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Non-ablative removal of sub surface damages in ground optical glass substrates by controlled melting of thin surface layers using CO2-laser radiation

: Jung, Manuel; Trum, Christian; Schmidbauer, Beate; Willenborg, Edgar; Rascher, Rolf


Fähnle, Oliver W. (Hrsg.); Fütterer, Gerald (Hrsg.); Rascher, Rolf (Hrsg.); Haberl, Alexander (Hrsg.) ; Society of Photo-Optical Instrumentation Engineers -SPIE-, Bellingham/Wash.:
Seventh European Seminar on Precision Optics Manufacturing : 30 March – 1 April 2020, Teisnach, Germany
Bellingham, WA: SPIE, 2020 (Proceedings of SPIE 11478)
ISBN: 978-1-5106-3865-5
ISBN: 978-1-5106-3866-2
Paper 1147804, 10 S.
European Seminar on Precision Optics Manufacturing (POM) <7, 2020, Teisnach>
Fraunhofer ILT ()
laser processing; glass; optics manufacturing; removal; surface damage

The form generation of optical surfaces by grinding and mechanical polishing results in small sub surface damages in theform of micro cracks which conventionally have to be removed by further removal of the damaged surface layers. In orderto reduce process time and material cost non-ablative methods for removal of micro cracks are desired. Utilising the lowoptical penetration depths of less than 10 μm for CO2-laser radiation in most optical glasses, the laser energy can be usedto heat up and melt thin surface layers. Using a 1.5 kW CO2-laser, a quasi-line focus formed by a scanner unit and aconstant 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 Ø 30 mm N-BK7 lens, respectively 7.5 seconds forfused silica. With a Sa as low as 50 nm and low distortion from the original shape the surfaces can directly beconventionally polished, further reducing the process chain complexity.