Efficient and sensitive detection of absorptive defects in fused silica optics based on laser thermal pumping combined with micro-interferometric imaging

A novel, efficient, and sensitive method for detecting absorptive defects in fused silica optics, called laser thermal pumping combined with micro-interferometric imaging (LTP-MII), is proposed. First, the thermal deformation simulation of fused silica optics under a 355-nm LTP was calculated using a multiphysics field approach. Even with ppm-level absorption on the surface, a 600-W/mm2 LTP power density could induce a 2.1-nm deformation within the detection range of the MII. Subsequently, an LTP-MII system was constructed, comprising a 355-nm LTP spot with a 0.45-mm diameter and a white-light MII. Results from different fused silica samples showed that, at the same LTP power density of 19.5 W/mm2, the induced thermal deformation followed this order: 5% absorption thin film (75.3 nm) > a full brittle fracture pit defect (27.8 nm) > a scratch defect (5.2 nm). However, no thermal deformation was observed for the super-smooth polishing low-absorption substrate under these conditions, though it exhibited a deformation of 13.31 nm when the LTP power density reached 110.7 W/mm2. In addition, thermal deformation was positively correlated with the LTP power densities. These results demonstrate that LTP-MII is an efficient and sensitive method for reflecting the photo-thermal absorption characteristics of surface defects in fused silica optics.