Detection of the laser induced damage using a He Ne laser reflective imaging technique

Abstract: Laser-induced damage is a serious challenge for optical components; therefore, determining the laser-induced damage threshold (LIDT) is a crucial step in the manufacturing process. In many cases, such as for space applications, it is also necessary to account for the vacuum environment. Since conventional damage detection methods face limitations under vacuum conditions, an alternative approach is required. This article introduces a He-Ne laser imaging system designed for in-situ damage detection within a LIDT station. The system enables fourfold magnification imaging of the test sample and its surroundings without placing imaging optics inside the vacuum chamber, thereby preserving the cleanliness of the chamber. The detection method can be applied to both transparent and opaque samples; in transparent optics, damage is observable from either side. To verify the system's functionality, two sample types were investigated: a silica wafer (non-transparent for He-Ne radiation) and a commonly used dielectric mirror for 1030 nm (transparent for He-Ne radiation). Particular attention was devoted to determining the minimum damage size that can be reliably recognized. The system successfully distinguished damage features as small as 35 ${\mu}$m.