Laser technology has become increasingly widespread in various research fields in recent years. Compared with continuous laser processing, femtosecond laser processing can improve or even eliminate the thermal effects caused by laser reactions, while being highly designable and controllable because of the wide range of materials that can be processed. Currently, the atomic force microscopy is widely used for the inspections of the morphology of femtosecond laser etching processes. This method can achieve nanoscale precision measurements of the sample morphology; however, the inspection process is slow and expensive and can only detect the physical dimensions of surface etching, which is a significant constraint when studying the morphology of transparent materials after femtosecond laser internal processing. A bright-field microscope can only qualitatively measure the edges of the process without information on the refractive index. In contrast, quantitative phase imaging (QPI) is an imaging method that can measure the phase information of transparent samples by allowing light beams to pass through the processed area while quantitatively detecting the optical properties around the processed area. Due to its non-contact nature, high sensitivity, and wide field of view, QPI has been used extensively in industrial inspection and biomedicine. However, to the best of our knowledge, its application in femtosecond-laser processing has not yet been reported. Therefore, this study proposes performing QPI measurements on femtosecond laser-processed glass samples. The results demonstrate the potential of this method in detecting the sizes and refractive indices of machined cavities inside glass cubes, as well as verifying the effects of different glass dopants with different femtosecond laser pulse energies.

In this experiment, a femtosecond laser was focused on a glass sample, creating linear cavities inside the glass with the aid of high pulse energy. Initially, the processed sample inside the calcium-sodium glass was characterized using a bright-field microscope and QPI system to determine the size of the machined cavity. During this process, the changes in the modified region around the cavity can be quantitatively measured using a QPI system. To analyze the three-dimensional physical characteristics of the laser processing area from a side view, a four-sided polished K9 glass cube was employed. Finally, to further investigate the effects of cavity processing on undoped glass materials, the same process was performed on fused silica and analyzed quantitatively using the QPI system.

Femtosecond lasers with different pulse energies were used to process cavities inside doped (calcium-sodium glass and K9 glass) and undoped (fused silica) glass cubes, and the cavity structures were characterized in three dimensions using QPI. After femtosecond laser processing, the doped glass exhibits a symmetrical area of tubes and bands in the top-view direction. In this region, the phase undergoes a semicircular change, with the phase falling in the center and rising at the edges of the cavity (Fig. 3). In the side-viewing direction, there is an extension, and the phase first increases and then decreases. By analyzing the processing area inside the glass from various angles, we restore the morphological changes in the modified area around the processing location inside the calcium-sodium and K9 glasses and describe them in three dimensions (Fig. 5). For undoped glass, the phase decreases in the processed area in the top-view direction and increases on both sides. However, there is no semicircular modified area or abrupt phase change at the edge of the processed cavity. In the side-view direction, the phase drops and rises rapidly in the machined area, whereas the average phase is slightly higher than that in the unmachined area (Fig. 6).

QPI is an important technique for analyzing optical-microscopic characteristics and has the potential to be a valuable tool in ultrafast laser processing. Unlike atomic force microscopy, QPI can probe the interior of transparent materials and recover their internal morphology using quantitative phase information. Through the three-dimensional analysis of the machined areas inside the glass, it is possible to restore and depict the morphological changes around the modified areas of calcium-sodium and K9 glasses. The results indicate a significant difference in the range of the modified areas produced by different doped glass materials, when processed at the same energy. When the undoped fused silica is subjected to femtosecond laser processing, a “pearl chain” structure appears and the semicircular modification of the refractive index around the processed position is not readily apparent. This phenomenon is related to a change in the refractive index of the glass itself caused by the doped materials. In conclusion, QPI holds promise for playing an important role in the field of laser processing inspection.