Ultraprecision grinding of optical materials for high-power lasers

Abstract

Grinding is considered to be a rough machining process in the field of optics; a polishing process must follow the grinding process for getting optical-quality surfaces. An ultraprecision surface grinder with hydrostatic oil bearings and a glass-ceramic spindle of extremely low thermal expansion was developed to get smooth optical surfaces without any polishing process. Various optical materials such as NbF1, BK7, LHG08 fused silica, KTP, KDP and CLBO were ground into optical surfaces after empirically determining the conditions required to attain ductile-mode grinding. An extremely smooth surface less than 0.1 nm rms was obtained on BK7 glass by the ultraprecision grinding process. The laser-induced damage threshold was measured on variously finished LHG-8 laser glass at (lambda) equals 1.053 micrometers and 1-ns pulse width. The damage threshold was measured at 22.2 J/cm 2 on a ground surface with the polarization parallel to the grinding direction. This number is higher than that obtained by optical polish. The damage threshold of 293 J/cm 2 was also obtained on a ground LHG-8 glass surface at (lambda) equals 1.053 micrometers and 30-ns pulse width.