micro cracks are desired. Utilising the low optical penetration depths of less than 10 μm for CO_2-laser radiation in most optical glasses, the laser energy can be used to heat up and melt thin surface layers. Using a 1.5 kW CO_2-laser, a quasi-line focus formed by a scanner unit and a constant feed speed, it is possible to close all micro cracks present in the rough ground test surfaces (max. SSD-depth ～ 63 μm), while achieving a process time of less than 2 seconds for a O 30 mm N-BK7 lens, respectively 7.5 seconds for fused silica. With a Sa as low as 50 nm and low distortion from the original shape the surfaces can directly be conventionally polished, further reducing the process chain complexity.