Quantitative Evaluation of Subsurface Damage by Improved Total Internal Reflection Microscopy

Subsurface damage (SSD), having a great impact on the laser-induced damage threshold (LIDT) of ultra-smooth optics applied in high-power laser systems, should be tightly controlled. An improved total internal reflection microscopy (TIRM), combined with digital image processing techniques, is proposed to quantitatively inspect SSD. With the characteristic that there is a relatively small depth of field (DOF) for a microscope at high magnification (50×), a series of SSD images are captured along with the microscope focusing at different depths under the surface by means of micro-focusing control. The definition of each image is calculated through wavelet transformation. By simulation, the relationship between the definition of TIRM images and the depth of the SSD has been established. According to the definition curve, the SSD depth is acquired. Fused silica glasses polished after fine grinding are measured non-destructively by our TIRM setup. The results show that the improved TIRM is a useful method to evaluate SSD. It is helpful to improve the efficiency of optical fabrication.