Effect of numerical aperture on molten area characteristics in micro-joining of glass by picosecond pulsed laser

Glass products with precise and sophisticated shapes are highly demanded in the field of MEMS due to their excellent properties. Ultrashort pulsed laser has been expected to be a powerful and reliable tool for micro-welding of glass. Focusing condition such as numerical aperture (N.A.) is a critical parameter that controls how ultrashort laser pulses interact with and propagate in glass, and it has a great influence on the laser micro-welding characteristics of glass. In order to investigate the quality of welding process, it is important to understand the dependence of the mechanical strength of molten area created in glass specimen with various numerical apertures. Therefore, the mechanical strength of molten area with various numerical apertures was evaluated in micro-welding of glass by picosecond pulsed laser. Higher bending strength could be obtained under an appropriate volume ratio of molten area and glass specimen, when continuous molten areas were formed. In addition, high density and large size of molten area without crack led to higher breaking stress. It is concluded that superior focusing characteristics such as N.A. 0.65 enable a long region of high power density in beam axis, which can satisfy both high mechanical strength and high processing speed.